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COLLEGE PHYSIOGRAPHY 



THE MACMILLAN COMPANY 

NEW YORK • BOSTON • CHICAGO • DALLAS 
ATLANTA • SAN FRANCISCO 

MACMILLAN & CO., Limited 

LONDON • BOMBAY • CALCUTTA 
MELBOURNE 

THE MACMILLAN CO. OF CANADA, Ltd. 

TORONTO 



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COLLEGE 
PHYSIOGRAPHY 



BY 



RALPH STOCKMAN TARR 

LATE PROFESSOR OF DYNAMIC GEOLOGY AND PHYSICAL GEOGRAPHY 
IN CORNELL UNIVERSITY 



PUBLISHED UNDER THE EDITORIAL DIRECTION 
OF 

LAWRENCE MARTIN 

ASSOCIATE PROFESSOR OF PHYSIOGRAPHY AND GEOGRAPHY 
IN THE UNIVERSITY OF WISCONSIN 



THE MACMILLAN COMPANY 
1914 

All rights reserved 



4 



<w 



Copyright, 1914, 
By THE MACMILLAN COMPANY. 



Set up and electrotyped. Published September, 1914. 



SEP 24 1814 



NortoootJ IPregs 

J. 8. Cushing Co. — Berwick & Smith Co. 

Norwood, Mass., U.S.A. ■ 



EDITOR'S PREFACE 

This text-book of College Physiography is written for use in elemen- 
tary physical geography courses in universities, colleges, and normal 
schools, for supplementary reference-reading by high school students 
who are using a more elementary text, and for general reading by lay- 
men of mature years. 

The plan of the book is to present, in order, (i) the geographical 
features of the earth as a planet, (2) the processes in operation and 
the topographic forms in existence on the lands, (3) the physical 
geography of the ocean, and (4) the nature and effects of the atmos- 
phere. Combined with each of these are illustrations of the relations 
of physical geography to life and especially to man. 

No attempt has been made to cover all topics simply because they 
are usually included in a course in physical geography. Instead, each 
topic is discussed where it naturally comes up in the logical develop- 
ment of the subject. It is assumed that a certain elementary knowl- 
edge, for example of latitude, longitude, standard time, the seasons, 
etc., is retained from grammar school geography or high school phys- 
iography. They will naturally be reviewed in the laboratory work of 
a good course in college physiography or, when necessary, can be 
looked up in a school geography. For schools desiring a shorter list 
of assignments than is here presented, an abridgment might be ac- 
complished by omitting such matters as the Specific Instances of 
Volcanic Eruptions (pp. 449-475), Relief Features of the Earth (Chap- 
ter XVI), or the Earth's Interior (Chapter XVII). It is desirable 
that both field and laboratory work accompany the study of the text. 
For courses in advanced physiography or geomorphology, the study of 
the maps and the reading of selected papers from the original sources 
listed at the ends of the chapters is recommended. The photographs 
and diagrams in the text have been chosen with great care and are as 
well worth study as the text itself. 

The book was written by the late Professor Tarr, chiefly during the 
winter 1 9 ig-u, although there are indications that he began work on it 
as early as 1895. He had completed the first draft of the manu- 
script dealing with the earth as a planet, the lands, and the ocean be- 
fore his death on March 21, 19 12. He died suddenly and left no 
directions as to the disposal of this manuscript With the approval of 
Mrs. Tarr and after conference with several of Professor Tarr's more 
intimate friends among the geographers of the United States, the 
editor undertook the task of preparing the book for publication. He 
edited the existing manuscript, added data in connection with new 
discoveries in physical geography, prepared the illustrations and the 



vi EDITOR'S PREFACE 

bibliographies and map lists at the ends of the chapters, and wrote 
seven chapters to complete the book. These are the chapters dealing 
with the atmosphere and with terrestrial magnetism. In writing the 
original twenty chapters the author had followed the outline of the 
printed syllabus of his course in elementary physical geography at 
Cornell University. Accordingly this syllabus has been followed in 
the new chapters, amplified by as many as possible of the illustrative 
features which Professor Tarr used in his work of instruction at 
Cornell. In preparing the bibliographies and map lists, use was made 
of such materials as were left by Professor Tarr, some of them partly 
drawn up on catalogue cards for use in this book and some published 
in earlier books, together with some from the editor's own materials 
used in instruction at the University of Wisconsin. In all respects he 
sought to carry out the plan and style of presentation which he thought 
Professor Tarr would have followed in completing the book. In addi- 
tion to five years of study and teaching at Cornell University the edi- 
tor had the privilege of intimate association with Professor Tarr in 
four summers of field work in Alaska and in New York. He also 
assisted the author in clerical work while he was preparing three of his 
text-books and collaborated with him in writing two scientific books 
and a number of technical and popular articles. With this experience 
in mind constantly, the editor has striven to complete the book along 
the lines which the author would have followed, although with obvious 
imperfections in execution. 

As a matter of professional acknowledgment the editor does not 
feel that he can do better than to quote the author's own words, from 
the preface of Tarr's New Physical Geography, written eleven years ago. 
" It goes without saying that the author is profoundly indebted to the 
host of workers in physiography, from whom he has drawn so much 
inspiration, suggestion, and fact : Gilbert, Davis, Powell, Geikie, Penck, 
de Lapparent, Russell, Shaler, Dutton, Chamberlin, Hayes, Campbell, 
Salisbury, Brigham, Dodge, Dryer, and many others. From the writ- 
ings of these physiographers the author has culled whatever seemed 
to him suited to a scheme of elementary instruction ; and so numerous, 
and often so unconscious, is the influence of these fellow-workers, that 
specific acknowledgment would be quite impossible. Doubtless the 
most profound influence upon the author is that of his two teachers, 
Professors Shaler and Davis, the importance of which to him cannot 
be overestimated. Together with other physiographers, the author 
further recognizes in Professor Davis a leader in American physi- 
ography, from whom even some of the fundamental principles of the 
subject have been derived. An examination of the following pages 
would show the influence of this physiographer in many places, an 
influence not confined to the pure science, but extending to the 
pedagogy of the subject as well." 

The illustrations, many of which are new, are taken from photo- 
graphs by the author, or pictures taken under his direction by J. O. 
Martin of Wilbraham, Mass., by O. D. von Engeln of Ithaca, N.Y., 



EDITOR'S PREFACE vii 

by the editor of this volume, and others. A number were purchased 
for use in this and earlier books by the author, the collections of cer- 
tain American physiographers, and of the United States Geological 
Survey, the department of geology at Cornell University, W. H. Rau 
of Philadelphia, F. J. Haynes of St. Paul, S. R. Stoddard of Glens 
Falls, N.Y., and the Detroit Photographic Company supplying a great 
many. The photographer's name, where known, appears in the legends 
of the illustrations. Most of the foreign photographs were purchased 
in Europe by the author. Many of the photographs by the author and 
the editor were taken under the auspices of the National Geographic 
Society of Washington, the American Geographical Society of New 
York, and the United States Geological Survey. Most of the models 
are by the late E. E. Howell of Washington. A large proportion of 
the block diagrams were drawn by C. W. Furlong of Boston. Mr. E. 
F. Bean of the University of Wisconsin helped materially in prepar- 
ing the illustrations'. 

Acknowledgment is particularly due to Professor R. de C. Ward of 
Harvard University, who was good enough to read the manuscript of 
the six chapters dealing with the atmosphere, and to Dr. L. A. Bauer 
of the Carnegie Institution of Washington, who kindly read the man- 
uscript of the chapter on terrestrial magnetism. Each of these gentle- 
men made valuable criticisms and suggestions, but the editor takes 
full responsibility for any shortcomings in these chapters. 

The editor feels keenly the responsibility which he is incurring by 
preparing this book for the press, for the author's reputation is so high 
that nothing should be done that could possibly mar it. Professor 
Tarr prepared only the first draft of twenty out of the twenty-seven 
chapters. His experience in twenty years of college teaching, his 
facility in writing, and especially in writing three successful high 
school physiographies, a laboratory manual of physical geography, 
an elementary geology, an economic geology of the United States, a 
series of grammar school geographies, a regional geography of his 
own state, three scientific books on physiography, glaciers, and earth- 
quakes in Alaska, and scores of technical and popular articles, based 
upon his investigations in various parts of North America and Europe, 
all qualified him to produce a book of the first quality. Had he lived 
to complete it, there would surely be an improvement along many 
lines ; but, as it is, the book must stand upon its merits. 

LAWRENCE MARTIN. 

Madison, Wisconsin, 
July 13, 1914. 



CONTENTS 



Introduction 



PAGE 

xv-xxii 



PART I. THE PLANET AND THE LANDS 

CHAPTER 

I. Fundamental General Facts 

II. Weathering and Rock Disintegration. 

III. The Work of Winds 

IV. The Work of Underground Water 

V. Rivers and River Valleys 

VI. River Deposits 

VII. The River Valley Cycle ... 

VIII. Glaciers and Glaciation . 

IX. The Glacial Period . 

X. Lakes and Swamps 

XI. Shorelines . 

XII. Movements of the Earth 

XIII. Vulcanism . 

XIV. Plains and Plateaus 
XV. Mountains . 

XVI. Relief Features of the Earth 

XVII. The Earth's Interior 

XVIII. Terrestrial Magnetism 



s Crust, or Diastrophism 



i 
37 
57 
76 
100 
141 
171 
197 
256 
308 
342 
389 
438 
497 
525 
583 
611 
629 



PART II. THE HYDROSPHERE 

XIX. The Ocean 

XX. Life in the Ocean 

XXL Movements of the Oceanic Water 



637 
669 
682 



XXII. 
XXIII. 
XXIV. 

XXV. 

XXVI. 

XXVII. 

INDEX 



PART III. THE ATMOSPHERE 

Characteristics of the Atmosphere 
Light and Warmth in the Atmosphere 
Rain and Other Forms of Water 

Winds 

Storms . . 

Climate . ' ' . 



709 
7'5 
733 
746 

759 
783 

815 






LIST OF COLOURED MAPS 



PLATE 


NAME 




FEATURES SHOWN FACING PAGE 


I. 


Turtle Mountain and Frank, Alberta landslide 


5 2 


II. 


Niagara Falls and Gorge 


waterfall 


130 


III. 


Mississippi River . 




floodplain . 


144 


IV. 


Bering Glacier 




piedmont glacier 


242 


V. 


Harriman Fiord 




mountains and fiord 


286 


VI. 


Coast of California . 




shorelines 


348 


VII. 


Vesuvius 




volcano 


454 


VIII. 


Grand Canyon of the 


Colorado 


canyon and plateau 


516 


IX. 


Yosemite Valley 




mountain valley . 


548 


X. 


Boston and Vicinity 




peneplain and harbours 


598 



ACKNOWLEDGMENT OF ILLUSTRATIONS 

In addition to the credit for illustrations given in the Preface and in the 
legends of the half tones and text figures, acknowledgment is due the follow- 
ing authors and publishers for kindly permitting the use of illustrative material 
and, in several cases, for supplying electrotypes. The numbers below refer to 
figures in this book. 

Bowman's Forest Physiography, John Wiley & Sons, New York, — Figs. 
28, 183, 290, 351, 358, 362, 367, 378, 468. 

Davis's Elementary Meteorology, Ginn & Co., Boston, — Figs. 429, 500. 

Davis's Erkliirende Beschreibungder Landformen, B. G. Teubner, Leipzig, — 
Fig. 150. 

de Martonne's Traite de Geographic Physique, Armand Colin, Paris, — 
Figs. 379, 392, 412. 

Encyclopaedia Britannica, University of Cambridge Press, — Figs. 14, 77, 
170, 387,442. 

Herbertson and Taylor's Oxford Wall Maps, Henry Frowde, London, — 
Fig. 486. 

Hobbs's Earth Features and their Meaning, Macmillan Co., New York, — 
Figs. 36, 54, 59, 73, 139, 162, 164, 165, 166, 173, 181, 184, 190, 196, 221, 237, 
255, 257, 261, 278, 279, 280, 283, 289, 296, 322, 336, 384, 433. 

The editor of this volume, — (Photographs), Figs. 1, 122, 128, 136, 138, 
151, 153, 175, 176, 202, 203, 231, 360, 363, 376; (Maps and Diagrams), Figs. 
123, 126, 143, 154, 155, r 5 6 > *59< 195. 22 7< 272, 276, 286, 364, 377. 

Milham's Meteorology, Macmillan Co., New York, — Figs. 424, 457, 465, 
470, 477, 495> 49 8 - 

Moulton's Introduction to Astronomy, Macmillan Co., New York, — Figs. 
7, 8, 389. 

Murray and Hjort's Depths of the Ocean, Macmillan & Co., London, Fig. 398. 

Murray's The Ocean, Henry Holt & Co., New York, — Figs. 390, 391, 395, 
408, 413. 

Scott's Introduction to Geology, Macmillan Co., New York, — Figs. 17, 
211, 277, 292, 304. 

Todd's New Astronomy, American Book Co., New York, — Fig. 1 1 . 

Ward's Practical Exercises in Elementary Meteorology, Ginn & Co., Boston, 
— Figs. 460, 502. 

Ward's Climate, Considered Especially in Relation to Man, G. P. Putnam's 
Sons, New York, — Figs. 431, 435, 447, 483, 484, 492, 503. 

Wyoming Historical and Geological Society; also Connecticut Geological 
and Natural History Survey, diagrams by J. Barrell, — Figs. 345, 346. 



INTRODUCTION 

THE EARTH SCIENCES 

The earth consists of three quite distinct portions, a solid central 
mass, a partial envelope of liquid, and a complete blanket of gases, 
each of which is the seat of a series of interesting phenomena. Their 
investigation has attracted the thoughtful attention of many scientific 
men, both in the past and at present. The study of the gaseous por- 
tion, or atmosphere, has led to the development of Meteorology, and 
one phase of this study has been recognized as the science of Clima- 
tology. The science of the study of the waters is called Hydrography, 
and of that larger part of the liquid envelope which occupies the ocean 
basins, Oceanography. 

Several distinct sciences deal with the study of the solid earth itself. 
For example, Mineralogy concerns itself with the minerals of which 
this solid earth is composed ; and Petrology with the study of the rocks 
of the earth ; while the study of certain special phenomena has given 
rise to special sciences with limited scope, such as Seismology, which is 
concerned with a study of earthquakes, and Vulcanology, with volcanic 
phenomena. But the two chief sciences dealing with the solid earth 
are Geology and Geography, and each of these is concerned also, to a 
certain extent, with the waters and the air. These are, therefore, of 
broader scope than either of the other sciences, since they involve, to a 
degree, a consideration of the earth as a whole, not a single phase of it. 

Geology deals with the past history of the earth and its development 
through the ages. Geography, on the other hand, is concerned with 
the present condition of the earth in its relation to life. One of the 
divisions of geography is called Physical Geography, or, sometimes, 
Physiography, which may be defined as that science which investigates 
the physical features of the earth and their influence on life, especially 
man. It is a fundamental part of geography, and basal to any scien- 
tific study of that subject. To some, it seems difficult of separation 
from geology, and in certain of its aspects it might indeed be considered 
the latest chapter in geology, — the history of the present surface of 
the earth, or Geomor photo gy . But it is broader than this, for it deals 
not merely with the latest chapter in the history of the earth, but also 
with the influence of the surface features on human and other life, and 
the interaction and interrelation between air, water, land, and life. 

Physical geography is an integral part of geography, and not to be 
separated from it, having independent subrank under the larger whole, 
side by side with Political Geography, Anthropo geography, etc., and 



xvi INTRODUCTION 

having for its special field the more physical aspects of geography, 
as its name indicates. Dealing as it does with air, land, and water, 
physical geography of necessity draws from meteorology, geology, and 
oceanography for some of its facts and methods, and even for some of 
its field of investigation. Did it not do so, it could be little more than 
a descriptive science, telling merely what the earth's surface is, and 
leaving to other sciences the statement of how it came to be. In the 
study of the land in particular, it is necessary to borrow from geology, 
for no interpretation of the present surface features is possible without 
a knowledge of at least some of the past events by which they have 
come to be. 

The Principle of Vast Lapse of Time 

One of the most fundamentally important contributions to an under- 
standing of the history of the earth is the proof which geologists have 
presented of the vast lapse of time during which this earth history has 
been in progress. It is equally fundamental to an understanding and 
interpretation of the surface features of the earth with which physical 
geography has to do. So long as it was thought that the age of the 
earth was to be numbered in a few thousand years, no real progress 
was possible, either in unravelling its history or in interpreting the 
earth forms by which man is surrounded. It has now been demon- 
strated, by a series of proofs that are incontrovertible, that the age of 
the earth is to be reckoned in millions of years, and that even those 
slowly operating processes with which we are surrounded, and which, 
in a human life-time, may not cause visible change, are capable of 
performing vast tasks and of bringing about great changes, when in 
incessant operation through not merely hundreds of years, but tens 
of thousands and hundreds of thousands of years. 

The acceptance of this principle, which during the last century 
required long and heated argument to establish, and the patient 
accumulation of a great mass of observations before it was finally 
and universally accepted even by scientific men, is fundamental to an 
appreciation of the phenomena of the surface of the earth. It is as 
basal a principle in geology and physical geography as a broad concep- 
tion of the distances in space is basal to astronomy. In both cases the 
full appreciation of the conception is denied the human mind, for in 
his experience man deals only with inches, feet, and miles, and with 
seconds, minutes, and years. It is, therefore, quite beyond our power 
to fully realize the true significance of the 92! million miles which sep- 
arate the sun and earth, or the scores of millions of years which sep- 
arate us from the early ages of geological time. Yet the one is as truly 
a fact as the other, and neither the principles of astronomy nor of 
physical geography can be really appreciated without accepting as a 
basal principle the measure of the space or the time which lies far 
beyond our limited range of experience. The principle is so well 



INTRODUCTION xvii 

established that it may fairly be stated as such without a preliminary 
attempt at proof, leaving the verification to appear as the subject is 
developed. 

Development of Physical Geography 

The development of science, in general, up to its present standard 
was primarily the work of the last century, though it was preceded by 
a series of brilliant discoveries, notably of basal principles in astronomy 
and physics. The study of the earth, partly descriptive, had occupied 
the attention of many workers in the preceding centuries ; and natu- 
rally the phenomena of the earth upon which man lived, and by which 
he was surrounded, led to some investigation and to still more specu- 
lation, often most fantastic. Thus earthquakes, volcanoes, fossils 
in the rocks, and other phenomena early attracted attention and were 
subjects of investigation and speculation ; and naturally the question 
of the origin of the earth itself was a source of interest and wonder 
which led to speculation, as is proved by the cosmologies of the an- 
cients, and the even more vague speculations of more primitive peoples. 

Although much thought had been given to the subject, and much 
had been written upon it and some important facts and principles 
had been put forward by the beginning of the last century, there had 
been little real progress in the development of any phase of earth 
science up to the beginning of the nineteenth century. This lack 
of progress was in part due to the general unorganized state of science. 
This affected all sciences to almost equal extent. It was even further 
due to the prevalence of the fundamental fallacy that the cosmology 
presented in the first chapter of Genesis was to be taken literally, and 
was to serve as the basal principle in an interpretation of earth history. 
It required more than ordinary evidence to overcome this fallacy, 
for it was given the stamp of infallibility by theological dogma. Any 
facts that seemed to controvert the Jewish cosmology must needs be 
thrown out; and any argument based upon such facts was regarded 
as an attack upon the very foundation of religious belief. There 
arose, therefore, a conflict between science and religion, or, as White 
better phrases it, a " Warfare of Science with Theology." This 
conflict just alluded to led to bitter controversy, increasing in extent 
and intensity in the first half of the nineteenth century, and not com- 
pletely extinguished even in the second half, though now, happily, 
almost completely at an end. 

There were also bitter controversies among geologists, of which one 
of the most serious was between the school of Werner, a German, and 
Hutton, a Scotchman. The former held that the earth had developed 
its present form with rapidity through a succession of catastrophic 
phenomena in which water was the prime agent, and the Wernerian 
School became known as the Neptunists. Hutton held that the present 
earth form was the result of slower evolution in which both water and 



xviii INTRODUCTION 

heat were involved, and his school became known as the School of the 
Vulcanists. In its main elements, the theory of Hutton has prevailed; 
and to it we may look for some of the basal principles of the physical 
geography of the lands. Playf air's " Illustrations of the Huttonian 
Theory of the Earth," published in 1802, is the real beginning of the 
modern physical geography, for it postulates the idea of vast lapse 
of time in which " we can see neither the beginning nor the end," 
the importance of the forces at present in operation, when operating 
through long periods of time, the true origin of river valleys, and other 
basal principles of physical geography. 

For a generation, so bitter was the controversy, and so opposed was 
the Huttonian theory to the supposed demands of true religion, that 
the brilliant assemblage of facts and logical deductions from them put 
forward by Hutton, Playfair, and others, failed of acceptance and 
apparently left little or no impression upon the science of the time. 
The Huttonian theory was revived, elaborated, and amplified in 1830 
in Lyell's " Principles of Geology." He fostered it with all the vigour 
of his brilliant mind. Primarily by Lyell's work, aided by the re- 
searches of a number of other students of earth science, the Huttonian 
principle became established, and the doctrine of Uniformitarianism, 
as opposed to that of Catastrophism presented. With some modifica- 
tion in detail it is basal to the study of the development of the surface 
forms of the earth. This doctrine holds that by the processes of the 
present, working through the lapse of time, the present features of the 
earth have been evolved ; and that catastrophes, though probably 
occurring, are not essential to the underlying causes. 

In the further development of physical geography a multitude of 
workers have taken part in bringing it to its present standard. This 
is not the place to attempt to trace the development of the subject 
in detail, nor to list the names and contributions of the principal 
workers. The names of three Americans, — Gilbert, Powell, and Davis, 
— however, stand out with such special prominence in the history 
of the development of modern physical geography that they call for 
mention even in this generalized view. In two reports, written at 
about the same time, 1875, — Gilbert's chapter on Land Sculpture 
in " The Geology of the Henry Mountains " and Powell's " Explora- 
tion of the Colorado River of the West," — there are stated for the 
first time some of the underlying principles of land sculpture, upon 
which the scientific study of the surface of the earth is based. Pro- 
fessor Davis has added still other principles, has outlined and de- 
veloped the idea of the progressive stage in the development of land 
forms, and has given to physical geography an organization which 
has won many followers, including the writer of this book, who was 
fortunate enough to be one of his early pupils, and at the same time 
to come under the inspiring influence of that great teacher and geog- 
rapher, Professor N. S. Shaler. Some of Professor Davis's papers 
have been collected in a single volume entitled " Geographical Essays," 



INTRODUCTION xix 

1909 ; see also Davis's " Physical Geography," 1898 ; " Practical 
Exercises in Physical Geography," 1908 ; " Grundziige der Phys- 
iogeographie," 191 1; " Erklarende Beschreibung der Landformen," 
1912. 

In a study of the air and of the oceans, as well as of the lands, many 
men have been at work, and the development of the sciences of the 
air, ocean, and land is dependent upon the combined effort of them 
all, though with some more potent than others in the discovery, veri- 
fication, and exposition of underlying principles. Modern physical 
geography has developed out of the work of this army of students, 
specific reference to some of whose contributions will appear in the 
succeeding chapters of this book. 

References to Literature 

The literature of physical geography is extensive. Among the writ- 
ings upon the subject are elementary school textbooks, special articles 
upon particular processes or areas, books upon special topics such as 
rivers, earthquakes, etc., books and articles relating specifically to 
the atmosphere and the oceans, and books of a general nature. Ref- 
erence to all but the first of these classes of publications will be found 
in later pages, but there are a number of publications of such a general 
nature that they are listed below, mainly of books and magazines, 
relating specifically to the Physical Geography of the Lands, deferring 
reference to the atmosphere and oceans to those sections dealing spe- 
cifically with these topics. The list also includes a few books on human 
geography. This subject is discussed incidentally throughout the 
book along the line of the splendid contributions by Friedrich Ratzel, 
Elisee Reclus, J. Brunhes, and others in Europe, and Miss E. C. Semple, 
A. P. Brigham, and others in America. It is not claimed that the fol- 
lowing list is complete, nor that it has included all that are of impor- 
tance and value. 

PHYSICAL GEOGRAPHY 

John Playfair. Illustrations of the Huttonian Theory of the Earth, Edin- 
burgh, 1802. 

Karl Ritter. Die Erdkunde, Berlin, 1817. 

Sir Charles Lyell. Principles of Geology, 1st edition, 1830; nth edition, 
2 vols., New York, 1873. 

A. von Humboldt. Cosmos, 1844 ; edition in English, 5 vols., London, 1871-1872. 

J. P. Lesley. Manual of Coal and its Topography, Philadelphia, 1856. 

J. W. Powell. Exploration of the Colorado River of the West, Chapters 
XI and XII, Washington, 1875. 

G. K. Gilbert. Report on the Geology of the Henry Mountains, Chapter V, 
Washington, 1877. 

T. H. Huxley. Physiography, London, 1877. 

O. Peschel and G. Leipoldt. Physische Erdkunde, 2 vols., Leipzig, 1880. 

F. von Richthofen. Fiihrer fur Forschungsreisende, Berlin, 1886, 1901. 

A. Geikie. The Scenery of Scotland, London, 1887. 



xx INTRODUCTION 

G. de la Noe and Emm. de Margerie. Les Formes du Terrain (with atlas of 

plates), Service Geographique de l'Armee, Paris, 1888. 
N. S. Shaler. Aspects of the Earth, New York, 1889. 
S. Gtinther. Lehrbuch der Physikalischen Geographie, Stuttgart, 1891 ; 

Handbuch der Geophysik, 2 vols., Stuttgart, 1897, 1899. 
H. R. Mill. The Realm of Nature, New York, 1892. 
T. G. Bonney. Story of Our Planet, London, 1893. 
J. Geikie. Fragments of Earth Lore, Edinburgh, 1893. 
N. S. Shaler. Sea and Land, New York, 1894. 

A. Penck. Morphologie der Erdoberfiache, 2 vols., Stuttgart, 1894. 
J. W. Powell and Others. Physiography of the United States, National 

Geographic Monographs, New York, 1896. 
Andrew D. White. History of the Warfare of Science with Theology in 

Christendom, 2 vols., New York, 1896. 
A. de Lapparent. Lecons de Geographie Physique, Paris, 1896. 
C. R. Dryer. Studies in Indiana Geography, Terre Haute, 1897. 
E. Bruckner. Die Feste Erdrinde und ihre Formen, Leipzig, 1897. 
N. S. Shaler. Outlines of the Earth's History, New York, 1898. 
J. Geikie. Earth Sculpture, London, 1898. 
Henry Gannett. Topographic Atlas of the United States. U. S. Geol. Survey, 

— Folios 1 and 2, Physiographic Types, 1898, 1900; Folio 3, Physical 

Geography of the Texas Region, by R. T. Hill. 
J. E. Marr. The Scientific Study of Scenery, New York, 1900. 
A. J. Herbertson. Outlines of Physiography, London, 1901. 
R. S. Tarr. The Physical Geography of New York State, New York, 1902. 
J. Lubbock (Lord Avebury). The Scenery of England, New York, 1902. 
A.. Robin. La Terre, Paris, 1902. 

H. Wagner. Lehrbuch der Geographie, 7th edition, Leipzig, 1903. 
E. Suess. Das Antlitz der Erde ; edition in English, " The Face of the Earth," 

translated by Sollas, 4 vols., Oxford, 1904-1912; the French edition, 

" La Face de la Terre," translated under the direction of the eminent Emm. 

de Margerie, is the best of the three because of the many additional 

illustrations and references to literature. 
A. Hettner. Grundziige der Landerkunde, Vol. I, Europe, Leipzig, 1907. 
J. van Baren. De Vormen der Aardkoorst, Groningen, 1907. 
R. D. Salisbury. Physiography, New York, 1907. ' 
R. D. Salisbury and W. W. Atwood. The Interpretation of Topographic 

Maps, Prof. Paper 60, U. S. Geol. Survey, Washington, 1908. 
J. W. Gregory. Geography — Structural, Physical, and Comparative, Lon- 
don, 1908. 
W. M. Davis. Practical Exercises in Physical Geography (with atlas), 

Boston, 1908. 
E. de Martonne. Traite de Geographie Physique, Paris, 1909, 1913. 
Gen. Berthaut. Topologie, Etude du Terrain, 2 vols., Service Geographique 

de l'Armee, Paris, 1909. 
W. M. Davis. Geographical Essays, Boston, 1909. 
A. Supan. Grundziige der Physischen Erdkunde, Leipzig, 191 1. 
W. M. Davis. Erklarende Beschreibung der Landformen, Leipzig, 191 2. 
S. Passarge. Physiologische Morphologie, Mitt. Geog. Gesell. in Hamburg, 

Band 26, Heft 2, 191 2. 
J. Brunhes, E. Chaix, Emm. de Martonne and Others. Atlas Photo- 

graphique des Formes du Relief Terrestre, Geneva, 1914 to date. 

HUMAN GEOGRAPHY 

Arnold Guyot. The Earth and Man, Boston, 1849. 

Karl Ritter. Geographical Studies, Boston, 1863; Comparative Geography, 
Philadelphia, 1865. 



INTRODUCTION xxi 

G. P. Marsh. The Earth as Modified by Human Action, New York, 1863, 

1874- 
O. Peschel. Races of Man, New York, 1876. 
E. Reclus. La Terre, 2 vols. ; Nouvelle Geographie Universelle, 19 vols., 

Paris, 1878-1895, — published in English as The Earth and Its Inhabitants ; 

L'Homme et La Terre, 6 vols. 

E. A. Freeman. Historical Geography of Europe, London, 1881, 1903. 
J. Lubbock (Lord Avebury). Origin of Civilization, New York, 1886. 

A. Kirchhoff. Unser Wissen von der Erde, 5 vols., Leipzig, 1886-1890; Man 

and Earth, London, 1906. 
G. G. Chisholm. Handbook of Commercial Geography, 1st edition, 1889; 6th 

edition, London, 1906. 

F. Ratzel. Anthropogeographie, 2 vols., Stuttgart, 1891, 1899; The History 

of Mankind, 3 vols., New York, 1896-1898; Die Erde und Das Leben, 

2 vols., Leipzig, 1901 ; Politische Geographie, 2d edition, Munich, 1903. 
N. S. Shaler. Nature and Man in America, New York, 1891; Man and the 

Earth, New York, 1905. 
E. Stanford. Compendium of Geography and Travel, 2d edition, 12 vols. 

1893-1901. 
A. H. Keane. Ethnology, 2 vols., New York, 1896; Man, Past and Present, 

New York, 1899. 
W. Z. Ripley. Races of Europe, 2 vols., New York, 1899. 
H. R. Mill and Others. International Geography, New York, 1899. 
H. B. George. Relations of Geography and History, Oxford, 1901. 
H. J. Mackinder. Britain and the British Seas, New York, 1902, 
A. P. Brigham. Geographic Influences in American History, Boston, 1903 ; 

From Trail to Railway Across the Appalachians, Boston, 1907. 
J. Partsch. Central Europe, New York, 1903. 
E. C. Semple. American History and Its Geographic Conditions, Boston, 

1903; Influences of Geographic Environment, New York, 1911. 
A. Geikie. Landscape in History, and Other Essays, London, 1905. 
J. Brunhes. La Geographie Humaine, Paris, 1910. 
H. E. Gregory, A. G. Keller, and A. L. Bishop. Physical and Commercial 

Geography, Boston, 19 10. 
C. R. Van Hise. Conservation of Natural Resources in the United States, 

New York, 1910; Mineral Resources in Civilization (in press). 
J. R. Smith. Industrial and Commercial Geography, New York, 1913. 



PERIODICALS 

Bulletin of the American Geographical Society, New York. 

Annals of the Association of American Geographers. 

Geographical Journal, London. 

Scottish Geographical Magazine, Edinburgh. 

Annates de Geographie, Paris. 

La Geographie, Paris. 

Revue de Geographie, Paris. 

Petermann's Mitteilungen, Gotha, Germany. 

Zeitschrift der Gesellschaft fur Erdkunde zu Berlin. 

Geographische Zeitschrift, Leipzig. 

Mitteilungen der Geographischen Gesellschaft in Wien. 

Bollettino delta Reale Societd Geografica, Rome. 

Bulletin of the Geographical Society of Philadelphia. 

Appalachia, Boston. 

National Geographic Magazine, Washington. 

Geographical Teacher, London. 

Journal of Geography, Madison, Wisconsin. 



xxii INTRODUCTION 

BIBLIOGRAPHIES 

U. S. Geological Survey. Catalogue and Index of Contributions to North 

American Geology, 1732-1891, — Bulletin 127, U. S. Geol. Survey, 

1896, by N. H. Darton; the same continued from 1892 to 1912 by F. B. 

Weeks and by J. M. Nickles as Bulletins 188, 189, 301, 372, 409, 444, 495, 

524, and 545. 
Royal Society of London. International Catalogue of Scientific Literature, 

annual bibliographies of Geography and Geology. 
Gesellschaft fur Erdkunde zu Berlin. Bibliotheca Geographica, published 

annually. 
Geographisches Jahrbuch and Geographen Kalendar, Gotha, Germany. 
Annates de Geographic Bibliographic Geographique annuelle. 
Geographical Association. Guide to Geographical Books and Appliances, by H. R. 

Mill, A. J. Herbertson, and others, London, 19 10. 



COLLEGE PHYSIOGRAPHY 



PART I. THE PLANET AND THE LANDS 



CHAPTER I 



FUNDAMENTAL GENERAL FACTS 



The Earth as a Planet 



The Solar System. — The earth is one of a vast number of spheres 
in space, about most of which relatively little is known. A small 
group of these spheres, revolving about a central body, the star which 
we know as the sun, are better known, and together constitute the 
solar system. Omitting (a) occasional visitors to the solar system, or 
comets, (b) the small spheres or asteroids, (c) the still smaller meteo- 
rites, and (d) the rings of Saturn, there remain three quite distinct 
classes of bodies as constituent parts of the solar system : (i) . the 
central sun, (2) the planets, (3) the satellites. 

Similarities of Members of the Solar System. — Among the 
spheres that revolve about the sun, and especially the eight moderate- 
sized spheres called planets, there 
is a striking uniformity in some 
important respects. First and 
foremost, each has a spherical 
form. This is familiar in the 
.case of the earth from the proofs 
in connection with (a) the cir- 
cumnavigation of the globe, (b) 
the method of disappearance of 
ships upon the sea, and (c) the 
curved shadow of the earth 
during an eclipse of the moon 
(Fig. 2), as was well known to 
some of the ancients. Each 
planet is distorted by protuber- 
ance in the equatorial region into 
the form of an oblate spheroid. 

Secondly, all are rotating about 
an axis inclined to the plane 
through which they are revolving about the central body; but the 
inclination of the axis and the rate of rotation vary from sphere to 
sphere. In the third place, they are all engaged in a revolution^bout 




Fig. 2. — Proof of the roundness of the earth 
from curved shadow during eclipse of moon. 
(Photograph by Harvard College Observa- 
tory.) 



COLLEGE PHYSIOGRAPHY 




Fig. 3. 



Relative sizes of the four larger planets. 



EARTH 



the central body, the sun, following an elliptical path, or orbit; while 
the satellites, in addition, are revolving about the planet to which 

they are attached. A 
fourth resemblance is 
that they all receive 
their light and heat 
from the central sun, 
though in amounts 
varying with the dis- 
tance. Finally, it is 
probable, though not 
certainly proved, that 
all these spheres are 
composed of essentially 
the same materials. 

Contrasts within the 
Solar System. — While 
there are these resem- 
blances, there are also 
notable differences. The spheres differ greatly in size (Figs. 3, 4), 
ranging from the sun, with a diameter of 860,000 miles (Fig. 9), to 
the earth, with about y-J-g- this diameter, and the satellites with diam- 
eters of but two or three thousand miles, 
and to the still smaller asteroids. They 
differ also in their distance from the 
sun, and consequently in the length of 
the orbit through which they circle 
about it, as well as in the time required 
to complete the revolution (Fig. 5). 
Thus Mercury, the planet nearest the 
sun (Fig. 6), being approximately 
36,000,000 miles distant, requires about 
88 days for its journey about the sun ; 
the earth, 92,750,000 miles distant, re- 
quires a little over 365 days, determin- 
ing the length of our year; and Neptune, 
the most distant planet, 2,775,000,000 
miles from the sun, requires about 165 
years for its revolution. 

A third noteworthy difference among 
the members of the solar system is the 
different periods of rotation, the earth 
turning on its axis in about 24 hours, 
and, therefore, determining the length of 
a day, while the sun rotates in 25 days, 

the moon in 27^ days, and Jupiter in 9 hours and 55 minutes. That 
the earth does rotate from west to east upon its axis was long ago 




Fig. 4. — Sizes of 
planets, given 
miles. 



the four smaller 
in diameters in 



FUNDAMENTAL GENERAL FACTS 



demonstrated by Galileo. In investigating the behaviour of objects 

falling through the air, he discovered that they always fell a little 

to the east of a point 

directly below that 

from which they were 

dropped (Fig. 7). At 

the Leaning Tower of 

Pisa, for example, the 

rotation of the earth 

causes an object at the 

top of the tower to 

move faster than one 

at its base, as Galileo 

correctly reasoned. 

The proof of the 
earth's rotation by Fou- 
cault's pendulum (Fig. 8) was first carried out in 1851 and is repeated 
every year in the physics or the geography departments of many 




Fig. 5. 



Diagram showing the time required for each 
planet to revolve around the sun. 



>h>-rt- 



! --isK»° L -430,000,000 
- -G7.000.000 
136,000,000 



---SSI. 000,000 



Fig. 6. — Diagram showing the distance from the sun to the various planets in miles. 



colleges. Foucault's method was to suspend a heavy weight from 
the dome of the Pantheon in Paris, and set it to swinging. A pendu- 
lum will continue to swing indefinitely in exactly 
the same plane. After being set in motion 
it appears to cease to vibrate parallel to a 
mark on the floor, gradually comes to a posi- 
tion of swinging at right angles to the mark, 
and, in 24 hours or a little more, depending on 
the part of the earth, it seems to shift until it 
once more swings parallel to the mark. This 
is because the building turns around the 
pendulum as the earth rotates. 

Still another contrast within the solar system 
is the condition of the spheres. On some, like 
the earth and Mars, there is an atmosphere, 
while on others, like the moon, there is no gas- 
eous envelope. There seems also to be a grada- 
tion in temperature from the highly heated 
sun to the completely cold moon, with intermediate stages, such as 
Jupiter, which is evidently highly heated, though not glowing, and the 
earth, which, though cold at the surface, is apparently heated within. 




deviation of falling bod- 
ies. (After ■ Moulton.) 
An object dropped from 
the tower MF reaching 
the earth at P rather 
than at F'. 



COLLEGE PHYSIOGRAPHY 




Fig. 8. — To show the rela- 
tions of Foucault's pendu- 
lum to the rotating earth 
at the pole P, if it were 
started swinging in a plane 
parallel to the meridian m. 
(After Moulton.) 



While it does not fall within the province of Physical Geography 
to study the other members of the solar system, no presentation of 
this subject would be complete which ignored the resemblances and 
relationships which exist between the members 
of the great family of spheres which constitute 
the solar system. Nor would any attempt 
to understand the origin of the phenomena of 
the earth itself be fruitful without utilizing at 
least some of the facts which astronomers have 
contributed as a result of their study of the 
members of the solar system. 

The Earth in the Solar System. — The earth 
is an integral part of this system ; its move- 
ments in space are influenced and guided by 
its relation to other members of the great 
family of spheres ; its light and heat, its tides, 
and winds and rains, together with the changes 
of the earth's surface, which result from their 
presence and action ; and even the direct result of the astronomical 
relations of the earth and its history of development as a planet, — ■ 
can be understood only by considering it as one of a series of spheres 
of common character and common origin. 

Earth and Sun. — There is a difference in the degree of importance 
of the relationships between the earth and its fellow members of the 
solar system, and from the stand- 
point of the study of Physical 
Geography we may ignore all other 
relationships excepting those be- 
tween the earth and the moon 
and the sun. To the sun the 
earth is bound by the tie of gravi- 
tation, which holds it to its ellip- 
tical orbit, as the moon is held to 
its orbit around the earth. Across 
the space of about 92 J million 
miles, radiant energy passes from 
the sun, which shines in the heav- 
ens and like the other stars is fiery 
hot, to the surface of the earth, 
which merely reflects sunlight as 
the moon does. This radiant en- 
ergy produces the phenomena of 
heat and light. Magnetic waves 
also span the distance, giving rise to phenomena upon the earth whose 
full significance is not yet understood. 

By the inclination of the earth's axis to the plane of the ecliptic, the 
plane in which the earth moves in its revolution around the sun, the 




Fig. 9. — Diagram to show vast size of the 
sun compared with the earth. If the 
earth and moon and orbit of the moon 
were placed inside the sun, the relation- 
ship would still be as shown above. 



FUNDAMENTAL GENERAL FACTS 5 

distribution of light and heat, which on a sphere would otherwise 
vary regularly from equator to pole, varies within other limits. 
These limits are constantly changing during the revolution of the 
earth about the sun; and, since the inclination is 23-J- from the verti- 
cal, shift from a point 23^-° north of the equator to a point 23^-° south 
of the equator. Thus arise our seasons with all their momentous 
consequences. 

Among the consequences of inclination is this. It happens that we 
have found it convenient to bisect the distance between the North and 




STANDARD TIME IN THE UNITED STATES 

Fig. 10. — The belts usually adopted in the United States. 



South Poles, at the ends of the earth's axis, by an equator, and to sub- 
divide it further along the line of parallels of latitude, among which 
the Tropics of Capricorn and Cancer and the Arctic and Antarctic Circles 
are definitely related to the inclination of the earth's axis. 

Because of the period of daily rotation of the earth (23 hours and 
56 minutes) the point upon which the sun's rays strike vertically is 
constantly and steadily shifting eastward, and thus in a day a line is 
traced around the earth upon which the sun's rays strike vertically, 
making it convenient, among other things, to have meridians of longi- 
tude, reckoned from the arbitrarily chosen prime meridian at Green- 
wich. The parallels and meridians are divided into degrees, and these 
are divided into minutes and seconds. A degree of longitude at the 
equator is about 69! miles ; in the latitude of Philadelphia, Denver, 
Madrid, Peking, and New Zealand it is only about 53J miles ; in the 
latitude of St. Petersburg it is about 35 miles ; and at the poles it has 







|uN» s£ 



Fig. ii. — Diagram to show why the sua appears to rise farther north in summer than in 
winter. (From Todd's " New Astronomy.") 







Fig. 



Diagram to show the portions of the earth illuminated at various seasons during 
revolution around the sun. 



Earth ( Moon215o\ 7900 



FUNDAMENTAL GENERAL FACTS 7 

no length. A degree of latitude varies in length from 68.7 to 69.4 
miles, because of the earth's polar flattening. As a result of the earth's 
daily rotation it is also necessary for us to have Standard Time (Fig. 

From the point of verticality of the sun's rays the angle at which the 
rays reach the earth is lower and lower in each direction. With the 
change of seasons (Fig. 12) 

the line of verticality shifts / \ 

northward and southward, 
from the Tropic of Cancer 
on the north to the Tropic 
of Capricorn on the south. 
Accordingly the sun appears 
to rise farther north in sum- 
mer in the northern hemi- 

■1 ,1 • , /T7« Fig. 13. — Diagram showing relative sizes of the 

Sphere than in Winter (Fig. earth and the moon, diameters in miles. 

11). Likewise the polar 

regions are without light throughout the' period of earth's rotation 
during parts of the year, and continuously lighted at other periods 
(Fig. 12) at all points within 23 J° of the poles, that is, inside the 
Arctic and Antarctic Circles. 

To day and night, and to seasons, with the resulting alternations of 
temperature and other conditions, are to be ascribed some of the most 
significant phenomena of Physical Geography, and some of the most 
momentous consequences to the surface of the earth and to life upon 
it. 

Earth and Moon. — The moon, though near, is both small and cold. 
It gives to us only reflected light and a negligible quantity of heat. It 
is small (Fig. 13), but is very near the earth (average distance 240,000 
miles, least possible distance 221,000, greatest 259,600 miles). De- 
spite its smallness it nevertheless exerts an important effect upon the 
earth by the attraction of gravitation, most noticeable in the liquid part 
of the earth, the great oceanic envelope. Thrown into undulation by 
this attraction, the ocean surface rises and falls in tides which follow 

TuitmNC Su m's GLOBC.J- 

TmicHINC S un -^ 




Touchwg Sun)s Globc 

Fig. 14. — Diagram to show positions of earth, E, and moon, M, during eclipses. (After 
Encyclopedia Britannica.) 

the moon in its passage through the heavens. Though a far larger 
body, the sun, owing to its greater distance, is much less effective than 
the moon in tide generation, but a distinct solar tide is nevertheless 
produced, thus modifying the lunar tide. There are some reasons for 
believing that there are other influences of the moon with important 
consequences in the operation of physical forces on the earth, but the 



8 COLLEGE PHYSIOGRAPHY 

operation of these is so obscure that their full significance is not under- 
stood. Among these are the possible tide produced in the atmosphere 
and a possible relation between earthquakes and lunar attraction. In 
the revolution of the moon around the earth, and of the earth around 
the sun we have lunar and solar eclipses (Fig. 14) at certain times. 

The Earth in Space 

Importance of Uniform Conditions. — As a mere sphere of rock the 
earth might maintain its individuality and chief characteristics even 
though the conditions surrounding it were greatly changed ; but as a 
body inhabited by a complex series of organisms, the earth is to be con- 
sidered as dependent for its very existence as a habitable globe upon 
the maintenance of a balance in which there are a variety of factors. 
No one of these factors can be seriously disturbed without a complete 
alteration of the conditions upon which the existence of life depends. 

Atmospheric Protection from the Cold of Space. — Passing rapidly 
through space, the earth is surrounded by such low temperatures that, 
if the supply of heat from the sun were cut off or greatly diminished, 
the temperature of the earth would quickly descend so low that life 
could not exist. Even a diminution in the atmospheric blanket would 
so upset the balance that, during the intervals of darkness, the earth, 
through radiation, would be exposed to the influence of the surround- 
ing coldness. It is estimated that the temperature of space is but 
little (5 Centigrade) above that of absolute zero, or 459 below zero 
(Fahrenheit) ; the moon, on the side away from the sun, is under its 
influence ; the earth is protected from it by its blanket of atmospheric 
gases, warmed by the sun during the earth's daily rotation. 

Other Uniform Conditions. — The maintenance of daily rotation, 
and of an annual revolution, the preservation of favourable distance 
between earth and sun, and the continuation of a supply of heat from 
the sun, neither too great nor too little, are all factors upon which the 
earth as a habitable globe depends. The atmosphere, from which oxy- 
gen is being constantly extracted both by life and by inorganic pro- 
cesses of rock alteration, must maintain a supply sufficient to the 
needs of abundant life ; and carbon dioxide, both extracted from the 
atmosphere and returned to it, cannot vary in quantity, excepting 
within narrow limits, without upsetting the balance. The distribution 
of land and water upon the earth, and the elevation of the land above 
the sea, are other factors, which, though capable of variation within 
limits, could not vary to an extreme, without giving rise to profound 
modification of the relation of life to the earth. 

Habitability Long Maintained. — That at a given time the earth 
presents a set of conditions of complex kind, all conspiring to render 
it suitable as the home of a vast and complicated series of organisms, 
is perhaps not remarkable ; but when it is considered that this favour- 
able balance has been preserved through the long ages of the past with 



FUNDAMENTAL GENERAL FACTS o 

which geological study has made us familiar, through untold millions 
of years in fact, it is certainly noteworthy, to say the least. In the 
present we see the past reflected through the vista of the ages. But 
this is not the same as saying that the past has in no important way 
differed from the present. There is good reason for believing, and the 
evidence of it is steadily accumulating, that there have been periods 
in the past history of the earth when conditions were greatly different 
from the present; but through it all, so far as the facts now known 
permit us to judge, there has been no time when the steady develop- 
ment of life on the earth was interrupted, or even seriously jeopardized. 
This is certainly a wonderful fact, and one that may well set us to 
serious thinking upon the mysteries of nature by which we are sur- 
rounded. 

The Earth Elements 

Air, Water, and Earth. — The earth consists of three quite different 
parts : (1) the air or atmosphere; (2) the waters of the earth or hydro- 
sphere; and (3) the solid earth, or lithosphere. Speaking generally, 
these parts of the earth are not only different but distinct from one 
another and fairly definitely separated. Yet it is not to be over- 
looked that both air and water penetrate into the solid earth ; that 
water and solid earth enter the atmosphere ; and that air and earthy 
materials find their way into the hydrosphere. There is, therefore, an 
intimate commingling of the three elements of the earth, though only 
within narrow limits and not enough to cause confusion in the attempt 
to distinguish them as definitely separate parts of the terrestrial sphere. 
It is not unreasonable to add still a fourth element, as some have pro- 
posed, — namely, the organisms of the earth, forming the biosphere, 
which occupies parts of the hydrosphere, the lower layers of the atmos- 
phere, and the surface and outer portion of the lithosphere, and which 
depends upon the presence of these three elements for its existence. 
Physical Geography deals with a study of these four elements of the 
earth in their natural relation to one another and their reaction upon 
one another, under the influence of a series of forces from both within 
and without the earth, primarily radiant energy from the sun, and 
gravity in the earth itself. 

The Atmosphere. — The atmosphere completely envelops the earth, 
rising certainly as much as 100 miles above the surface of the litho- 
sphere, and perhaps as much as 200 miles, or even more. Because it is 
drawn to the earth's surface by the pull of gravity and compressed in 
its lower layers, fully half the air lies within about three and- a half 
miles of the earth's surface, upon which it rests with a pressure of 
about 15 pounds to the square inch at sea level. Consisting of elastic 
gases which are readily set in motion by changes of temperature, the 
atmosphere is the theatre of incessant changes. In some of its activ- 
ities the atmosphere exerts important influence upon the surface of the 



IO 



COLLEGE PHYSIOGRAPHY 



•A 




lithosphere and the hydrosphere; and it is vitally essential to the 
organisms of the earth. The composition of the air is also respon- 
sible for important consequences, especially through the influence 
of three of its constituents, — oxygen, carbon dioxide, and water 
vapour. Since these effects of the atmosphere are stated with some 
fulness on later pages (Chapters XXII to XXVII), their considera- 
tion may for the present be 
deferred. 

The Hydrosphere. — The 
hydrosphere is only a partial 
envelope of the earth, by far 
the greater part of it being in 
the oceans. They cover nearly 
three fourths of the earth's 
surface to an average depth 
of about 12,000 feet, with a 
maximum depth near the Phil- 
ippine Islands of 32,114 feet. 
Like the air, the oceans are 
the seat of incessant activities ; 
and where they come in con- 
tact with the lands, at their 
borders, the effect of these 
activities is extended to a modification of the land 
itself. 

The ocean is profoundly affected by the atmos- 
phere ; and it, in turn, is greatly influenced by 
the oceans. The ocean modifies the temperature 
of the air and supplies by far the greater part of 
its water vapour. Thus there is an intimate, 
mutual reaction between the two elements of air 
and water, and between these and the surface of 
the lithosphere. One important result of this is 
the development of a series of phenomena of 
fundamental importance in Physical Geography (Chapters XIX to 
XXI). 

The Lithosphere. — By far the greater part of the earth sphere is 
the solid lithosphere, a body of rocky material with an equatorial 
diameter of 7926 (7926.5) miles, a polar diameter of 7900 (7899.7) 
miles, a circumference of about 25,000 miles, and a total volume of 
about 260,000,000 cubic miles. At the surface it consists of a complex 
series of rocks and minerals more or less completely oxidized, with an 
average specific gravity of 2.7, and with a temperature varying with 
the season. Fractures exist in this outer portion of the lithosphere, 
and, when stresses are applied, the consolidated rocks and minerals 
suffer breakage. This portion of the lithosphere is commonly known 
as the earth's crust; it has also received the name of the zone of fracture. 



Fig. is- — The 
relative thick- 
ness of the 
earth's hydro- 
sphere and at- 
mosphere. The 
figures indicate 
depths in miles, 
5! miles being 
one of the deep- 
est points in the 
ocean, though 
recently a point 
with a depth of 
over 6 miles has 
been discovered. 



FUNDAMENTAL GENERAL FACTS 



ii 



/VTMO^PMERE. 



Heated Interior of Earth. — Those portions of the lithosphere 
which have come under the direct observation of man are in this zone, 
and no essential difference is noted between the deepest parts so far 
exposed and the surface, excepting a difference in temperature. Below 
the zone influenced by the seasonal changes the temperature of the 
earth's crust is found to rise with increasing depth ; and while the rate 
of increase in temperature varies greatly from place to place, it is found 
to be on the average about i° F. for about 50 feet of descent. From 
this it has long been inferred that the interior of the earth is highly 
heated, and if the observed rate 
continues, this conclusion is, of 
course, necessitated. That the 
earth's interior is in the state of a 
highly heated body has also been 
inferred from the condition of other 
members of the solar system — 
notably Jupiter and the sun, which 
are thought to have had a similar 
history to that of the earth, but 
not to have progressed so far in the 
state of cooling, while smaller bod- 
ies, like the moon, have gone even 
much farther than the earth. Still 
another basis for the inference of a 
highly heated interior is the fact 
that molten rock and hot waters 
emerge from within the earth at 
various points on the surface. 

This conclusion has been contro- 
verted, and rival hypotheses have 
been put forward, a discussion of 
which will not be undertaken at 
present. Direct observation of the 
earth below the limit of a few 
thousand feet being prohibited, our 
knowledge of the interior is, of 
necessity, limited. That the in- 
terior is not a highly heated liquid, 
as once supposed, seems established 
now (a) by the evidence of the be- 
haviour of the earth toward the 
moon and other members of the 
solar system, (b) by the absence of 
interior tides, and (c) by the nature and rate of movement of earth- 
quake waves. It is apparently a solid mass, not very different from 
steel in specific gravity and rigidity, for while the average specific 
gravity of the crust is about 2.7, the specific gravity of the earth as 




Fig. 16. — The relative thicknesses of the 
lithosphere and the atmosphere. 



12 COLLEGE PHYSIOGRAPHY 

a whole is about 5.66, and steel is about 7. It is even possible that 
the interior of the earth is unoxidized metal, for there are certain facts 
suggestive of this, notably the magnetic phenomena of the earth, and 
the composition of lavas which bring to the surface a larger proportion 
of metallic elements than is common in the minerals of the crust. 

Flowage in Earth's Interior. — Of one characteristic of the hidden 
interior of the earth there seems good evidence, — namely, that, even 
though solid, and possibly cold, it nevertheless behaves as a viscous 
fluid when under stress. When under differential pressure, it yields 
and flows. This has long been suspected upon the basis of evidence 
that rocks, formerly deeply buried in mountain areas, but now re- 
vealed to view by erosion, have, under the stresses of mountain for- 
mation, moved by viscous flowage, instead of mechanical breaking such 
as the rocks of the surface are subject to. This has also been inferred 
upon the basis of theory, for under the heavy load of superincumbent 
layers, and especially if the temperature be high, mechanical breaking 
becomes an impossibility. The studies of the behaviour of metals 
under pressure, and more recently the brilliant experimental researches 
of Adams upon the effect of differential pressure upon various rocks 
under heavy load, have given to this theory satisfactory support. 
The conclusion seems, therefore, warranted that at a sufficient depth 
in the earth, all cavities become closed and fractures impossible, and 
that in that zone differential stress finds relief in rock flowage. To 
this part of the lithosphere, therefore, the name zone of flowage has been 
applied by Van Hise. It does not commence at a uniform and definite 
depth, but varies with the pressure and with the nature of the rock. 
It begins, in general, at depths of between 90,000 and 105,000 feet 
below the surface. 

The Form of the Earth 

The Oblate Spheroid. — The earth, in the largest sense, is a sphere, 
but, owing to rotation, it is distorted by flattening at the poles and 
spreading, or bulging, in the equatorial region, becoming therefore 
an oblate spheroid. As a result of this distortion of the sphere, the 
equatorial diameter is about 27 miles longer than the polar. While 
this is the greatest departure of the earth from the figure of a true 
sphere, it is by no means the only distortion. If the ocean, which 
tends to restore the oblate spheroidal form to the distorted earth, be 
ignored, and only the lithosphere be considered, the earth is found to 
depart so widely from the perfect form of an oblate spheroid, that it 
has been thought by some to deserve the special name of geoid. 

Continents and Ocean Basins. — The greatest departures from the 
spheroidal form are those of the great continental elevations and 
oceanic depressions, the full extent of which is hidden from view by the 
ocean water which occupies the great depressions. The continents, 
which occupy about one-fourth of the total surface of the earth, rise 



FUNDAMENTAL GENERAL FACTS 



13 



to an average elevation of about 2300 feet above sea level, though 
in places reaching elevations of ten, fifteen, and twenty thousand feet, 
and culminating in Mount Everest with an elevation of 29,000 feet. 
The ocean basins, on "the other hand, with an average depth below 
sea level of about 12,000 feet, have extensive areas with a depth much 
greater than this, and at their deepest point attain a depth of 32,114 
feet, in which Mount Everest might be placed with a half mile of water 
over its summit. Since the mean surface of the Hthosphere is about 
7500 feet below sea level, it will be seen (Fig. 17) that a very large part 
of the surface of the earth falls below that limit, while the continents, 




Fig. 17. — Relative proportions of land above and below sea level. Elevations in metres. 
Figures on horizontal lines in millions of square kilometres. (After Penck.) 

together with some of the peripheral sea bottom, lie above it. How 
great and extensive are the oceanic depressions is indicated by the fact 
that if the Hthosphere were perfectly spheroided, the waters now in the 
oceanic basins would overspread the entire earth with a hydrosphere 
nearly two miles in depth. 

The diversity of the earth's surface due to the ocean basin depres- 
sions and continent elevations attains a maximum of over eleven miles, 
measured from the deepest known point in the ocean to the highest 
point on the land, the crest of Mount Everest ; though the average dif- 
ference between ocean basin depth and continent elevation is only 
about two and a half miles. The boundary between the continent 
elevations and the ocean depressions is commonly not at the line of 
contact between ocean and land, for the ocean overflows and floods 
the edges of the continents to a variable width. If, therefore, the 



14 



COLLEGE PHYSIOGRAPHY 



oceans were removed, the extent and outlines of the continents would 
be materially modified ; yet, in the main, they would retain their 
present figures, being extended, and modified in detail . Their borders 
would be the great, fairly steep slopes which now lie beneath the sea 
just outside the continent margins. On the land side of their slope 
the plains, hilly lands, and mountain ranges rise above the level of the 




Fig. i 8. — Model of the earth, showing the continents of North and South America in relief 
above the adjacent ocean basins. The real border of the continental plateau is beyond 
the coast, as on the Grand Banks east of Newfoundland. (Copyright, 1894, by Thomas 
Jones, Chicago.) 



sea ; on the ocean side there is a steep descent to the depth of the ocean 
with its broad expanse of ocean bottom plains. 

Relief Features. — Both the continent elevations and the ocean 
basin depressions are diversified by irregularities of secondary rank. 
The dominant surface feature of the earth is the plain, both on the con- 
tinents and in the ocean basins, but portions of the crust rise above 
these plains in linear bands, forming mountain ranges and chains. 
Although occupying but a small proportion of the earth's surface, 
these mountain uplifts give rise to great, though local, departures 



FUNDAMENTAL GENERAL FACTS 15 

from the mean sphere level. In the opposite direction, still smaller 
portions of the earth's surface are depressed by the downsinking of 
limited areas, as in the depressions partly occupied by the Dead Sea 
and the depression in which the Mediterranean lies. 

Still a third notable irregularity of the earth form is that which 
results from the repeated emission of molten rock from within the earth 
through an orifice of limited extent, giving rise to volcanic cones. 
These rise both from the sea floor and from the continents, mainly 
from near the continent borders, and usually along lines and in associa- 
tion with mountains. In some cases these volcanic cones, though 
occupying but a very limited proportion of the earth's surface, intro- 
duce a great departure from the spheroidal form, as, for example, in the 
Hawaiian Islands, a great volcanic mountain range rising fully 30 000 
feet above the surrounding sea floor. 

Erosion Features. — The earth's surface is still further diversified 
by a multitude of minor irregularities, especially in the lands, where 
the work of running water and other agencies of change have sculp- 
tured the surface into a complex series of forms, varying greatly in 
character and in magnitude. Most of these are so minute that, when 
compared to the earth as a whole, they are negligible ; and even the 
greatest of the irregularities of the spheroid are but minute undulations 
on the surface of the great sphere, and exceedingly minor departures 
from the spheroidal form. Yet, when viewed from the standpoint 
of an occupant of the earth's surface, they stand out as great irregu- 
larities, impressive partly because of the limited range of vision. 

Earth Activities 

Conflict of Activities. — The irregularities of the earth's surface are 
the result of the operation and interaction of a series of processes at 
present actively at work, and active through a long distant past. 
The earth is the theatre of ceaseless activity and incessant change, and 
the departure of its form from that of a sphere is the result of the slow, 
long-continued operation of these activities. There are two sets of 
processes, in the main in conflict, one set inherent in the earth itself, 
the other derived from without the earth — hence one set terrestrial, 
the other extra-terrestrial, in origin. These processes, though in the 
main separable, are so completely interrelated in origin, activity, 
and resulting modification of the earth's surface, that it is only in the 
most general sense that they may be put apart. 

Terrestrial Forces. — Inherent in the earth is the great force of 
gravity, tending to hold all objects in appropriate relation as inherent 
parts of a sphere, arranged according to specific gravity. Thus we 
have the three layers of (a) air, (b) water, and (c) rock, and, in the 
lithosphere itself, a lighter crust upon a denser interior. To a certain 
degree opposed to the operation of gravity is the centrifugal force 
introduced by rotation, as a result of which the earth has been given 



16 COLLEGE PHYSIOGRAPHY 

its greatest departure from the form of the true sphere, that equatorial 
bulging which makes it an oblate spheroid. 

Because of conditions within the earth, the exact nature of which is 
not yet understood, still further diversity is given to the surface form 
of the lithosphere. Over large areas the crust is depressed below the 
main sphere level, while elsewhere portions rise above it, and the 
outline and elevation of these areas are even now undergoing change, 
as they have throughout past ages. Here and there the continent 
margins are rising, or are sinking, and a study of the past history of the 
earth proves that these variations have been in progress throughout 
geological time. Parts of the surface of the lithosphere have been 
thrown into great undulations along relatively narrow bands, as a 
result of which portions of the surface have been pushed high above 
the surrounding levels as mountain ranges and chains. These changes 
are even now in progress in certain parts of the earth, as they have 
been during the geological ages. Likewise, now, as in the past, molten 
rock is being extruded to form local elevations in parts of the litho- 
sphere surface. 

The great result of the operation of these terrestrial activities has 
been to give to the surface of the lithosphere the greatly diversified 
outline already mentioned ; and, with the exception of the oblate 
form, the tendency of time has been to add to the diversification. 

Extra-terrestrial Forces. — At all times there have been opposed 
to the tendency to produce diversification the operation of a series of 
activities whose main source of energy is derived from outside the 
earth, aided, however, by the force of gravity, by rotation of the earth, 
by revolution, and by the presence of air and water envelopes upon the 
lithosphere. Radiant energy from the sun is the chief of the extra- 
terrestrial forces, which, under various modified forms, becomes an 
agent of vast change in the lithosphere. It induces rock disintegra- 
tion, and modifies and aids the work of the atmosphere in the same 
directions; it sets the atmosphere in motion, and either directly or 
indirectly through the atmosphere it sets up motions in the ocean also, 
giving rise to winds in the atmosphere, and to waves and currents in 
the ocean, all processes operating to modify the surface of the litho- 
sphere ; it aids in the introduction of vapour into the air, and by the 
winds which it generates it guides the distribution of this vapour, 
and in important ways, also, it helps to determine the fall of the vapour 
upon the earth, where, gathering into rills and rivers, it runs away under 
the pull of gravity, causing great modifications of the surface of the 
earth when operating through long periods of time ; it is one of the 
vital factors upon which life on the earth depends, and life is in various 
directions one of the agencies of change in the surface of the litho- 
sphere. 

Gravitation, operating to hold the members of the solar system in 
their place, and to keep the earth and moon in their paths of revolu- 
tion, is to be reckoned as another highly important extra-terrestrial 



FUNDAMENTAL GENERAL FACTS 17 

force, upon which depend many of the activities of the earth resulting 
from the influence of radiant energy. A more direct effect of gravita- 
tion is the disturbance of the ocean by the tidal waves which twice 
each day sweep over its surface, performing much work in modi- 
fying the surface of the lithosphere, especially along the continent 
margins. 

Balanced Result of Upbuilding and Tearing Down. — In a general 
sense the forces from within the earth, and those derived from outside, 
may be considered to be in some important respects in opposition, or 
conflict. Those operating from within the earth are tending toward 
diversity of surface form, those from outside the earth, cooperating 
with gravity and utilizing the air and water as agencies, are tending 
toward reduction of irregularities, tearing down the higher portions 
here and building up the depressions. Were the terrestrial activities 
to operate unchecked, the surface of the lithosphere would attain a 
far more striking degree of irregularity than now, as is the case on the 
surface of the moon; were internal activities to cease, while those gen- 
erated from without the earth continued, the earth's surface would 
diminish in irregularity. With both sets of activities in operation, there 
is a double cause for irregularity, for those diversities introduced by the 
terrestrial activities are only partly removed, and they bear the scars 
of the attacks made upon them by the activities whose main source 
of power is sent across space to the earth. The battered and scarred 
surface of the earth, bearing the marks of the conflict and interaction 
of terrestrial and extra-terrestrial activities, is the subject of the study 
of the physical geography of the lands. The processes are still in 
operation all about us, and the results which they have produced 
during the ages that are past, are to be understood and interpreted 
only through a knowledge of the nature of the activities at present in 
operation. 

Another Classification of Activities. — The processes by which the 
surface of the earth has been given its present shape in departure from 
that of a perfect oblate spheroid may be grouped in another way into 
three divisions, diastrophism, vulcanism, and denudation. 

Diastrophism deals with the nature and effects of movements of the 
crust by which some parts are raised and others lowered in relation to 
one another. Vulcanism deals with the nature and results of the 
movements of molten rocks from one part of the earth to another, and, 
from the standpoint of physical geography, primarily a movement 
from some point within the earth to the surface of the lithosphere. 
Denudation includes the operation and results of a complex series of 
processes by which the surface of the lithosphere is attacked — it is 
the expression of the tendency to lower the level of the lithosphere to 
the perfect spheroidal form by removing those parts that are too high 
and filling those parts that are too low. Because diversities are ever 
being added, the work of denudation, though in operation during the 
millions of years of geological time, accomplishing vast results, has 



1 8 COLLEGE PHYSIOGRAPHY 

failed to even approximate the ultimate end toward which gravity is 
tending to lead it. 

Denudation. — Under denudation is included two quite different 
classes of processes (i) weathering, or rock disintegration, (2) erosion, 
or rock removal and transportation. The former tends to prepare 
the rock for the more efficient work of the latter. Erosion and weather- 
ing each operate both by mechanical and chemical means, and in some 
of these phases they are aided by organic processes, while throughout 
they are dominantly influenced by gravity. Weathering, per se, goes 
no further than the disintegration of rock, though by the aid of life, 
of gravity, or of wind and water, the weathered rock products may be 
moved from their place of origin to a place of deposit. Erosion, on 
the other hand, involves three stages : (a) removal, (b) transportation, 
and (c) deposition of rock fragments, or degradation, transportation, 
and aggradation. 

The agencies of erosion are several, as follows : (a) gravity, (b) organ- 
isms, (c) air movements, (d) running water, (e) glaciers, (/) waves, 
tides, and currents in lake and ocean. The operation of the agencies 
of erosion and weathering are more or less intimately interrelated in the 
general work of the denudation of the land, and it is only for the pur- 
pose of clearness of presentation that it is warranted to separate them 
and treat them independently. 

The nature and results of diastrophism and vulcanism are treated in 
later sections, after the study of the processes and effects of denudation. 
Though complex in their interaction, the agencies of denudation treated 
independently, and as if they were actually working separately, offer 
a simpler beginning in a study of the physical geography of the litho- 
sphere than vulcanism and diastrophism ; and, moreover, they are the 
processes with which we may begin with abetter basis of acquaintance, 
since they are in operation round about us and in some of their phases 
are more or less familiar to the observant student. The agencies of 
denudation are actively at work, in one form or another, on all parts 
of that portion of the lithosphere which rises above the oceans, and to 
some, though variable, degree in the ocean basins also, at points away 
from the coast lines, however, mainly by deposition. The work of 
denudation is by no means uniform, being influenced by a great va- 
riety of conditions, such as slope, climate, and the composition, struc- 
ture, and attitude of the rocks which are being attacked. Before 
considering the agencies of denudation in detail, therefore, it is neces- 
sary to gain a general view of the manner in which the rocks of the 
earth's crust vary. 

The Rocks of the Earth's Crust 

The Nature of Minerals and Rocks. — The chemical elements, 
sometimes singly, as in native copper or in sulphur, sometimes in com- 
bination, as in the silicon and oxygen which make quartz, occur in the 



FUNDAMENTAL GENERAL FACTS 



19 



earth's crust as minerals. The elements silicon, oxygen, aluminum, 
and potassium make up one variety of the mineral feldspar. A min- 
eral may be defined as_a_ single -element, or two or more elements chem- 
ically combined, forming a part of the earth's crust. 

The most common rock-forming minerals and their composition are 
listed in the following table. 



TABLE SHOWING ROCK-FORMING AND OTHER COMMON 

MINERALS 

The Minerals and their Composition 



Quartz (Si0 2 ) 

(Silicon, Oxygen) 



Orthoclase Feldspar 
(KAlSi 3 8 ) 

(Potassium, Aluminum, Silicon, 
Oxygen) 



Plagiociase Feldspar 
[(NaAlSi 3 8 ) + 
(CaAl 2 Si 2 8 )] 

(Sodium, Aluminum, Silicon, 
gen, Calcium) 



Oxy- 



Muscovite Mica 
[H 2 (K)Al 3 (Si0 4 ) 3 ] 
(Hydrogen, Potassium, 
Silicon, Oxygen) 



Al 



uminum, 



Biotite Mica 

[(H-K) 2 (Mg-Fe) 2 Al 2 (Si0 4 ) 3 ] 
(Hydrogen, Potassium, Magnesium, 
Iron, Aluminum, Silicon, Oxygen) 



Hornblend3 

[(Ca(Mg-Fe) 3 (Si0 3 ) 4 .Al 2 03)] 
(Calcium, Magnesium, Iron, Silicon, 
Oxygen, Aluminum) 



A ugite 

[(CaMgFe)0(AlFe) 2 34 (Si0 2 )] 
(Calcium, Magnesium, Iron, Oxy- 
gen, Aluminum, Silicon) 



Calcite (CaC0 3 ) 

(Calcium, Carbon, Oxygen) 



Dolomite 

[CaMg(C0 3 ) 2 ] 
(Calcium, Magnesium, 
Oxygen) 



Carbon, 



Salt (NaCl) 

Sodium, Chlorine) 



Gypsum (CaS0 4 , 2 H 2 0) 

(Calcium, Sulphur, Oxygen, Hydro- 
gen) 



Pyrite (FeS 2 ) 
(Iron, Sulphur) 



Magnetite (Fe 3 4 ) 
(Iron, Oxygen) 



Hematite (Fe 2 3 ) 
(Iron, Oxygen) 



Limonite (2 Fe 2 3 , 3 H 2 0) 
(Iron, Oxygen, Hydrogen) 



Siderite (FeC0 3 ) 

(Iron, Carbon, Oxygen) 



Kaolin 

[H 4 Al 2 Si 2 9 ] 

(Hydrogen, Aluminum, Silicon, Oxy- 
gen) 



These minerals are identified in a general way by various features of 
(a) colour, (b) lustre, (c) hardness, (d) number and arrangement of 
crystal faces, (e) cleavage faces and their directions, (/) fracture, 
(g) solubility in water and various acids, and (h) associations in rocks. 
They may be determined with greater refinement (a) by specific 
gravity, (b) by relation to heating tests under the blowpipe and in the 
presence of various chemical reagents, or (c) by grinding rocks to a 
thin section in which the constituent minerals may be determined under 



20 COLLEGE PHYSIOGRAPHY 

the high-power microscope by certain significant phenomena, including 
their behaviour with regard to the light passing through them under 
various conditions. 

Quartz. — Quartz, or silica, forms the most common mineral in rocks 
and soils of the earth. Although slightly soluble in underground 
water, quartz does not decay, because its silicon and oxygen are so 
firmly united. The minerals opal, chalcedony, agate, and jasper are 
impure varieties of silica, as is the rock flint, or chert. Crystalline 
quartz occurs in six-sided prisms terminated by six-sided pyramids, but 
not all quartz is crystalline. Its lustre gives it a glassy appearance 
and its colours vary from clear to milky white, blue, rose, red, and 
variegated. It cannot be scratched with a knife, and although hard 
enough to scratch glass, it is brittle and, when broken, has a con- 
choidal fracture. 

The Feldspars. — These minerals, which are silicates and therefore 
include silica in their composition, are among the most common sub- 
stances in the earth, occurring in all the main classes of rocks. Feld- 
spar is nearly as hard as quartz, and is not soluble, as quartz is, but is 
less durable. It decays when exposed to air and water, and in the 
course of time crumbles to kaolin, a dull, whitish clay. Decayed feld- 
spar is common in many soils and is the source of the best pottery clay. 
Bauxite, a form of kaolin, is the source of aluminum. Orthoclase and 
plagioclase feldspar differ in that the latter has the elements sodium 
and calcium instead of potassium. There are still other feldsp'ars. 

In none of the feldspars are crystals common, but cleavage planes 
are conspicuous, extending through the feldspar, causing it to break 
along smooth faces, and facilitating its decay. Many feldspars are 
light-coloured. 

The Micas. — The colourless variety of mica is familiar in the " isin- 
glass " of stove doors, splitting into thin sheets because of its excep- 
tional cleavage. It is the Muscovite mica which is transparent, and 
this is because it lacks the iron and magnesium of the dark biotite 
mica. All micas are easily scratched with a knife and some of them 
decay rapidly, while others persist after the rocks in which they origi- 
nally occurred are destroyed, appearing as shiny flakes in soils and in 
such rocks as shale and sandstone. 

Hornblende and Augite. — Hornblende, as the table indicates, is 
of complex chemical composition. It is hard, black, lustrous, often 
crystalline ; and with well-defined cleavage. It decays upon exposure 
to air and water, often staining the rock because one of its important 
elements is iron. 

Augite is difficult to distinguish from hornblende, especially in small 
particles. It is usually green rather than black, its cleavage faces 
meet at a different angle, and its crystal form is different from horn- 
blende. It also decays readily. 

Calcite and Dolomite. — These carbonate minerals are alike in being 
easily scratched by a knife and in having cleavage in three directions, so 



FUNDAMENTAL GENERAL FACTS 21 

that they break into readily recognized rhombs. Calcite, like quartz, 
is of variable light colours. It may be distinguished from quartz by 
its softness and by its solubility, which permits it to effervesce freely 
in acid. It is one of the most soluble of common minerals, and its 
cleavage planes allow water to enter and dissolve it, if carbon dioxide 
is also present. Thus a rock containing calcite is much less durable 
than one made up of feldspar and quartz. Calcite has pearly lustre 
and often has perfect crystals. 

To the calcium of calcite (carbonate of lime), magnesium is often 
added, forming dolomite, which is less soluble. If the calcium is 
replaced by iron, then the heavy mineral siderite is formed, the brown 
" spathic " iron ore. 

Salt and Gypsum. — Crystals of rock salt are most easily identified 
by their saline taste. They are cubes, and the cleavage is also cubical. 
The mineral is soluble in ordinary water and soft enough to be 
scratched by the ringer nail, but not as easily as gypsum. 

Like calcite, gypsum is a common constituent of " hard " water 
because of its solubility. It is often white, sometimes crystalline, 
splits into thin flakes because of perfect cleavage, but these flakes are 
not elastic as in mica. Salt and gypsum may also be regarded as 
rocks composed of a single mineral. 

Iron and Iron Ores. — More common and valuable than siderite 
are the ferruginous minerals, magnetite, hematite, and limonite. The 
former may be identified by the fact that a magnet will pick up parti- 
cles of it. It is heavy, usually crystalline, and of a metallic lustre. 
Hematite is heavy and may be red. It is sometimes crystalline 
{specular iron ore), sometimes earthy, and sometimes in smooth, 
rounded masses. Limonite is yellow. The common iron rust is 
limonite, which also occurs sometimes as an ore, one variety being 
bog iron ore. Hematite is the most important of the iron ores, sup- 
plying nine-tenths of the iron produced in United States. Limonite 
gives a yellow streaky hematite red, and magnetite black when 
scratched on a piece of china or on white quartz. 

Iron pyrite is not an iron ore, though sometimes a source of sul- 
phuric acid. )Vhen copper is added (c hale pyrite) , this is often a 
valuable copper ore. Gold also occurs in pyrite, though very rarely, 
but pyrite is often mistaken for it and hence is called " fool's gold." 
This resemblance is striking, because pyrite is a heavy, golden-yellow 
mineral. They may be distinguished because of the hardness of pyrite 
and the softness of gold, which is easily scratched with a knife. Py- 
rite often has cubical crystals. 

Since small quantities of iron are present in a great number of min- 
erals and rocks, and since the oxidation or rusting of the iron goes on 
rapidly, a red stain is given to many rocks and many soils are red with 
hematite stain or yellow with limonite stain. 

Minerals in Rocks. — The common rocks, which will now be dis- 
cussed, are constituted chiefly of the dozen or so minerals listed above 



22 COLLEGE PHYSIOGRAPHY 

(see tables, pp. 24, 26, and 28). Only one or two hundred of the 2000 
or more known minerals are abundant. Locally the others may occur 
in considerable amounts, but all except the common and rock-forming 
minerals are relatively rare in the rocks of the earth. Some of these 
rare minerals, for example, the gems, the ores of gold, silver, copper, 
lead, zinc, tin, platinum, and the ore containing radium, are highly 
prized by mankind. A rock is an aggregate of minerals, sometimes 
chiefly or entirely of a single mineral species, as in the case of rock 
salt, ice, and some limestones, but more commonly of two or more 
different minerals. In common usage a rock is something hard, but 
in the geological usage hardness and consolidation are not necessary 
characteristics of a rock. Thus sand is as certainly a rock as is a sand- 
stone used in building ; and there is every gradation in the earth's 
crust between the loose sand, and the sand which, through deposit of a 




Fig. 19. — The gradation in sedimentary rocks in the sea from pebbles near shore, to sand 
in deeper water, and clay still farther from the coast. A similar gradation and inter- 
calation takes place in sedimentary rocks deposited on the land by rivers. 

mineral cement, such as silica, calcite, or iron, has the grains bound 
together to make the sandstone. In a similar way, the line cannot be 
drawn between the liquid rock which flows as lava from a volcano 
and the solidified lava on the slopes of the volcano. 

Kinds of Rocks. — In the crust of the earth there are a multitude 
of minerals, and these have been assembled in a multitude of ways, 
giving rise to a great variety of rock species and varieties. Three 
great groups of rocks are commonly recognized, on the basis of their 
origin : (1) sedimentary rocks, (2) igneous rocks, (3) metamorp hie rocks. 
The members of one of these rock groups differ from those of either 
of the other groups in significant respects ; and the members of a single 
group differ from one another in more or less notable ways. The full 
study of these differences forms the science of petrology, but physical 
geography is concerned with some of the more significant differences. 

Sedimentary Rocks. — The most widespread of the rocks are those 
that have been accumulated from the disintegration, transportation, 
and deposition of previously existing rocks. The principal agents of 
such transportation are air and water, ^r the latter operating in the 
form of rivers, lakes, oceans, and glaciers. During the transportation 
of the rock fragments there is a more or less perfect assortment of the 



FUNDAMENTAL GENERAL FACTS 



23 



fragments according to their specific gravity, or weight, and the trans- 
porting power of the agent of transportation ; and in their deposition 
there iiS|an arrangement, more or less perfect, according to the size 
of the particles. Thus, there are deposits of pebbles, of sand, and clay, 
and as the supply varies, or the transporting power varies, these layers 
may alternate one above the other (Fig. 19). This assortment, and 
the resulting deposit of layers, gives rise to stratification, which is such 
a characteristic feature of sedimentary rocks, that they are often called 
stratified rocks. The 
layers, or strata, vary 
greatly in kind and in 
thickness, sometimes 
occurring in massive 
strata having great 
thickness and uniform- 
ity, at other times in 
thin layers with rapid 
alternations from one 
kind of rock to another. 
Although derived 
from the waste of pre- 
viously existing rocks, 
and originally depos- 
ited in unconsolidated 
state, the sedimentary 
layers are commonly 
changed to the consoli- 
dated state, primarily 
by the deposit of min- 
eral matter which acts 
as a cement to hold the 
individual grains to- 
gether. Thus beds of 
pebbles are changed to 
conglomerate; sand to 
sandstone, and clay to clay rock and shale. 

as they do of fragments derived from the waste of other rocks, are 
often called fragmental, or clastic rocks. There are two other ways 
in which rocks of this class may be formed, the first by deposit from 
solution, as in the case of rock salt, the second by the work of organ- 
isms, both plant and animal. Plant remains, for instance, give rise 
to coal_ strata ; petroleum and natural gas, which of course are not 
rocks, thougrl found in sedimentary strata, are complex compounds 
of carbon and hydrogen whose origin is not fully understood, though of 
great value to man ; the shells and limey parts of various animals, 
notably shell-fish and corals, cause deposits of limestone, one of the most 
common and widespread of the sedimentary strata, also thought to be 




Fig. 20. — A fossil leaf from the coal beds of the Carbonif- 
erous, showing that there may be plant as well as ani- 
mal fossils, and terrestrial as well as marine sediments. 



Such rocks, consisting 



24 



COLLEGE PHYSIOGRAPHY 



sometimes formed by direct precipitation of lime in the ocean. Cement 
rock and some phosphate rock are also marine sedimentary rocks, 
though the latter is afterwards altered. A magnesian limestone is 
called dolomite. 

Both the clastic rocks and the limestones of organic origin are now 
being accumulated mainly in the oceans ; and the same has been true 
of those rocks formed in the earlier geological ages, as is proved by the 
presence of marine fossils entombed in them. That the most wide- 
spread rocks of the lands are the sedimentary strata, and that the 
greater portion of these were deposited in the oceans, testifies to the 
fact, otherwise abundantly proved, that the relative position of land 
and sea have undergone great change in the past. There are also 
terrestrial deposits, their fossils being non-marine (Fig. 20). 



SEDIMENTARY ROCKS 



Origin 


Name 


Composition 


Fragmental, 
or clastic 
rocks. 


Gravel beds. 
Conglomerates. 
Sand beds. 
Sandstones 
Clay beds. 
Shale. 


Made of pebbles derived from other rocks. 

Consolidated masses of pebbles. 

Finer fragments, usually quartz grains. 

Consolidated sand beds. 

Disintegrated feldspar, hornblende, etc. 

Consolidated clay beds, splitting readily. 


Chemically 1 
formed 
rocks. 


Stalactite, oolite, 
calcareous tufa. 
Iron deposits. 
Silicious sinter. 
Salt. 
Gypsum. 


Carbonate of lime, deposited in water. 

Some ores of iron, especially bog iron ore. 
Silica deposited from water. 
Sodium chloride. 
Sulphate of lime. 


Organic 

rocks. 


Most limestones. 
Dolomite 

Coal (bituminous, 
lignite, peat). 


Carbonate of lime, made of shells, etc, 
Magnesian carbonate of lime. 
Made of plant remains. 



Igneous Rocks. — The igneous rocks have all been in a melted 
state, having been forced upward from within the earth (Fig. 22), 
and, on cooling, solidified in the position where they are now found. 
The igneous rocks differ from the sedimentary rocks in the general 
absence of assortment and stratification and in being made up of inter- 
locking mineral grains clinging together, not by the deposit of a cement, 
but by consolidation on changing from the liquid to the solid state. 
The latter quality gives to them a crystalline structure (Fig. 21) — 
being made of an aggregate of crystals ; the former gives them a mas- 
sive structure as distinguished from the stratified structure of the 
sedimentary rocks. 

The differences among the igneous rocks themselves are numerous, 



FUNDAMENTAL GENERAL FACTS 



25 



and are due primarily to two quite different causes. The first of these 
is the difference in the kind of mineral of which the rocks are made, 
and on this basis many different species of igneous rocks have been 
recognized. The chief underlying reason for these differences is the 
chemical composition of the lava from which the igneous rocks are 
made, and this varies from one part of the earth to another. 

A second difference among igneous rocks is dependent upon the 
position in the earth's crust in which the lava cooled. In volcanic 
regions the lava is expelled into the air, where it cools ; but lava has 
also risen toward the surface, without 
actually reaching it. That which reaches 
the air cools quickly, and the minerals 
of which it is composed do not have time 
to grow to the size that is possible in those 
buried masses which, protected by a blan- 
ket of overlying rocks, require a much 
longer time for cooling and solidification. 
Thus, lavas that flow out at the surface 
are dominantly finer grained than those 
intruded into the strata, and later re- 
vealed by the wearing away of the over- 
lying layers. 

Granite (Fig. 23) is one of the most 
common of the intruded rocks, thrust into 
the strata in great masses, sometimes 
called bosses (Fig. 22) ; but there are 
other intruded rocks, some in bosses, 
others in sheets between the layers, some 
in dikes which cut across the layers, and 
some in other forms of intrusion. There 
are also numerous kinds of lava, one of 
the most common of which is basalt. 
Besides varying in mineral composition, 
lava differs much in texture, some, like obsidian, being so fine grained 
as to form a natural glass, others with sufficiently coarse grain for 
the individual minerals to be recognized. There is also a difference 
according to the effect of the expansion of steam included in the lava 
at the time of its emission. In some cases the expansion of the in- 
cluded water blows the lava to bits, making a volcanic ash which, 
drifted by the wind, may settle on land or water and build up 
layers of stratified rock. In other cases the lava is blown so full 
of holes by the expanding steam, that the solidified lava is porous, 
and even spongy in texture, as in the case of pumice (Fig. 23). 

Igneous rocks are dominant in the neighbourhood of active vol- 
canoes, as would be expected; but they are also found, often over 
wide areas, as in western United States, where volcanic action is no 
longer present. This proves clearly that vulcanism has in previous 




Fig. 21. — A specimen of diabase, 
a fine-grained igneous rock, en- 
larged to show the interlocking 
crystals of feldspar and augite. 
Some trap rock is diabase. 



26 



COLLEGE PHYSIOGRAPHY 



ages been present in areas now free from it. For example, the Rhine 
valley crosses a region of former volcanic activity ; northern Ireland 
and western Scotland, as well as other parts of the British Isles, have 
witnessed great volcanic outflow and intrusion ; and the Palisades of 
the Hudson, in the very suburbs of New York City, are made of lava. 
Furthermore, as the surface of the land is slowly wasted by denuda- 




FlG. 22. — Variable conditions of deposit of igneous rock as in the volcano on the left with 
its dikes, sheets, and ash deposits, and the bosse on the right where the molten rock cools 
deep below the earth's surface. 

tion, it is not uncommonly the case that in or beneath the sedimentary 
strata volcanic intrusions are revealed that were thrust into the strata 
in bygone ages, and often in places where no other sign of volcanic 
action has been discovered. All these igneous rocks, both extruded 
and intruded, slowly crumble when exposed to the air, as other rocks 
do, and supply materials for the formation of clastic rocks. 

IGNEOUS ROCKS 



Texture 


Name 


Chief Mineral Components 


Coarse 
grained 


Granite. 
Syenite. 
Diorite. 


Quartz, feldspar (orthoclase), and horn- 
blende, or mica, or both. 

Feldspar (orthoclase) and either mica, or 
hornblende, or both. 

Feldspar (plagioclase) and either horn- 
blende, or mica, or both. 


Either coarse 
or fine 
grained 


Diabase. 


Feldspar (plagioclase) and augite. 


Fine grained. 


Rhyolite (quartz 

porphyry). 
Trachyte. 

Andesite. 

Basalt. 


Quartz, feldspar (orthoclase), and horn- 
blende, or mica, or both. 

Feldspar (orthoclase), and either horn- 
blende, or mica, or both. 

Feldspar (plagioclase), and either horn- 
blende, mica, augite, or two of these. 

Feldspar (plagioclase) and augite (often 
still other minerals). 



FUNDAMENTAL GENERAL FACTS 



27 



Metamorphic Rocks. — The third class of rocks, the metamorphic, 
as the name indicates, are derived from alteration, or metamorphism, 
of other rocks, whether igneous or sedimentary. In a sense, it is 
metamorphism when clastic fragments are cemented to form a solid 
layer of sedimentary rock ; but this is not what is commonly meant 
by metamorphism. The continued action of water, especially if heated, 
may so alter a rock as to quite completely change its character, and it 




23- 



The banded appearance of gneiss (on the right), in contrast with granite (on the 
left) and pumice (above). 



then becomes a metamorphic rock. But more commonly it is heat and 
pressure, such as accompanies mountain folding, that introduces those 
extensive changes by which the metamorphic rocks are formed. 
Then sandstone may become so compact as to resemble massive quartz, 
forming the metamorphic rock, quartzite; or clay rocks may become 
slate; or limestone be altered to crystalline marble. Even further 
alteration may so transform a rock as to completely destroy the original 
characteristics, so that it is impossible to tell what the original rock 
was, or even to determine whether it was an igneous or a sedimentary 



28 



COLLEGE PHYSIOGRAPHY 



rock before subjected to metamorphism. Such highly transformed 
rocks are of many different types, but for our purposes they may be 
considered as either schist or gneiss. The schist rocks are laminated, 
with a structure resembling stratification ; but differing from stratified 
rocks in the absence of the clastic structure, and the possession, in its 
stead, of a crystalline structure resembling in certain respects that of 
the igneous rocks. Gneiss (Fig. 23) is far more massive, and in 
significant ways resembles granite, having a coarsely crystalline, mas- 
sive structure, and often having the same minerals as granite. It 
differs from granite in the possession of a more or less perfect banding 
of the minerals, roughly simulating stratification. 

The metamorphic rocks are, in the main, confined to mountain 
regions, and are, therefore, less widely distributed than the sedimen- 
tary strata. Having been formed in mountains, and deep beneath 
the surface, where pressure and heat were sufficient for their trans- 
formation, they are found at the surface only where the upper layers' 
have been stripped off by denudation. But metamorphic rocks 
abound in areas, like much of eastern Canada and New England, 
which are not commonly recognized as mountains. Their presence in 
such places, together with other evidences, proves that these sections 
were in former ages the seat of extensive mountain uplift and folding 
now quite extinct. These old mountain regions, long exposed to 
denudation, and worn to their very roots, reveal the deep-seated strata 
which were changed, far below the surface, by heat and pressure during 
the extensive mountain folding of early geological periods. Thus 
metamorphic as well as igneous and sedimentary rocks testify to the 
mighty changes that have been in progress in the lithosphere during the 
long periods of the geological past. 



METAMORPHIC ROCKS 



Name 


Source 


Mineral Composition 


Quartzite. 


Altered sandstone. 




Quartz. 


Slate (argillite). 


Altered clay rock. 




Partially crystallized micaceous 
minerals developed out of the 
clay particles. 


Marble 


Altered limestone. 




Calcite. 


Anthracite 


Altered coal. 




Mainly carbon and carbon com- 


(graphite). 






pounds. 


Schist. 


Altered from various 


Variable — usually two or more 




rocks, e.g. shale, 


con- 


of the following : feldspar, 




glomerate, diorite, 


etc. 


quartz, hornblende, or mica. 


Gneiss. 


Altered from various 


Variable — usually two or more 




rocks, e.g. shale, 


con- 


of the following : feldspar, 




glomerate, granite, 


quartz, hornblende, or mica. 




diorite, etc. 







FUNDAMENTAL GENERAL FACTS 29 

Rock Structure and Position. — It is in their power of resistance 
to the attacks of denudation that rocks are of prime interest to the 
student of physical geography. In this respect there is much dif- 
ference among rocks, due to a variety of conditions. There is, in the 
first place, a great variation in hardness from the soft, unconsolidated 
clays and sands on the one hand to the massive quartzite, so hard 
that it cannot be scratched by steel, on the other hand. Some rocks, 
like limestone, are quite easily soluble by water that penetrates into 
the earth, while others, like clay rocks, are either insoluble, or so little 
open to solution that they may, for all practical purposes, be classed 
as insoluble. Others still, perhaps neither soft nor soluble, are, never- 
theless, easily worn away because some or all of their minerals are 
readily altered and disintegrated, causing the rock to crumble. Such 
a change is often called rock decay, and the use of the word is war- 
ranted because much of the change is due to oxidation as in other forms 
of decay. Even the hardest rocks are sometimes subject to rapid decay 
because of the unstable condition of one or more of their constituent 
minerals on exposure to the air or to percolating water. Lavas furnish 
illustration of this, for the minerals that separate out from the molten 
rock in its cooling are not always compounds that can retain stability 
in the air. 

Usually the degree of resistance that rocks present to denudation is 
due to a combination of two or more of the conditions mentioned in the 
preceding paragraph. A rock that easily disintegrates is called soft, 
though these terms are not used to signify actual hardness or softness. 
It is better to use the term weak or non-resistant for those rocks that 
are easily worn away and resistant for those that withstand the attacks 
of denudation. Granite, quartzite, and gneiss are resistant rocks; 
limestones, clays, sands, and many lavas are weak rocks. Since there 
are those differences in resistance of rocks, as the surface of the lands 
is worn away, it is worn at variable rates, according to the nature of the 
surface rock. Therefore many of the details of topographic forms are 
the result of the condition of the underlying rock. 

Besides composition there are other factors of importance in guiding 
the rate of removal of rocks by denudation. Some rocks, for example, 
are porous, while others are quite dense, and therefore the rate at 
which water can enter the rock and help in its disintegration varies. 
Many rocks are crossed by natural breaks, called joint planes, into 
which water freely enters, and sometimes these joint planes are very 
numerous and close set, aiding greatly in opening the rocks to the 
attacks of the agents of disintegration. The laminse of such meta- 
morphic rocks as schist and the layers of the sedimentary strata are 
also aids to percolating water, often furnishing paths of entrance into 
the rocks. There is much difference in the effect of this influence 
according to the attitude of the layers. 

When deposited in the sea or lake or river, sedimentary strata are 
commonly laid down in horizontal or nearly horizontal position; 



30 



COLLEGE PHYSIOGRAPHY 




Fig. 24. — An unconformity along the line AB. 



but when uplifted above the sea, especially in mountains, these layers 
are often tilted out of the horizontal, and even into vertical position, 
forming folds. When rock layers are broken and then uplifted on one 
side of the break, there is said to be a fault. When folded rocks are 
truncated by erosion and horizontal beds are afterward laid upon the 
eroded edges of the inclined layers, the feature produced is spoken of 
as an unconformity (Fig. 24). 

Since rocks in different attitudes, or with different degrees of poros- 
ity, or with different development of joint planes are subject to the 
attacks of denudation at different rates, these are also important fac- 
tors in determining details in land form. Denudation works selec- 
tively, removing most rapidly those rocks which present the weakest 
resistance, whether as a result of mineral composition, or structural 

weakness, or attitude, or 
a combination of two or 
more of these. 

Mantle Rock or Rego- 
lith. — While in moun- 
tains, and in other places 
of steep slope, the rocks 
of the earth's crust out- 
crop, elsewhere the rocks 
are commonly mantled by a layer of unconsolidated rock frag- 
ments of varying thickness for which the names mantle rock and 
regolith have been proposed. Over much of the land surface this 
mantle of rock debris has been derived directly from the decay and 
disintegration of the rock on which it rests ; but over large areas it 
has been transported by wind, streams, glaciers, or other means from 
a place of origin to its present place of rest. This layer of rock waste, 
mantling a large part of the land surface, may be but a few inches 
thick, or it may be scores or even hundreds of feet thick. It is of 
vast importance in physical geography, for it acts as a blanket pro- 
tecting the underlying rock from rapid disintegration; it furnishes 
sediment to streams ; and it is in its upper portion that most of the 
vegetation of the land grows. 

The upper portion of the regolith, in which the vegetation grows, is 
called the soil, a loose mixture of rock fragments prevailingly of small 
size, ordinarily somewhat porous, and with a greater or less admixture 
of plant fragments. In some swampy places the soil is mainly of 
organic origin, but such soil is of a different origin from that which is 
now being considered. There is much difference in the texture of the 
soil, which varies from compact clay to sand and gravel ; in porosity, 
which ranges from almost impervious clay to loose sand and gravel ; 
in colour, which may be black, brown, red, or yellow ; in thickness, which 
may vary from an inch to three or four feet ; and in mineral and chem- 
ical composition. According to these variations the adaptability of 
the soil to cultivation varies greatly. Some soils are very fertile and 



FUNDAMENTAL GENERAL FACTS 31 

the seat of thriving agricultural industries, while others are thin or 
sterile and quite unsuited to agriculture. 

The soil grades downward into the subsoil, which closely resembles 
and is of the same origin as the soil ; but it lies below the zone of plant 
growth, and contains little or no admixture of organic matter. The 
subsoil, in turn, grades into the underlying rock from which it is 
derived, where the mantle rock has been formed by disintegration of 
the bed-rock ; or, if transported, the subsoil rests directly upon the 
rock upon which it was deposited. Everywhere beneath the mantle of 
soil lie the rocks of the earth's crust, in some places sedimentary, in 
others igneous, or metamorphic. 

Geological Ages 

For convenience of reference geologists have divided the strata of 
the earth's crust into groups, systems, series, and stages, corresponding 
to eras, periods, epochs, and ages of geological time. This is done 
chiefly upon the basis of the fossils contained in some rocks, especially 
those of sedimentary origin. As is shown in the table below, there 
was a time when there were no animals living upon the earth which 
were higher in development than the fishes. Accordingly if strata 
contain remains of birds, it is clear that these are not of such ancient 
date. Careful studies of all sorts of animals and plants of the past, 
as preserved in the rocks, make it possible for the paleontologist to 
determine the relative ages of the rocks with considerable precision, 
although no attempt is made to show how long ago, in years, the strata 
were formed. The relative ages of igneous and metamorphic rocks 
are made out from the structural relationships, the unconformities, 
etc. Indeed it is possible for students of stratigraphic geology to 
determine not only the nature of ancient life, but in some cases, from 
a study of the rocks themselves, to describe the conditions under which 
this life existed on land and sea, and even the climate of past geological 
ages. Such a description of ancient conditions on the earth's surface 
is called paleo geography. The presence of walrus bones in the sands 
of New Jersey, the existence of the musk ox as a fossil in Arkansas, 
and of the reindeer in southern New England, tell a definite story of 
colder climate in these parts of the United States not very long ago. 
Similarly, the fossil plants found near Toronto suggest a milder 
climate than the present. The growth of vegetation which was 
subsequently consolidated into coal in Antarctica and Greenland, the 
precipitation of gypsum and salt in Kansas, the formation of ancient 
glacial deposits in South Africa, of extremely old delta accumula- 
tions in the Appalachians, and of sandstone of eolian origin in Aus- 
tralia, all furnish facts for deciphering the paleogeography of one or 
another of the remote geological periods. 

The use of such a standard geological column as is given on page 32, 
in which the oldest eras and periods are printed at the bottom, is a 
matter of international agreement. 



V 



COLLEGE PHYSIOGRAPHY 
GEOLOGICAL COLUMN 



Era 


Period 


Condition of Life 


CENOZOIC 


Quaternary, or 
Pleistocene 


Man assumes importance, particularly in 
latter part of the Quaternary which is 
known as the Recent Period. Glacial 
Period in first half. 


TIME 

(Age of 


Pliocene 


1 




Mammals) 


Miocene 


Mammals develop in remarkable variety, 




Oligocene 


and to great size, while reptiles diminish. 




Eocene 




MESOZOIC 


Cretaceous 


Birds begin to be important ; reptiles con- 
tinue ; and higher mammals appear ; 
land plants and insects of high type. 


TIME 

(Age of 


Jurassic . 


Reptiles and amphibia predominate. 


Reptiles) 


Triassic 


Amphibia and reptiles develop remark- 
ably; low forms of mammals appear. 




Permian 
Carboniferous 


Land plants assume great importance. 


PALEOZOIC 
TIME 

(Age of 
Invertebrates) 


Devonian 


Fishes are abundant. They began in the 
Silurian and continue, though with 
many changes, to the present time. 


Silurian 
Ordovician 


Invertebrates prevail. They continue 
abundant to the present time, but are 
of different kinds. 




Cambrian 


No forms higher than invertebrates. 


PRE-CAMBRIAN 
TIME 

(Few fossils 
known) 


Algonkian 
Archean 


Mostly metamorphic rocks ; perhaps, in 
part, original crust of earth. 



Some geologists and geographers would change this column in minor 
details. In France and England, for example, the Paleozoic and 
Mesozoic are still sometimes called " Primary " and " Secondary," 
but Ordovician is not separated from the Silurian, while " Liassic " 
may be introduced after Triassic ; in England, Devonian may be called 
" Old Red Sandstone " ; in Germany, Permian may be called " Dyas," 
and Quaternary may be divided into " Diluvium " and " Alluvium " ; 
in the United States there are proposals to divide the Carboniferous 
into " Mississippian " and " Pennsylvanian " ; not to separate Per- 
mian from Carboniferous ; to separate the " Comanchean " from 
the Cretaceous ; to divide the Tertiary merely into Eocene and Neo- 



FUNDAMENTAL GENERAL FACTS 33 

cene; and to divide Quaternary into " Pleistocene " and " Recent." 
Various substitutes have been suggested for Pre-Cambrian, including 
" Azoic " and " Proterozoic," but Pre-Cambrian is most commonly 
used. 

Maps and Map Projection 

The ideal way to represent the features of the earth's surface is by 
a model or relief map. The globe is a small model of the earth and 
photographs of the models or relief maps of the continents are shown 
in Figs. 380, 381, 382, 383, and 385. Flat maps, however, are neces- 
sary for use in books ; but these, though convenient, have disad- 
vantages, especially in the difficulty of representing the third dimen- 
sion, — height or depth. When we come to represent a globular area, 
like the earth, upon a flat map, there are great difficulties to overcome, 
and the whole earth has to be shown in two halves or hemispheres 
(Figs. 18 and 277). As we cannot flatten out the curved surface it 
is, even then, necessary to represent it as if projected upon a flat sur- 
face, and there are several such plans, or projections, the details of which 
may be found in a text-book of mathematical geography. 

One scheme is to project the parallels and meridians, and the 
features of the earth x upon a piece of paper, rolled up like a cylinder. 
The unrolled map has the meridians all parallel instead of converging, 
and the parallels all of the same length. This is Mercator's projection 
(Fig. 321). The distances east and west are all distorted, except on 
the equator, and are too great near the poles, as is seen by comparing 
the width of Greenland on Fig. 321 and on a globe (Fig. 18). Ortho- 
graphic, Stereo graphic, Globular or Equidistant, Homolo graphic, and 
many other projections vary with the assumed position and distance 
of the observer when projecting the lines, and result in less distortion 
than in Mercator's projection. This is seen by comparing Greenland 
in Fig. 486 (Homolographic projection) with the same area in Fig. 
321 (Mercator's projection). There are several Conical Projections, 
which suppose the area of the map to be projected upon a cone or a 
series of cones, similar to the cylinder of Mercator's projection. These 
result in far less distortion (see Greenland in Fig. 172, on a conical pro- 
jection). The projection most commonly used for small areas is the 
polyconic. 

Scale. — It is also necessary to use a scale of reduction in repre- 
senting areas upon a map, an inch upon the map equalling so many 
miles or so many feet in the area mapped. These scales are sometimes 
represented on maps by a printed statement, as one inch equals one 
mile, and also by a graphic scale of miles, as on Figs. 42 and 52, where 
there is a line at the bottom, divided into miles and parts of miles, 
for measuring distances upon the map. Sometimes the scale is also 
represented by a fraction or a ratio, as in the fractional scale 63 3 60 
or 1 : 63,360, meaning that one inch on the map is equal to 
63,360 inches in nature. This and its multiples are much used in 



34 



COLLEGE PHYSIOGRAPHY 



Great Britain. As 63,360 inches (12 times 5280) make a mile, the scale 
of this map is exactly one inch to one mile. The scale 1 : 62,500 (Fig. 
115) is close to that of 1 inch to 1 mile, and this and directly related 
scales are commonly used in United States. The departure from the 
1 : 63,360 scale is based upon convenience in relation to the decimal sys- 
tem ; for 1 : 62,500 is related in a simple way to the scales of 1 : 1 25,000 
and 1 : 250,000 and 1 : 1,000,000. In France, Germany, and other parts 
of Europe the decimal scale of 1 : 100,000 is very common on govern- 
ment maps. 

There is usually no scale of miles on a map of Mercator's projection 
because of the distortion. Scales vary according to the degree of 





Fig. 25. — A shaded map on the left, showing hills, valleys, and a lake. On the right is a 
contour map of the same area with a contour interval of 20 feet. 



detail to be represented on the map, and the use to which it is to be 
put. 

Relief. — Heights and depths may, of course, be represented upon 
the map by figures, but it is also desirable to show the actual shapes of 
the land forms upon the surface of the earth. This may be accom- 
plished in several different ways. On the model it is, of course, done 
by actually carving a minute representation of the real thing, with 
some scale of reduction (see the Rocky Mountains, for example, on the 
photograph of the model of North America, in Fig. 382). On a flat 
map this is impossible except by photography, and a variation of the 
same method is a system of shading the northwest or some one side of 
all elevations, giving an effect similar to what one would see if the 
area were seen in daylight, with one side of each hill and valley il- 
luminated by the sun (Fig. 25). 

Contours. — Another method is to draw lines through all points 
equally high above sea level. These lines are called contours, and they 
represent the edges of parallel planes, such as sea level, and the suc- 
cessive levels to which the slopes would be submerged if the land sank 
20 feet, 40 feet, 100 feet, etc. The vertical distance between these 



FUNDAMENTAL GENERAL FACTS 35 

lines is called contour interval. Contours are closely spaced on steep 
slopes and are far apart on gentle slopes. They bend up-stream in 
valleys and down hill on ridges. Figures 25, 314, 315, and 318 are 
contour maps. 

The space between contours may be shaded, as in Fig. 391, where 
the deeper and deeper tints show deeper and deeper water. The 
height of the land may be shown in a similar way (Fig. 332). 

Hachures. — Another plan of showing relief is to omit the contour 
lines, but to draw short lines down the slopes, as in Fig. 26. These 
short lines are called hachures, and hachure maps (as Fig. 306) give a 
vivid idea of the way a region looks, for the little lines are short and 
close together on steep slopes, long and far apart on gentle slopes, and 




Fig. 26. — A hachure map. (U. S. Coast and Geodetic Survey.) 

are omitted on flats. They are commonly used on United States 
Coast and Geodetic Survey maps, for example, because they give the 
mariner a general idea of the appearance of the country near the coast. 
Hachures do not give as specific information as contours, by which one 
can tell exact heights throughout the area of the map, and the latter 
are generally used on United States Geological Survey maps, as well 
as in many other countries. 



References to Literature 

Simon Newcomb. Elements of Astronomy, New York, 1900. 

C. A. Young. Manual of Astronomy, Boston, 1902. 

David Todd. New Astronomy, New York, 1897. 

F. R. Moulton. Introduction to Astronomy, New York, 1906. 

A. Geikie. Text-book of Geology, 4th edition, 2 vols., New York, 1903. 

J. D. Dana. Manual of Geology, 4th edition, New York, 1895. 

Joseph Le Conte. Elements of Geology, New York, 1877 ; 5th edition, edited 

by H. L. Fairchild, New York, 1903. 
T. C. Chamberlin and R. D. Salisbury. Geology, 3 vols., New York, 1905. 
W. B. Scott. Introduction to Geology, 2d edition, New York, 1907. 
W. H. Hobbs. Earth Features and their Meaning, New York, 191 2. 
A. W. Grabau. Principles of Stratigraphy, New York, 1913. 
F. W. Clarke. The Data of Geochemistry, Bulletin 330, U. S. Geological 

Survey, 1908; 2d edition, ibid., Bulletin 491, 191 1. 



36 COLLEGE PHYSIOGRAPHY 

Mineral Resources of the United States. An annual publication of the 

United States Geological Survey. 
E. S. Dana. Minerals and How to Study Them, New York, 1895. 
L. F. Pirsson. Rocks and Rock Minerals, New York, 1908. 
J. F. Kemp. Handbook of Rocks, New York, 1896, 1900. 
B. Willis, R. D. Salisbury, and Others. Outlines of Geologic History, Chicago, 

1910. 

B. Willis. Index to the Stratigraphy of North America, Prof. Paper 71, U. S. 

Geological Survey, 191 2. 

C. Schuchert. Paleogeography of North America, Bulletin Geological Society 

of America, Vol. 20, 1910, pp. 427-606. 
W. E. Johnson. Mathematical Geography, New York, 1907. 
S. Giinther. Handbuch der Mathematischen Geographic, Stuttgart, 1890. 
Henry Gannett. Manual of Topographic Methods, Monograph 22, U. S. 

Geological Survey, 1893; ibid., Bulletin 307, 1906. 
E. A. Reeves. Maps and Map Making, London, 1910. 



CHAPTER II < 

WEATHERING AND ROCK DISINTEGRATION 

Instances of Disintegration 

Freshly quarried rock seems hard and indestructible ; but so too 
does a piece of steel from a blast furnace. Yet it is a well-known fact 
that the latter, on exposure to the air and dampness, rusts, decays, and 
slowly crumbles. So, too, do rocks, though the time required for their 
disintegration may be longer. The process might escape the casual 
observation in the brief period of a human lifetime, but it becomes very 
noticeable when its effects are magnified by the passage of time. For 
example, headstones placed in cemeteries a century or two ago have 
often so crumbled that the inscriptions upon them are now quite 
obliterated. In old buildings the effects of disintegration of the 
building stone are often very noticeable. For instance, the stone in 
Westminster Abbey in London, put in place in the thirteenth century, 
has during the seven centuries of exposure to the atmosphere so 
crumbled that even the most casual observer must be struck by the 
change ; and the gargoyles and other ornamental parts have so 
crumbled that many of them now are mere shapeless masses of stone. 
Evidence that the same changes are in process in the rocks of the 
earth's crust is clear and complete. 

As stated in the last section, the rate of rock disintegration varies 
with the nature of the rock. That in Westminster Abbey is a loose, 
porous, weak rock which crumbles readily ; but even the most resist- 
ant of rocks are subject to the same changes, though at slower rates. 
There is a variation in rate also according to the nature of the exposure 
and to the climate. The latter point is illustrated by the case of the 
Obelisk in Central Park in New York City. After remaining unchanged 
for many centuries in the desert climate of Egypt, this obelisk was 
brought to the damp, frosty climate of New York, and almost at once 
it began to crumble, and at such an alarming rate that it became 
necessary to protect it by a coating of glaze. In Brazil the decomposi- 
tion of the rock goes to a depth of ioo to 300 feet. 

Agents of Weathering 

Relation to the Atmosphere. — Such rock decay, or disintegration, 
is often called weathering, for the chief agents by which the decay is 
brought about are, directly or indirectly, related to the atmosphere, 

37 



38 COLLEGE PHYSIOGRAPHY 

and to weather conditions. It is, however, a complex process, in 
which the weather is only one of the elements, and from that stand- 
point the term may be somewhat misleading. 

The Three Chief Agencies. — There are three primary agents in- 
volved in weathering : (i) water, (2) atmospheric gases, (3) organisms. 
Independently or in cooperation these work toward the one result of 
rock disintegration ; and each of the agents operates both by mechan- 
ical and by chemical processes. Cooperating with weathering in 
making it more effective are gravity and the agents of erosion, by 
means of which the disintegrated rock fragments are more or less 
completely removed from the rock from which they are derived. 
Where this removal is incomplete, the rock fragments remain as 
mantle rock, thus serving as a partial protection to the bed rock 
against some of the agencies of weathering. 

The Work of Water 

Although the process of weathering is the combined result of the 
action of several agents, the nature of this process will be more easily 
understood, if the agents be studied one by one, and their action 
considered, for the time being, as if weathering were the sole process 
at work. 

Chemical Work of Water. — Among the agents, that of water is 
surely by far the most important. Moisture in the air together with 
the atmospheric gases are potent causes for chemical change, but this 
action is mainly confined to the very surface of the exposed rocks. 
Percolating water, on the other hand, enters into the interior of rocks 
and, accordingly, greatly extends the process of weathering. The 
rate of weathering is, therefore, greatly influenced by the porosity of 
the rocks. Some rocks are quite impervious to water, but the great 
majority are sufficiently porous for the fairly easy entrance of per- 
colating water. Some, indeed, are so porous that the rain-water 
quickly soaks into them, as in sand beds. In the Bermuda Islands, 
where, although in a rainy climate, no fresh water is to be found, the 
supply of drinking water must be obtained from rain-water stored in 
cisterns. 

Rock porosity is dependent primarily upon the fact that the minerals 
or grains of which the rock is composed are not thoroughly bound 
together. There are minute pores or cracks around the grains or 
minerals, through which the water finds more or less ready passage 
according to the size of the cavities. The minerals themselves are 
often traversed by minute cracks, such as open cleavage planes, along 
which water can penetrate, thus finding admission into the very heart 
of the minerals. Furthermore there are larger cracks, such as joint 
planes, along which still greater volumes of water find entrance into the 
earth. 

If the percolating water were pure, the chemical change or solution 



WEATHERING AND ROCK DISINTEGRATION 39 

resulting from its entrance into the rocks would not be great ; but no 
percolating water is pure, for in its passage through the air it carries 
with it atmospheric gases, notably oxygen and carbon dioxide, and in 
its passage through the soil it is also armed with organic acids. Thus 
charged, water becomes a potent agent of solution of some minerals 
and of decay of others. Dissolving one kind of mineral in a rock, 
even to ever so slight an amount, and only around its boundaries, helps 
to reduce the cohesion of the mineral grains, and thus induces crum- 
bling. In some rocks the soluble minerals are present in such amounts 
as to make this cause for rock disintegration of much importance. 

Minerals that are not directly soluble are often open to change in 
chemical composition in the presence of water. The nature of this 
change is not unlike that to which iron is subject. A nail, for example, 
exposed to damp air, first becomes dull, then rusty, and ultimately is 
reduced to a powder of iron rust. In this case water and oxygen from 
the air have formed a chemical compound with the metallic iron, 
changing the form, the chemical composition, the hardness, and even 
increasing the weight of the original iron. This change is oxidation 
and hydration, and the resulting iron rust is the hydrated oxide of 
iron, consisting of iron, oxygen, and water, chemically combined. 
Similar changes occur in some of the rock-forming minerals. There 
are, for example, certain minerals containing iron ; and oxygen and 
water acting upon these produce an iron rust of the same kind as that 
resulting from the rusting of a nail. The red and yellow colours so 
common in rocks and soils are stains due to the rusting or decay of iron 
minerals. 

Other minerals, such as feldspar, so hard that they cannot be 
scratched with the knife, and so clear and transparent as to be glassy, 
slowly suffer chemical change in the presence of water, oxygen, carbon 
dioxide, or other substances, so that they finally lose their glassy 
appearance, are no longer hard, and are transformed to a white powder 
which crumbles between the fingers. As in case of solution, so here, 
the change is most readily carried on along the crevices through which 
water percolates. Thus it happens that the minerals of rocks, when 
studied under the microscope, are often found to be decayed around 
the boundaries, or along the cleavage planes, while elsewhere they are 
fresh and unweathered. By the crumbling of some of the mineral 
grains in a rock it is so weakened that disintegration naturally results. 
Furthermore some of the decayed mineral may be actually removed 
from the rock by the percolating water, for among the products of 
these chemical changes there are often produced compounds which 
are easily soluble, although the original mineral was insoluble. 

The rate at which the chemical work of water proceeds, naturally 
varies greatly according to conditions, such as the amount of rainfall, 
the composition of the rock, the porosity of the rock, the exposure 
of the rock, the chemical composition of the water, and the tempera- 
ture. Speaking generally, this form of rock disintegration is most 






40 



COLLEGE PHYSIOGRAPHY 



rapid and most effective in warm, humid climates, where there is an 
abundance of water, there is much decaying vegetation to supply 
carbon dioxide and organic acids, and the temperature of the perco- 
lating water is high. It thus happens that in such regions the mantle 
rock is often very thick, and the bed rock is decayed to a great depth, 
even scores of feet below the surface. It should be noted, however, 
that one reason for this thick blanket of disintegrated rock in tropical 
regions is the protection that vegetation gives the unconsolidated 
mantle rock against such agents of erosion as the winds and running 
water. The chemical work of water in rock disintegration is least 

effective in arid climates, where there 
is little moisture, and in frigid zones, 
where the soil is frozen so that perco- 
lating water is unable to enter. 

Mechanical Work of Water. — The 
freezing of water is a potent agent of 
rock disintegration, for when ice is 
formed in rock crevices and cavities 
(Fig. 27) it exerts such a strong press- 
ure against the cavity walls that the 
rock may be broken. As the water 
changes to ice, it expands and must, 
therefore, have more room, which, if 
in a closed or nearly closed cavity, it 
can obtain only by enlarging the cavity. 
It is for this reason that bottles of 
water burst when exposed to freezing ; 
and even bombshells and cannon have 
been broken by the powerful force of 
expanding water during freezing. At 
a temperature of 30 F. a pressure of 
about 138 tons to the square foot is exerted by the ice forming in a 
closed cavity. 

To this enormous force rocks near the surface of the earth in regions 
of frost are frequently subjected. The water in the microscopic 
crevices, as well as that in the larger cracks and openings, must, on 
freezing, find space for increase in volume. Where the cavities are 
open, the full force of the expansion will not be exerted ; but even here 
some force is applied against the cavity walls, the amount depending 
upon the difficulty with which the ice is forced out of the cavity. 
As a result of the freezing of water in the rock cavities, disintegration 
is caused, both by the breaking off of minute bits, and by the disrup- 
tion of large masses. With alternate thawing and freezing, such as 
accompanies the succession of warm days and cold nights, the frost 
work is repeated again and again during a single season. This cause 
for rock disintegration is, of course, confined to the colder regions of the 
earth, and it naturally assumes greatest importance in the polar regions 




Fig. 27. — Percolating water that has 
frozen after seeping out of the rock 
along joint planes. Within the rock 
it is a mechanical agent of weather- 
ing. . 



WEATHERING AND ROCK DISINTEGRATION 41 

and in high mountains (Fig. 30). In such places, in summer, there is 
frequent alternation of temperature from above to below the freezing 
point, and there is abundance of moisture. There, frost action is the 
principal agent of weathering. By it, the exposed bed rock is so dis- 
rupted, that its surface is covered with a layer of loose, angular, frost- 
riven blocks. For example, on certain low, flat-topped hills in Spitz- 
bergen one may walk for a mile or more over a field of angular rocks 
with no bed rock and no soil to be seen, while on the steeper slopes 
the large angular fragments may often be seen and heard to break away 
from the bed rock under the influence of the frost action, and to fall to 
the cliff base. This powerful quarrying work of frost is similarly 
potent among lofty mountains where there are extensive fields of 
frost-riven rock fragments (Fig. 30) on the more gentle slopes, and 
accumulations of them at the cliff base. Frost action is one of the most 
important and most rapidly acting causes for the lowering of lofty 
mountain peaks and ridges. 

Frost action is an effective agent of weathering in cool temperate 
regions, though its effects are far less noticeable than in higher lati- 
tudes and altitudes. The breaking away of rock fragments from cliffs, 
and the disruption of exposed rock occur here as in colder climates; 
and the effect of frost upon the soil is often very noticeable. As thaw- 
ing and freezing occur, the ice that forms in the porous soil, seeking 
relief on expansion by rising toward the surface, pushes before it soil 
particles, stones, and even boulders; and at the same time, the 
freezing of water in the soil particles themselves helps to break them 
up and make the soil finer in grain. Frost work, is, therefore, 
an effective agent in rock disintegration and in soil formation ; but 
its activity is limited to a much narrower zone than that of percolating 
water, for it is essentially confined to that surface film of soil or rock 
in which there can be frequent alternation of temperature above and 
below the freezing point. Therefore frost work is most effective in 
those places where the slope is sufficiently great for the frost-riven 
blocks to freely fall away when disrupted ; while on lesser slopes its 
effectiveness diminishes as the mantle of weathered rock accumulates. 

Atmospheric Work 

The air is an agent of erosion, a subject treated in a later section. 
It is also effective in rock disintegration, acting in this respect both 
directly and indirectly through its influence upon temperature. Like 
water, it works both chemically and mechanically. 

Chemical Work of the Air. — In a dry state the air has little power 
for chemical change in rocks, but with its vapour content, the gases, 
notably oxygen and carbon dioxide, become important agents of rock 
change. The processes which result from the action of water vapour, 
oxygen, and carbon dioxide in the air are essentially of the same kind 
as those of percolating water, already discussed — namely, solution, 



42 



COLLEGE PHYSIOGRAPHY 



oxidation, hydration, and other changes in the chemical composition 
of the rock-forming minerals. But this action is confined practically 
to the very surface, although, as we have seen, the extension of weather- 
ing into the rock beneath the surface is greatly aided by the oxygen 
and carbon dioxide supplied to the percolating water, which itself is 
supplied to the atmosphere as vapour condensed to rain. Thus in- 
directly the atmosphere is a very im- 
portant aid to rock disintegration by 
means of chemical change. 

In its direct influence upon the 
chemical changes in rocks, the air is 
of least importance in arid and desert 
lands where the water vapour content 
is least ; and it is most effective in 
humid regions. The chemical work 
of the air is increased by the addition 
of certain foreign substances. For 
instance, in large cities, the abun- 
dance of carbon dioxide, coal gases, 
and other impurities increases the 
activity of the air in this respect. 
The presence of salt in the air, 
brought to the lands by the east 
winds, produces a noticeable effect 
at Gloucester, Mass., where, after a 
few years' exposure, chimneys lean to 
the east because of the action of the 
damp salt air in removing the cement 
between the bricks. 

Mechanical Work of the Air. — 
Directly the atmosphere is of little 
importance as an agent of weather- 
ing by mechanical means, but indi- 
rectly, through its influence upon 
temperature, it is an effective agent. 
In parts of the earth, notably in dry 
perature (b) with expansion of the c ii mate s, there are great ranges in 

surface when heated, (c) with contrac- ' i • 1 i 

tion of the same surface when cooled, temperature during the day, some- 
times as much as 70 or 8o°, and 
even more. The rocks become even warmer than the air, due to 
absorption of heat, and their temperature in the full sunlight may 
rise to as much as 120 or 130 , while at night, by radiation, there 
is a rapid fall to 6o° or less. 

Warming causes expansion, and cooling contraction, so that there is 
a constant straining of the minerals, analogous in a lesser degree to 
that caused in a piece of glass when it is rapidly heated over a flame, 
when, as is well known, it will snap into pieces. This straining is 





Fig. 28. — Van Hise's diagram to show 
the effect of heating the surface of a 
rock : conditions (a) at uniform tern 



WEATHERING AND ROCK DISINTEGRATION 43 



differential, for some minerals expand and contract at one rate, some 
at another ; and some minerals, such as those that are black in colour, 
reach a higher temperature than others, such as the white and trans- 
parent minerals. Consequently, as rock is warmed there is, in the first 
place, an expansion of the entire mass whose temperature is raised, 
and there is also set up within the warmed area a series of strains of 
complex nature by which the minerals tend to pull apart along their 
boundaries. 

Exfoliation. — As a result of these changes, through alternate expan- 
sion and contraction, the outer layers of rocks are weathered and made 
to crumble. Not only are small grains loosened, but layers are cracked 
off and peeled away, giving rise to the phenomenon called exfoliation. 
Some varieties of rock exposed to this phase of weathering present a 
layered outer structure resembling that of an onion, from which it is 
possible to pry off one or more layers, already loosened, but not yet 
quite ready to fall away naturally (Fig. 28). 

Exfoliation is a phenomenon especially common in arid climates 
where the daily changes of temperature are especially rapid ; but it is 
also noticeably present on high mountains, and is not absent in humid 
regions. Although capable of production by temperature change 
alone, exfoliation is often the result of a cooperation of causes in which 
are included change of temperature, chemical change, and frost 
action. 

The Work of Organisms 

Many forms of life, both animal and plant, are contributing toward 
the disintegration of rocks, either directly or indirectly, and both by 
chemical and mechanical means. 

Work of Plants. — Lichens, 
clinging to rock surfaces, loosen 
and pry off particles by mechani- 
cal means as they grow, and they 
aid in chemical changes, both 
directly by abstracting mineral 
substances, and indirectly by 
conserving moisture on the rock 
surface and supplying organic 
acids to it. Higher plants oper- 
ate in the same directions both 
upon the bed rock and upon the 
soil. As the plant roots grow, 
they often exert a powerful force, 
sufficient to wedge off both small 
and large rock fragments (Fig. 
29), and to break up the soil particles ; and by the decay of the plant 
fragments on and in the soil, organic acids are produced which add 
to the efficiency of percolating water as an agent of chemical change. 




Fig. 29. 



— The work of tree roots in weathering. 
(Gilbert, U. S. Geol. Survey.) 



44 COLLEGE PHYSIOGRAPHY 

In the production and comminution of soil there is a process of high 
importance, the exact operation of which is at present only partly 
understood. This is the abstraction of mineral matter from the soil, 
by which a portion of the disintegrated rock is taken into the plant 
in solution, and, on the death of the plant, left on the surface in a 
finely-textured state. In this way, by the action of multitudes of plants, 
there is a slow reduction and wastage of the soil. At least a part of 
this process is performed by bacteria, but an efficient aid in it is the 
hairlike roots of the plant. 

Work of Animals. — Animals aid in rock disintegration mainly by 
work upon the already partly disintegrated rock of the soil. Among 
those animals which are efficient aids to weathering are the burrowing 
animals, such as the ground squirrel, the prairie dog, the woodchuck, 
the earthworm, and the ant. All of these aid to some extent by 
furnishing chemical substances to percolating water ; by bringing soil 
particles to the surface and hence to a zone of greater exposure ; and 
by rendering the soil more porous and, hence, more open to percolating 
water. Some of them, like the earthworm, also aid in the comminu- 
tion of the soil by passing it through their intestinal tract. Among 
the burrowing animals the earthworm in temperate climates and the 
ant in the warmer regions are by far the most important, and are to 
be reckoned as among the significant agencies in the production and 
comminution of soil, one of the final stages in rock disintegration. 

Man, in his civilized state, has come to be one of the most effective 
organic agents in weathering. By tunnelling into the earth, by quarry- 
ing, by excavating, by ploughing, and by removal of natural vegetation, 
he is aiding in the processes of weathering in most important ways ; 
but this work is only recent and, from the standpoint of the develop- 
ment of land forms, the influence of man as an agent of weathering 
may be ignored. 

Variations in Rate of Weathering 

While each of the agents mentioned above, acting in each of the 
ways described, is by itself a factor in weathering, the process of rock 
disintegration, viewed broadly, is the result of a complex interaction 
of all these agents, one dominating here, another there. The general 
result is to cause a slow wasting of the surface, the production of a 
layer of disintegrated rock of variable thickness, and the reduction of 
solid rock to sufficient degree of softness or comminution to permit of 
ready transportation by the agents of erosion. 

Influence of the Rock. — The rate at which weathering succeeds in 
this result depends upon a variety of conditions, some inherent in the 
rock, others dependent upon the nature and intensity of the effective 
agents in a given locality. The rocks themselves vary (a) in porosity, 
(b) in the solubility of the component minerals, (c) in the stability of 
the minerals, that is, in the ease with which they undergo chemical 



WEATHERING AND ROCK DISINTEGRATION 45 

change in the presence of water and atmospheric gases. Rocks com- 
posed of relatively insoluble and stable minerals, so closely set as to 
reduce porosity to a minimum, are disintegrated very slowly, while 
porous, soluble rocks, or rocks with unstable minerals, weather rapidly. 

Influence of Climate. — Both the rate and the nature of the 
weathering processes vary with the climate, one form, like frost, domi- 
nating in cold climates ; another, like solution and chemical change, 
in warm, humid climates ; and a third, like the effect of temperature 
change, in arid climates. Speaking generally, weathering is least 
rapid in arid climates, for there the work of water and of life is reduced 
to a minimum ; but whether the rapid disintegration by frost action in 
cold climates is a more effective agent of weathering than the solution 
and chemical change of warm, humid regions, we are not in a position 
to state. 

Influence of Structure. — In certain exposures rock weathers far 
more rapidly than in others. If, for example, there is a plane of more 
easy percolation in a rock, it is important whether that plane lies 
horizontally or vertically, for, if the latter, it offers a freer passage of 
water into the rock. Again, it is a matter of much importance, 
whether the rock is crossed by many (Fig. 27) or by few joint planes, 
for, if the former, the extent of the rock surface exposed to the air and 
to the effects of freely moving water is greatly increased. Even more 
important than either of those is the question whether the rock is 
exposed to the air or is protected by mantle rock. 

Influence of Slope. — The most widespread and efficient cause for 
the exposure of rock to the air, in spite of the constant disintegration 
to which it is subjected, is steepness of slope, as a result of which frag- 
ments fall away under the pull of gravity as fast as they are dislodged. 
On such slopes weathering may maintain its activity, and rock disinte- 
gration proceeds at a rapid rate. It is partly because of the abundance 
of such slopes among lofty mountains that the rate of weathering there 
is rapid. Other causes for the maintenance of bare rock surfaces are 
the action of the wind and running water, which strip away the rock 
fragments as fast as weathering dislodges them. 

Influence of Vegetation Cover. — A final important influence in rate 
of weathering is the extent of the vegetation cover. While plants 
are aids to weathering in significant respects, as outlined above, they 
exert, on the other hand, an important conserving effect, protecting 
both soil and rock from some of the agents of weathering, and, by their 
protective influence, and by the tangle of roots which they send into 
the soil, tending to hold the mantle rock in its place, and thus keep the 
bed rock protected by the blanket of products of disintegration which 
the agents of erosion tend to remove. It is probable that the general 
protective effect of vegetation is far more important than its destruc- 
tive effect in directly and indirectly assisting in the processes of 
weathering. Where a land surface bears but little vegetation, as in 
arid lands, erosion by wind and running water has a much greater 



46 COLLEGE PHYSIOGRAPHY 

tendency to remove the soil cover than in humid forested areas. 
The stripping off of vegetation in a humid climate has the same 
tendency of permitting the mantle of disintegrated rock to be removed, 
and the bare rock to be exposed, instances of which abound in regions 
where man has carelessly interfered in nature's balance. In this 
respect, as in others, man is to be reckoned as one of the animals con- 
tributing toward rock disintegration. 

Results of Weathering 

The prime result of weathering is the transformation of resistant 
rock to a fragmental condition, in which its removal becomes an easier 
task for the agents of erosion. It is, therefore, a cooperative process 
in the denudation of the land, preparing the rock for removal. Inci- 
dental to this are a series of phenomena, dependent upon the rate of 
preparation, the rate of removal, and the nature of the process of 
removal. 

Influence upon Topography. — Since the rate of disintegration 
varies, according to the climate, the slope, and the nature of the rock, 
the influence of weathering varies from place to place. In cold cli- 
mates gentle slopes are covered with a field of angular blocks of frost- 
riven rock ; in warm climates similar slopes are clothed in a mantle 
of soil ; on steep slopes, rugged rock cliffs appear, while gentle slopes 
are smoothed with a blanket of disintegrated rock. 

Even more noticeable than this, however, is the influence exerted 
by weathering upon rocks of different degrees of resistance. Weather- 
ing is a delicate tool of rock sculpture, detecting even minute differences 
in rock texture and composition, and etching them out into relief. 
A fossil, embedded in a rock, will be etched into relief if more resistant 
than the enclosing matrix, and even its delicate markings will be 
brought out clearly ; or, if less resistant, its site will be transformed to a 
cavity by the work of weathering. Minerals that are more resistant 
than their neighbours will be etched into relief, and cavities will be 
worked into the surface on the sites of the weaker minerals. 

Still larger differences in rock resistance are discovered by weather- 
ing, and the topography influenced thereby. In sedimentary strata, 
for example, the weaker beds are eaten away and the resistant beds 
left in greater relief. Here weathering is only the agent of disintegra- 
tion, but cooperating with it are agents of removal by which the 
effects of disintegration are continued and made more manifest. By 
the combined action of disintegration and removal of the products 
of disintegration by gravity, wind, or water, resistant beds are brought 
into prominence, on a small or large scale according to the degree of 
difference in resistance of the associated strata. 

The resulting topographic form naturally varies, not only accord- 
ing to the nature of the rock, but its attitude. Thus horizontal strata 
are etched along the horizontal outcrop, giving rise to alternate linear 



WEATHERING AND ROCK DISINTEGRATION 47 

cliffs of resistant rock and less steeply inclined intermediate slopes of 
weaker rock (Fig. 67). Inclined strata give rise to ridges where the 
resistant rock outcrops, and to intermediate valleys where the rocks 
are weaker. Massive rocks weather into forms quite different from 
those of bedded rocks in which there are alternations in degree of 
resistance from layer to layer; and minutely jointed rocks weather 
both at a different rate, and into different forms from those that are 
not greatly jointed. 

There is, therefore, a vast difference in the rate of weathering, 
according to structure, texture, and attitude of rocks, and these dif- 
ferences are of fundamental importance in the determination of topo- 
graphic form. It must be clearly understood, however, that the 
result is not the sole product of weathering, but of cooperation between 
weathering and erosion. It is a part of the lowering of the surface in 
which some portions go faster than others. This process may be 
stated as a law as follows : that in the general reduction of the surface 
of the lands those rocks which are resistant to weathering tend to lag behind 
those which are less resistant, and, therefore, to give rise to prominences. 
It is to the operation of this law that we owe most mountain peaks, 
most plateau escarpments, and other topographic forms, the nature 
of which will serve as topics for later discussion. A large part of the 
topographic detail of the lands is dependent in a basal way upon the 
discovery by weathering of differences in rock structure, texture, and 
attitude. 

The Aid of Weathering to Agents of Erosion. — As rock disintegrates, 
the products of disintegration are subject also to removal by gravity, 
wind, and water, and, in the course of their removal, these products 
are used by the agents of erosion as tools in their work of erosion. 
Thus weathering is an aid to erosion, not merely in disintegrating the 
resistant rocks so that they can be more easily worn away, but also in 
giving to the agents of erosion rock fragments with which they can 
scour and grind away the resistant unweathered rocks. Were it not 
for this aid of weathering, the rate of erosion by wind, waves, and rivers 
would be far less rapid. 

The weathered particles are transported by the agents of erosion to 
a place of rest, there accumulating in strata of sedimentary rock. 
Traced back to their ultimate source, most of the materials in the sedi- 
mentary strata have been derived from some previous state of consoli- 
dated rock, and the first step in the process of their removal to a place 
of deposit has been the disintegration of the solid rock by the agents of 
weathering. It is, therefore, proper to consider weathering as funda- 
mentally important in the derivation of the vast series of sedimentary 
strata which have accumulated in the upper layers of the crust of the 
earth during the long ages of past denudation. 

Deposits of Weathered Rock Fragments. — While some of the prod- 
ucts of rock disintegration are moved by the agents of erosion far 
away from their source of origin, other portions come to rest close by 



4 8 



COLLEGE PHYSIOGRAPHY. 



their source. Among these are deposits in whose movement gravity 
has been the prime agent of transportation. 

Talus. — This finds typical illustration in those deposits called talus, 
which accumulate at the base of cliffs by the frequent fall of rock 
fragments loosened by weathering (Fig. 30). They consist of angular 
pieces of rock of varying size, from minute bits to larger blocks whose 
origin is easily proved not merely by the nature of the deposit, but 
also by actual observation of the process of its production ; for the fall 
of fragments from the cliff is often seen. Among mountains where 





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Fig. 30. — Talus slopes in the Rocky Mountains of Colorado. (Cross, U. S. Geol. Survey. 



cliffs are high and weathering active, the rate of talus accumulation is 
rapid. In the St. Elias Range of Alaska, for instance, every morning 
for over a week when the sun's rays warmed the surface of a cliff 
and melted the ice that had formed in the crevices during the preceding 
night, sharp reports were heard by the author as rock fragments were 
dislodged, and the falling blocks could be seen descending the talus 
at the cliff base. 

If there is a stream or other transporting agent at the cliff base to 
remove the talus, its growth is limited, and the excess of supply is taken 
over by the agent of erosion, and removed to other places. But if 
there is no such agent of removal, the talus accumulates, forming a 



WEATHERING AND ROCK DISINTEGRATION 49 

curving deposit which slowly rises up the cliff base, protecting it 
from rapid weathering, and thereby limiting its own supply and 
growth. Ultimately the upward growth of the talus will reach 
a balance between supply from above and removal below. Some of 
the most pleasing curves in mountain topography are those of the 
talus slope, especially when the supply of rock fragments has become 
so diminished that vegetation is enabled to clothe it. Above the 
timber line, and below it wherever the talus supply is rapid enough, 
the talus slopes are almost, if not quite, free from vegetation cover. 
They often attain the slope of loose rock fragments at rest, up which it 
is difficult to climb, and on which one may start a boulder rolling 
whose motion sets up a sliding of an extensive area of unstable talus. 

Deposits due to Creep. — Even on slopes so gentle that the mantle 
rock is able to completely cover them, there is a downhill movement 
of rock fragments, often so slow that it cannot be observed, though its 
effects are noticeable. This movement, called creep or soil flow — 
solifluction — is determined primarily by gravity, though it is aided 
by percolating water, which lubricates the particles and, therefore, 
makes slipping more easy. By slow creep (Fig. 31), the mantle rock 
is steadily moving from higher to lower positions, and this is one of the 
reasons why the layer of disintegrated rock is commonly thicker at 
hill base than on hill slope and hill side. As a result of creep, trees 
growing on hill slopes are sometimes inclined because of downhill 
movement, and the layers of the bed-rock over which the creep has 
passed are bent down hill. Creep, though so slow as to escape casual 
observation, is so widespread and so. continuous that it is to be 
reckoned as one of the leading causes for the removal of rock waste. 
Although far less spectacular, creep is probably much more important 
as a general agent of removal of disintegrated rock than talus forma- 
tion, which is confined to the relatively rare areas of cliff outcrop. 

Creep is closely related to talus formation in that it is dependent 
primarily upon the downhill pull of gravity ; and in a sense it might be 
considered the analogue of talus development upon gentler slopes. 
It is, however, a more complex process, for, besides mere gravity pull 
upon previously disintegrated rock fragments, there is the action of 
percolating water and rain wash, thawing and freezing, expansion and 
contraction with change of temperature, and the push of the wind, 
especially on tree-covered slopes. All these causes cooperate to aid 
gravity in its task of drawing the loose fragments down the gentle 
slopes. 

Avalanches and Landslides. — A third expression of gravity work 
upon more or less completely weathered rock is the occasional fall of 
large masses of rock, called avalanches or landslides. This is especially 
common in lofty mountains, though observed on lesser scale in regions 
of gentler slopes. There is every gradation from the loosening and 
falling of a small piece of rock from a cliff to a huge landslide involving 
hundreds of cubic yards of rock and earth. Here, as in so many other 




Fig. 31. — Features due to creep. Upper view (Hardin) shows rock layers bent down hill 
in Pennsylvania. Lower view (Atwood, U. S. Geol. Survey) shows the displacement 
of a railway by hillside creep near the Yukon River. 



WEATHERING AND ROCK DISINTEGRATION 51 

phenomena of the earth's surface, however, the same phenomenon 
may result from more than one cause. Landslides, for example, may 
be started by the undermining action of a stream flowing at a cliff 
base ; or by the ocean waves cutting back at the base of a sea cliff ; 
or by a glacier steepening a valley slope by its erosive action ; or by 
underground water dissolving out a cavern whose roof falls in ; or 
the lubricating effect of water percolating along planes of easy passage 
in the strata, or even by man in mining at the base of a cliff. It is 
not always easy to tell the exact cause by which sufficient instability 
was given a slope to permit its avalanching to lower levels, and often 
it is apparently the result of more than a single cause. 

Whether due to weathering or to other cause, landslides in general 
may be considered a part of that disintegration by which the rocks 
are being reduced to fragmental condition. In many landslides, and 
if the smaller ones are included, in the vast majority, the primary 
causes for the downfall are weathering and the pull of gravity. On 
steep valley sides in mountain regions, weathering has opened up 
joint planes and other cavities, into which water finds its way, to 
carry the work further either in the form of frost or of underground 
water. Ultimately a part of the steep valley wall becomes so unstable 
that gravity pulls it down, though it sometimes happens that the final 
cause for movement is the pressure of a strong wind or the weight of a 
heavy snowfall, or the melting out of the frost, or the vigorous shaking 
of an earthquake. In all cases the landslide is the final descent of a 
mass of earth or rock which, through previous preparation, has been 
given so unstable a condition that it can no longer remain there. 
Among the agents of preparation, weathering is one of the most effec- 
tive and is, therefore, to be considered as one of the landslide causes. 
The fall of the landslide itself is to be classed as a part of the process 
of weathering from another point of view, since by it, whatever the 
cause, the process of weathering is thereby aided by exposure of fresh 
rock to the air and percolating water. 

While landslides are common and spectacular phenomena, and the 
underlying causes are several and widespread, they are not among the 
leading phenomena of denudation, for they can develop only under 
exceptional conditions, and after a long period of preparation. For 
example, in Quebec, a cliff separates the upper and lower town. For 
several centuries the houses of the lower town extended up to the 
cliff base, and upon the cliff crest rested the fortifications, but in 1800 
a mass of rock slipped away from the cliff face beneath the citadel, 
overwhelming houses upon which it was avalariched. Several cen- 
turies had been required for the preparation necessary for this land- 
slide. A similar conclusion is to be drawn from the landslide which 
descended the steep mountain slope at Amalfi in Italy, removing rock 
from beneath one end of the old monastery which had stood there for 
seven or eight centuries. 

When vigorous earthquakes occur, unstable conditions which ulti- 



52 COLLEGE PHYSIOGRAPHY 

mately would express themselves in landslides are discovered, and 
in a few minutes numerous landslides are precipitated, which under 
normal conditions would descend one by one whenever the degree of 
instability became sufficient for gravity to act. 

In the Canadian Rockies a tremendous rock mass nearly half a mile 
square fell from a mountain side in 1903, partly destroying the town of 
Frank, Alberta, and killing about 70 persons. The landslide com- 
pletely crossed a broad valley and partly ascended the opposite slope, 
burying nearly a mile and a half of railway and producing a new 
configuration of the earth's surface, the landslide topography shown in 
PL I and Figs. 32, 33. Gravity, aided by man's activity in mining in 




Fig. 32. — View of the Frank landslide. (Canadian Geological Survey.) 

the mountain, in combination with certain natural features of topog- 
raphy and structure, caused this landslide. The adjacent peaks were 
so weakened that gaping cracks are now visible on the mountain side, 
and it is quite within the realms of probability that another landslide 
may take place. It would be most wise to move the town because of 
this danger. 

This is one of the largest landslides in the world. It brought down 
nearly as much rocky material as that of Rossberg in Switzerland, and 
many times as much as the Elm landslide of 1881 and the Simplon 
avalanche of 1901. The Rossberg landslide in 1806 resulted in the 
destruction of four villages and the loss of 457 lives. 

The Formation of Soil 

The Importance of Soils. — Among the results of weathering the 
formation of soil is to be classed as by far the most important from the 
human standpoint, for it is this softening and preparation of solid 
rock that makes possible the varied and extensive development of 



Plate I 



^JU RTU&jyiOU NTAI N 
South T^.eak 



North Peak 



; — — -- of, 



■--^ of n 




WILLIAMS ENGRAVING I 



TGPQGRAPHY BY CANADIAN GEOLOS CAL SURVEY 



TURTLE MOUNTAIN, AND FRANK, ALBERTA 



A landslide came from the area on Turtle Mountain between the North and South Peaks in 1903. It 
spread across the valley within the limits of the heavy dotted line. Contour interval 20 feet. 



WEATHERING AND ROCK DISINTEGRATION 



53 



life on the lands. The soil, together with the underlying subsoil, is the 
most extensive and widespread of all the deposits directly due to 
weathering (Fig. 34). It is that part of the disintegrated rock which 
remains at or near the source, — the excess of supply over removal. 




LAKt 



*- 4100' above Sea Les/ef 
Fig. 33. — Cross section showing the Frank landslide. (After McConnell and Brock.) 

Limestone Soils. — In the Bermuda Islands, there is a thin, bright 
red soil, absent on many of the slopes, but often fairly deep in the 
depressions. It rests directly on the. white limestone, forming so 
striking a contrast to it, both in colour and texture, that it was once 
thought to be an ocean bottom deposit. This soil has been derived 
from the slow wasting away of the white limestone, which is composed 
of grains of coral and shell. As the rain-water has fallen on and per- 
colated into this limestone, it has dissolved and carried off the carbonate 
of lime of the coral and shells ; but in these are certain foreign sub- 



-?1 




Fig. 34. — The gradation from solid rock to partly decayed rock and residual soil. 

stances not so easily dissolved, such as silica and salts of iron, and these 
have remained as a residue, which, being minute in quantity, has given 
rise to only a thin soil cover whose red colour is due to the presence of 
iron oxides. Such a soil is called a residual soil, being the residue 
left by weathering. 



54 COLLEGE PHYSIOGRAPHY 

Since limestone consists in the main of soluble minerals, the soil that 
results from its disintegration by the agents of weathering is commonly 
thin, for, even though it may be rapidly disintegrated, the residue that 
remains cannot be great in quantity. Moreover, since it consists in 
the main of minute impurities in the limestone, the texture is so fine 
that it is easily moved by the agents of erosion. 

Granite Soils. — Rocks of more complex composition tend to leave 
a much deeper and more varied product on disintegration. Granite, 
for example, consists of quartz, which is only slightly soluble, and 
which resists chemical change, and of feldspar, hornblende, and per- 
haps other minerals. The latter are insoluble, but their decay gives 
rise to fine-textured products, some of which are soluble, some in- 
soluble. Thus a residual soil derived from the disintegration of 
granite varies in texture, containing grains of quartz and clay, side by 
side ; and the residue from the disintegration of a given thickness of 
granite is many times greater than that from limestone of the same 
thickness. Even in the case of soil produced from granite, however, 
there is some removal of soluble parts, so that the disintegrated product 
is much less in amount than the bulk of the rock from which it was 
derived. 

Variations of Soils. — All rocks being subject to disintegration, the 
formation of residual soil can result from the action of weathering 
upon any kind of rock ; and since rocks vary in mineral composition 
the resulting soil naturally varies both in texture and composition, 
according to the kind of rock from which it is derived. In the soil 
itself, that is, the upper layer of mantle rock, these differences in texture 
and composition tend to disappear under the continued comminution 
to which they are subjected by the action of plants, animals, and per- 
colating water. The soils of different origins still further tend toward 
uniformity by reason of the admixture of organic materials, which 
commonly amounts to six or eight per cent of the whole, and in places 
to far more, notably in swampy areas. Yet there are notable dif- 
ferences among residual soils, some being coarse, others fine, in texture, 
some sterile, others fertile. For instance, in the Blue Grass region of 
Kentucky the soil derived from limestone containing phosphatic shells 
is so fertile as to give the basis for a special agricultural industry, 
whereas round about it are areas of infertile soil derived from other 
rocks and hence sparsely settled and of little agricultural value. 

Soil, Subsoil, and Weathered Rock. — The soil proper is only the 
upper portion of the disintegrated rock, in which there is an appre- 
ciable admixture of organic matter. It varies in depth from a few 
inches to two or three feet. The soil grades downward into the sub- 
soil, which is ordinarily less comminuted, and in which there is little 
or no organic matter. The depth of the subsoil varies greatly, but 
in regions where it is derived by rock disintegration it is not commonly 
more than a score or two of feet in depth, and often much less. It 
grades imperceptibly into the rock bed from which it is derived (Fig. 



WEATHERING AND ROCK DISINTEGRATION 55 

34), and the line of division between subsoil and bed rock is ordi- 
narily difficult to draw and very irregular, for the zone of decay 
extends deeper in some places than in others. Where joint planes offer 
paths of entrance to percolating water, or where easily disintegrated 
rock layers occur, the zone of decay descends farther into the bed rock 
than in the intermediate areas of better resistance. 

At and near the bed-rock surface, unconsumed remnants of the 
decaying rock exist as rounded boulders embedded in the disintegrated 
material. Percolating water, passing along joint planes and bedding 
planes, has decayed the rock on either side of the planes, and has eaten 
into the rock at the angles where these planes meet, thus rounding off 
the angles. But decay has not penetrated to the heart of the hard 
rock, and the core remaining is often quite fresh, though completely 
surrounded by disintegrated rock. When the products of disintegra- 
tion are removed, as by wind or running water, these undecayed 
remnants stand out upon the surface as rounded boulders. 

Thinness of Soils. — Gravity prevents the accumulation of dis- 
integrated rocks upon steep slopes, and by creep it tends also to cause 
the mantle rock to move away from even lesser slopes. Rain wash, 
wind action, and other agents of erosion tend also to carry away the 
unconsolidated rock, while percolating water is steadily engaged in 
removing soluble constituents. It is as a result of these movements 
that the thickness of the mantle rock is kept down to the present limits, 
for although it is always forming, it is also always moving away from 
the place of origin. That which remains represents merely the excess 
of formation over removal. Much of that which is removed finds 
temporary accumulation on the lesser slopes, and there it may attain 
depths of several hundred feet ; but far more of it finds its way into 
the streams and is borne far away. 

Transported Soils. — Some of the rock material thus transported 
finds lodgment on the land, there giving rise to soil far from the source 
of its origin. Such are wind-blown deposits, and river deposits on 
floodplains and deltas. These are transported soils, for although their 
component parts may have been originally derived by weathering, 
they owe their present position, texture, and composition largely, if 
not entirely, to the agent of transportation. Among transported 
soils some of the most important are those that have been brought 
to their place of deposit by glacial action. The soils of a large part 
of densely settled northwestern Europe and northeastern North 
America are of this origin. Transported soils differ. from residual 
soils in having no necessary relation to the rock upon which they 
lie, and in resting upon this rock with an abrupt boundary (Fig. 176) 
instead of grading into the bed rock. 

Soil the Basis of Agriculture. — Whether transported or residual in 
origin, the soil as the seat of plant growth is of vital interest to man ; 
for it contains mineral substances which he needs, and the plants have 
developed the power of abstracting these from the soil and transform- 



56 COLLEGE PHYSIOGRAPHY 

ing them to a condition in which they can be incorporated into the 
human body. For the utilization of the mineral substances the first step 
must needs be the crumbling of the hard rock under the processes of 
weathering, then the intervention of plants that find a foothold in 
the soil. 

References to Literature 

J. G. Andersson. Solifluction, a Component of Subaerial Denudation, Journ. 
Geol., Vol. 14, 1906, pp. 91-112. 

J. C. Branner. Ants as Geological Agents in the Tropics, Journ. Geol., 
Vol. 8, 1900, pp. 151-153; Geological Work of Ants in Tropical America, 
Bull. Geol. Soc. Amer., Vol. 21, 1910, pp. 449-496; Decomposition of Rocks 
in Brazil, ibid., Vol. 7, 1896, pp. 255-314; Bacteria and the Decomposi- 
tion of Rocks, Amer. Journ. Sci., 4th series, Vol. 3, 1897, pp. 438-442. 

A. P. Brigham. A Norwegian Landslip, Bull. Geog. Soc. Philadelphia, Vol. 4, 
1906, pp. 292-296. 

H. O. Buckman. The Chemical and Physical Processes Involved in the 
Formation of Residual Clay, Trans. Amer. Ceramic Soc, Vol. 13, 191 1, 

PP- 336-384- 
Charles Darwin. The Formation of Vegetable Mould, New York, 1883. 
C. Davison. On the Amount of Sand Brought up by Lobworms to the Surface, 

Geol. Mag., new series, Vol. 8, 1891, pp. 489-493. 

0. A. Derby. Decomposition of Rocks in Brazil, Journ. Geol., Vol. 4, 1896, 

pp. 529-540. 
A. Geikie. Rock- Weathering Measured by the Decay of Tombstones, 

Geological Sketches, London, 1882, pp. 159-179. 
Albert Heim. Uber Bergsturze, Zurich, 1882; Der Bergsturz von Elm, 

Zeitschrift der Deutschen Geol. Gesell., Vol. 34, 1882, pp. 74, 435-439. 
E. W. Hilgard. Soils, New York, 1906. 
C. G. Hopkins. Soil Fertility and Permanent Agriculture, Boston, 19 10. 

E. Howe. Landslides in the San Juan Mountains, Colorado, Prof. Paper 67, 

U. S. Geol. Survey, 1909, pp. 1-55. 
A. A. Julien. On the Geological Action of the Humus Acids, Proc. Amer. 
Assoc. Adv. Sci., Vol. 28, 1880, pp. 311-410. 

F. H. King. The Soil, New York, 1895. 

R. G. McConnell and R. W. Brock. Report on the Great Landslide at Frank, 
Alberta, Appendix to Part 8, Ann. Rept. Dept. Int. of Canada for 1902- 
1903, Ottawa, 1904, 17 pp. ; see also R. A. Daly, W. G. Miller, and G. S. 
Rice, Memoir 27, Dept. of Mines, Geol. Survey Branch, Ottawa, 191 2, 
34 PP- 

G. P. Merrill. Rocks, Rock Weathering, and Soils, New York, 1897; Prin- 

ciples of Rock Weathering, Journ. Geol., Vol. 3, 1896, pp. 704-724, 850-871. 
A. Penckand Others. Papers on soil flow in Spitzbergen, Iceland, and Central 
Europe. Zeitschrift Gesell. Erdkunde zu Berlin, 191 2, pp. 241-270. 

1. C. Russell. Subaerial Decay of Rocks, and Origin of Red Color of Certain 

Formations, Bull. 52, U. S. Geol. Survey, 1889, pp. 1-65. 
N. S. Shaler. Origin and Nature of Soils, 12th Ann. Rept., U. S. Geol. Survey, 

Part 1, 1891, pp. 213-345. 
R. S. Tarr. The Relation of the Secular Decay of Rocks to the Formation of 

Sediments, Amer. Geol., Vol. 10, 1892, pp. 25-44; Rapidity of Weathering 

and Stream Erosion in the Arctic Latitudes, ibid., Vol. 19, 1897, pp. 

131-136. 
C. R. Van Hise. The Belt of Weathering, A Treatise on Metamorphism, 

Monograph 47, U. S. Geol. Survey, 1904, pp. 409-561. 
T. L. Watson. Weathering of Granitic Rocks in Georgia, Bull. Geol. Soc. 

Amer., Vol. 12, 1901, pp. 93-108. 



CHAPTER III 

THE WORK OF WINDS 

Activities of the Wind 

We have seen that the atmosphere is directly and indirectly an 
aid to rock disintegration in the complex process of weathering. It 
is a potent agent of change on the earth's surface in other directions 
also, primarily through its movements which we call winds. Some 
of the most important effects of the winds are indirect, as in influ- 
encing temperature, in transporting water vapour, and in causing waves 
and currents in lakes and oceans. But the winds are of importance 
in a direct way, by their own work upon the surface of the lands, and 
it is this phase of the subject with which we are now concerned. 

The wind is one of the agents of erosion, which means that it is 
engaged in removing, transporting, and depositing rock fragments. 
Wind erosion is operating with greater or less effectiveness in all 
climates and regions, but it is least effective in humid lands, espe- 
cially where clothed with vegetation. There are four different types 
of land surface on which wind erosion is most active : (a) humid 
lands, (b) lofty mountain tops, (c) portions of the narrow strip of sea 
coast, (d) arid lands and deserts, the most extensive and important 
of all. These four different types of land surface- will be considered 
separately, beginning with the humid lands. 

Wind Work in Humid Lands 

Relation to Vegetation. — Where vegetation densely clothes the 
land, the loose, disintegrated rock is held in place and protected. Yet 
even here, strong winds now and then overturn trees, and winds of 
hurricane force, such as accompany tornadoes, may plough a path 
through the forest, even carrying the trees away bodily. The over- 
turning of the trees moves rock fragments a short distance, and, in 
the upturned roots, exposes soil or stone to the agents of weathering, 
but this is a minute effect. 

Aid from Man's Activities. — By his occupation of humid lands, 
man has created opportunity for the operation of the erosive work of 
the winds by the removal of the forest, and by exposing the soil in 
roads and ploughed fields. That this opportunity is taken advantage 
of is easy to see when gusts raise clouds of dust from field or road, and 
whirl it away. 

Dust in the Air. — Even with the aid of man, however, the erosive 
work of the wind in humid regions has not become an agent of great 

57 



58 COLLEGE PHYSIOGRAPHY 

change. It is, nevertheless, an effective agent of deposition, even in 
humid lands, for dust is ever present in the air and ever settling from 
it. Derived at some favourable point, minute fragments of minerals, 
together with other solid impurities, may be floated in the air for 
days and even months before settling to the earth in a place far distant 
from their source. Sometimes this dust slowly settles in days of 
calms, sometimes it is brought down in the raindrops and snowflakes, 
but more rises to take its place, buoyed up by the variable currents 
of the air. The heavy, hazy atmosphere common during periods of 
dryness is due to dust, of which mineral particles form a portion, and 
the cleanness of the air after rain is the result of temporary removal of 
the solid impurities by the falling rain. 

At times the fall of dust from the air is so noticeable as to attract 
widespread attention. For example, dust clouds and dust storms 
whose source is apparently the distant Sahara are occasionally ob- 
served in Italy and southern France ; and sometimes the rain is coloured 
red from the abundance of reddish mineral matter, giving rise to what 
is known as " blood-rain." 

Dust from Volcanic Eruptions. — Another source of mineral dust 
in the air is from volcanic eruptions. Volcanic ash erupted* from 
Vesuvius has fallen in Constantinople ; ash from Icelandic eruptions 
has fallen in Scandinavia ; and the ash thrown into the air by the 
great eruption of Krakatoa in the Straits of Sunda, in 1883, spread 
far and wide over the earth, and in such quantities as to cause brill- 
iant sunsets in Europe and America months after the eruption. 

Dust Settling from the Air. — From volcanic sources, from exposed 
surfaces in humid lands, from mountain tops, and from arid lands 
mineral dust is rising into the air, where it floats about, together with 
smoke particles and other foreign substances. Carried far and wide 
by the winds, this material returns to the earth on sea and land, in 
humid and arid regions. How important this supply of wind-drifted 
dust is, in humid lands in general, we are not in a position to say, 
though when considered as a process continuing through thousands 
of centuries it seems probable that its result has been noteworthy. 
Near the borders of arid lands, where there is much fine material 
within easy reach of winds from such regions, the importance of the 
fall of dust upon the land is certainly great. It is doubtless one of 
the important factors involved in the burial of the monuments of 
the ancient Roman civilization. That this is not an improbable 
statement is indicated by the fact that an inch of dust is reported to 
have fallen in parts of Italy during a single dust storm. 

Wind Work on Mountains 

Lofty mountains are exposed to strong winds, for they rise into the 
rapidly moving, upper air currents. These winds readily move light 
materials, such as snow and mineral fragments, and in exposed places 



THE WORK OF WINDS 59 

are, therefore, of importance in checking the accumulation of a pro- 
tective cover on the bed rock. The work of the wind is aided by the 
general absence of vegetation and by the steep slopes, both of which 
leave surfaces exposed to its action. As soon as a grain of mineral 
is loosened from a rock surface by weathering, and even before it is 
ready to fall away under the pull of gravity, the wind at exposed 
points is present to carry it away. Even particles of rock larger than 
sand grains may be carried by violent winds, and whirled with such 
force as to strike a painful blow on the face and hands. 

We have no measurements for the determination of the rate of 
work of the wind in such situations, and it would be difficult to measure 
in any event, since it is but one of several processes which are at work 
reducing mountain elevations and since its work is irregularly dis- 
tributed. But to one who has been much on the windward side of 
peaks, and in certain passes through which the wind persistently 
sweeps, the direct attack of the wind is one of the significant factors 
of denudation. 

Bearing upon this point is the evidence from the deposit of wind- 
drifted dust upon glacier surfaces and upon snow fields. In Green- 
land, for example, at a distance of several miles from the nearest 
land, dust is present on the glacier surface in such quantities that, 
gathering in little depressions, it forms dark-coloured deposits. By 
absorption of heat these melt holes in the ice, called dust wells, in 
the bottom of which the dust deposit stands. Some of this dust col- 
lected by the author and examined under the microscope was found 
to contain mineral fragments like those of the nearest mountains. 
A similar phenomenon has been observed in Spitzbergen and in Alaska. 

Wind Work along Shorelines 

Portions of the sea coast are especially favourable to effective wind 
work, for there are three favouring conditions : (a) an abundance of 
sand thrown up by the waves, (b) absence or sparseness of vegetation 
because of the porous sandy soil, its frequent movement, and the 
unfavourable effect of the salt spray and water, (c) the rapid drying 
out of the loose sand both between tides and in the zone above ordi- 
nary wave reach. All of these conditions, excepting the influence of 
the tides and of salt, may be found also on lake shores. 

The Formation of Sand Dunes. — With an abundant supply of 
dry sand, having little or no vegetation to hold it in place, the wind 
readily moves the sand before it, sometimes lifting it well above the 
surface, but more commonly drifting it along with the accompany- 
ing development of ripple marks (Fig. 35). Closely examined, 
the movement is seen to consist of a motion of the surface film of 
sand, rising up one slope of the ripple mark and dropping down 
over the other slope (Fig. 39), so that the ripple itself is constantly 
changing form and position. These sand movements are most effec- 



6o 



COLLEGE PHYSIOGRAPHY 



tive under the influence of drying winds, which need not necessarily 
be very strong ; but unless it is very wet, even damp winds cause 
the sand to drift, though in this case the wind force must be strong. 
Very violent winds whirl the sand before them, and are even able to 
pick up and transport shells and shell fragments. 

The sand is drifted in any direction that the wind may happen to 
be blowing, but since the great majority of winds blow either on shore 
or off shore, either directly or diagonally, the chief sand movements 
are either landward or seaward. In the latter case the sand comes 
within the reach of the waves to be thrown back again ; but in the 
former it accumulates on the land back of the beach. Here the ten- 




Fig. 35. — The checking of sand dune movement by vegetation on the coast of New Jersey 
Ripple marks on the sand. 



dency is for it to form a narrow strip of low sand hills, called sand dunes, 
whose width and height depend upon the extent of the land supply, 
and the force, direction, and dryness of the wind. Very often the 
dune strip is but a few yards wide, but in some cases it attains a width 
of several hundred yards, and even a mile or several miles. On the 
seaward side the dune face is fairly steep, due to the occasional attack 
of the waves at its base ; on the landward side it grades downward 
through lower and lower hills to a thin film of sand, or, if there is a 
lagoon behind the dune area, the sand grades into lagoon bottom 
sediments. In their highest portion the dunes are commonly not 
more than 50 or 75 feet, and often much less, but they may reach 
heights of 200 or 300 feet. 

The reason why the sand dunes are commonly highest near the 



THE WORK OF WINDS 61 

beach is, first, the nearness to abundant supply, and, secondly, the 
effect of sand-loving vegetation (Fig. 35) in checking the movement 
near the source of supply. The sand dunes have a very irregular form, 
though normally consisting of low, short ridges, or oval hills, with 
basin depressions between. Ordinarily they are subject to frequent, 
or even constant, change in form and size, for as the wind direction 
or force varies, deposition occurs at one time and removal at another. 
The sand layers which are thus deposited, partly removed, and then 
covered with other layers, assume various angles according to the 
slope on which they lie, and various positions with relation to one 
another. This gives rise to a cross-bedded structure known as 
wind-drift-structure, which is characteristic of wind-drifted sand 
deposits. 

Erosive Work. — As viscosity protects the sand grains it seems 
improbable that grains less than 0.755 millimetre could be well 
rounded under water ; when the sand leaves a beach, however, and 
is moved about by the wind the grains are ground together and reduced 
in size by slow attrition ; but little erosion is performed here by sand 
driven against the hard rock, because, ordinarily, there is little if 
any such rock in the sand dune areas. Here and there, however, 
there are rock cliffs at beach ends, and here the rock is polished and 
worn by the sand blast. That the driving of sand against rock may 
be an efficient agent of erosion is indicated by the fact that an arti- 
ficial sand blast is employed in chipping away the surface of glass 
in the process of manufacturing ground glass. On the island of 
Monomoy, south of Cape Cod, the glass in a window in a fisherman's 
house was so chipped by the natural sand blast that objects outside 
could be seen only indistinctly, and the fisherman told the author 
that it had been done in a period of twelve years. 

Encroachment of Dune Areas. — The movement in sand dunes 
is often very rapid, the complete form of a dune being changed during 
a wind of a few hours' duration. As the sand drifts about, it collects 
in lee spots, much as drifting snow does, and it is often necessary 
for a man living in a sand dune area to keep a path shoveled from his 
house door through the sand, just as, in winter, a path must be shovelled 
through the snow. The lighthouse-keeper at Ipswich, Mass., has a 
raised wooden walk from his house to the lighthouse, but, even from 
this, the sand must be shovelled away frequently. 

Even the dune area itself may migrate or extend its limits, usually 
when there has been some interference with natural conditions. 
This is well illustrated by the change at Coffin's Beach on Cape Ann 
in Massachusetts. Here, at the close of the eighteenth century, 
there was a broad sand beach facing westward, and backed by the 
normal fringe of sand dunes, behind which stood a farm and behind 
that the virgin forest. The owner of the farm stripped off the forest, 
and the sand began to move, inundating his farm with a flood of 
drifting sand, which to this day has not ceased its movement. 



62 COLLEGE PHYSIOGRAPHY 

On the coast of Europe there are numerous cases of the landward 
advance of sand dune areas. Around parts of the coast of the Bay 
of Biscay, for instance, the sand has marched inland at the rate of 
i6| feet per year. In their advance the dunes have overwhelmed 
farms, houses, and even villages ; and in some cases in their onward 
march these buried places have been partly or wholly uncovered 
(Fig. 36). The advancing sand encroaches upon forests, partly or 
completely burying the trees, then, with further changes, uncovering 
them again. Where the wind direction, supply, and topography are 
favourable the march of drifting sand is irresistible and overwhelming, 
carrying complete disaster with it. 

Yet in places it is possible to check or retard the destructive ad- 
vance. This may be done, for example, by establishing a forest 
windbreak in the rear of the dune area, or by planting trees, shrubs, 
or grasses that can grow in such a soil. European governments have 



/aoo 



/339 



/86£> 
Sca/e of Miles. 



Fig. 36. — Burial and uncovering of a village in northern Germany by sand dunes. 
(After Behrendt) 

done much to check the movements of the drifting sands, and latterly 
the United States Department of Agriculture has been doing a valu- 
able work in this direction, experimenting with different kinds of 
plants adapted to growth in the sand (Fig. 35) and with other means 
of holding the sand in place and protecting contiguous areas from its 
encroachment. 

Settlement of Dune Areas. — Most coastal sand dunes are composed 
of quartz sand, the material of the beaches, and they are, therefore, 
barren areas, for neither wild nor cultivated plants take kindly to 
such a soil. Ordinarily, therefore, the settlement of sand dune areas 
is sparse, and the inhabitants are mainly scattered fishermen, light- 
house-keepers, and others whose livelihood is connected with the 
sea rather than the land. Where the movement of the sand has 
nearly or quite ceased, the dunes may supoort a sparse pine or other 
forest ; and some dunes whose composition is of other than quartz 
fragments may have a considerable degree of fertility. Instances of 
such dunes are those in which fine-grained river sediment is incor- 
porated, or those made of shell and coral sand, like the Bermudas. 



THE WORK OF WINDS 63 

Aid of Wind in Formation of Sand Bars. — Coastal sand dune areas 
are widely distributed along ocean shores, and they are not uncom- 
mon along lake shores, especially those of large lakes. There is, for 
instance, an extensive area of this sort, in Indiana, at the head of 
Lake Michigan in which there are interesting conditions of encroach- 
ment upon contiguous forest. From New York southward to Mexico 
the coast line is fringed for much of the distance by sand beaches 
with associated sand dunes ; and north of New York there are local 
dune areas, the most extensive being on Cape Cod. Sand dunes are 
also accompanying features of sandy coasts in other parts of the 
world. 

The sand dune strip is a prominent part of many sand bars and 
low sand islands immediately off shore from the mainland. The 
waves throw the sand up to the limit of their highest reach, and the 
wind then piles the sand higher, raising the bars and islands and broad- 
ening them, thus making habitable land, free from danger of inunda- 
tion by the sea. The growth of these sand bars and sand dunes some- 
times partly encloses lagoons, and diverts the land drainage, de- 
flecting the stream mouths by forcing them to seek outlet around the 
end of the bar. Locally areas are completely shut in by the growth 
of the dune areas, giving rise to broad depressions and to ponds and 
small lakes. Still another effect is to act as a breakwater to protect 
the low-lying mainland behind from the attack of the ocean waves. 
A considerable part of the coast of Holland is efficiently protected 
in this way. 

Aid of Wind in Formation of Islands. — Many oceanic islands owe 
a part, if not all, of their area and height above sea level to the wind 
action. This finds illustration in the Bermuda Islands. Here lime- 
secreting animals thrive upon a submarine platform, whose founda- 
tion is probably a volcanic cone rising from deep water nearly to sea 
level. By their life and death these animals supply material, mainly 
corals and shells, for the waves to accumulate and grind up on beaches, 
forming coral and shell sand. This loose sand is drifted before the 
wind and has been piled up into an extensive sand dune area with 
the hillock and basin topography and the typical wind-drift-structure. 
The highest point to which the coral sand has been carried is about 
250 feet above present sea level. 

The formation of the Bermudas, though the work of the past, is 
not a completed process, for they are still being built up. Coral and 
shells still thrive on the offshore reefs; beaches of coral and shell 
sand still exist along the coast ; and the wind is still driving the sand 
inland. At one part of the south coast the sand has encroached upon 
gardens and fields, and has even overwhelmed and almost completely 
covered up a native house. 

Two conditions are adverse to the rapid shifting of sands here : (1) 
the fact that numerous plants, notably the oleander, thrive even in 
the shifting coral and shell sand, soon forming thickets which check 



64 



COLLEGE PHYSIOGRAPHY 



its movement, (2) the fact that the rain-water, percolating into the 
limey sand, dissolves carbonate of lime and deposits it around the 
grains, soon cementing them so that they resist removal by the wind. 
When the movement of the sand ceases, cementation soon binds the 
grains together into a rock which can be used for building. 




Fig. 37. 



Area of North America deserts cross-lined. Black indicates moister mountain 
areas. (MacDougal and Shreve.) 



This process is not confined to the Bermudas, but finds equal illus- 
tration in the Bahama Islands, and in many coral islands in the 
Pacific and Indian oceans. The wind, therefore, is to be reckoned 
as a potent island builder, taking materials supplied to it by the 
waves, and raising them above the reach of the sea. Many scores 
of thousands of people are living upon such wind-built islands. 



THE WORK OF WINDS 65 

Wind Work in Arid Countries and Deserts 

Extensive areas of the earth have a climate so dry that only sparse 
vegetation grows, and in some places it is so dry that vegetation is 
almost absent. Each of the continents has such areas, those in North 
America lying west of the 100th meridian, mainly in Mexico and western 
United States (Fig. 37). In such arid regions the soil exposed between 
the sparse vegetation is almost permanently dry and it is, therefore, 
subject to transportation by the winds, which are often very strong. 

Movement of the Sand. — Normally the loose soil is slowly drifted 
about, keeping close to the ground, and tending to accumulate around 




38. — Sand storm sweeping over the southeastern part of the Sahara near Khartum 

in 1906. 

obstacles, such as well-rooted plants. The surface is, therefore, made 
somewhat irregular by many small mounds, out of the top of which 
one or more plants grow ; but the mound is there because of the plant, 
not the plant because of the mound. This condition decreases toward 
the border of the arid lands, where, finally, plant growth so covers 
the surface as to effectively protect the soil from the wind. 

Now and then in the drier regions fierce winds sweep over the sur- 
face, and then the loose, fine-textured soil is raised in clouds of dust, 
which so fill the lower air as to completely shut out distant objects 
from view. In deserts these sand storms assume such proportions as 
to be dangerous to life, for the air is literally filled with sand, and even 
breathing is difficult. Extensive deposits are made, the local topog- 
raphy is altered in detail, and paths and trails are obliterated. The 
sand storm is one of the dreaded dangers of the caravan trade of the 
Sahara, and is one of the most vivid illustrations of the transporting 
power of wind when sweeping over desert lands (Fig. $8). 



66 COLLEGE PHYSIOGRAPHY 

Although more slowly acting, since it is a more continuous process, 
it is probable that the work of moderate winds is even more potent 
than that of occasional fierce winds. Still another means of trans- 
portation in the arid regions is that of the desert dust whirl. Look- 
ing down upon a desert lowland from some neighbouring elevation, one 
can often see several small columns of dust slowly moving across the 
lowlands, rising a hundred feet or more in the air with a diameter of 
but a few feet. These dust whirls develop on hot, calm days by the 
rising of the heated air, and the rapid inflow of the air, causing a move- 
ment of such force as to move along and even lift sticks of wood. 
Though covering but a small area, the dust whirls are formed so fre- 
quently that they are to be reckoned as important agents of transporta- 
tion, continuing the work of the wind even during periods of calm. 

Source of Sand Supply. — It is a common belief that deserts are 
typically expanses of constantly shifting sand dunes, but this is far, 
from the truth. It is true that over much of the surface there is a 
drifting of the sand, a rippled sand surface, and the development of 
mounds around vegetation ; and it is true that there are extensive 
tracts of sand dunes ; but there are also bare plateau tops, exposed 
rock ledges, barren mountain slopes, and fields of stream-borne stones 
at the mountain base (Fig. 39). The desert is a region of diverse 
topography and diverse surface conditions, having the one common 
feature of barrenness (Fig. 41) that aridity brings to all parts of it 
excepting the scattered watered spots, or oases. 

The Growth of Sand Dunes. — Over most, if not the entire, sur- 
face of a desert the work of the winds is active, though naturally it 
varies from time to time and from place to place. Some parts are 
much exposed to the wind work, while others are more or less pro- 
tected from it ; and some days have little or no wind, while in others 
the wind blows fiercely. The irregular work of the wind is especially 
well illustrated by the distribution of the sand dune areas. 

These occur only in areas of favourable conditions, notably the 
presence of abundant supply, of favourable wind direction, and of such 
protection as to permit the wind to deposit and prevent it from re- 
moving the sand as fast as it can be brought. The supply is the prime 
factor. Material is ready for movement from the barren wind-swept 
slopes, and from the soil between the scattered plants, and this is 
being constantly swept about, but it can accumulate in sand dune 
areas only where some topographic feature intervenes to check its 
further spread. 

By far the most extensive sand dune areas, however, are those with 
a still more abundant supply, coupled also, at times, with the pro- 
tective influence of topography. This supply is brought by the' 
streams that here and there flow across the desert or out into it. 
Many short streams descend from the mountains that border or rise 
out of desert areas, and there are even some of large volume. As 
these reach the drier lowlands, their water evaporates and a part or 



THE WORK OF WINDS 



67 



all of the sediment that they bear is left in the dry air at the mercy 
of the winds. With such an abundant sand supply, extensive tracts 
of dunes, often scores of miles in breadth, are built, in which the sand 
rises in a succession of ridges and hummocks with intervening depres- 
sions (Fig. 40), across which travel is most difficult and often dangerous. 
In the dunes the sand is in such constant movement that no vege- 
tation can find a foothold. When the wind blows fiercely, the sand is 
drifted in blinding sheets, giving rise to the dreaded sand storms. 
Such sand dune areas are found in all the great deserts of the world, 
— the Sahara, Gobi, Kalahari, in Australia, the Great Basin and the 
arid southwestern part of United States (Fig. 37), and many others. 




Fig. 39. — Sand dune ridg 



U. S. Geol. Survey.) 



Dunes along Rivers. — Dune areas of similar origin are developed 
even in semi-arid regions, where stream beds become partly dry 
during a portion of the year, thus exposing sandy tracts to the wind. 
Dune areas of this origin are found in western Texas, in the Pecos 
valley, in the Arkansas valley of western Kansas, in the Platte valley 
of western Nebraska, along the lower Columbia River, and in many 
other parts of western North America and other continents. In 
such places a strip of dune sand of variable width is found on the lee- 
ward side of the valley, and often at the base of the valley slope, up 
which the wind cannot bear great quantities of sand. 

Encroachment of the Sand. — The sand dune tracts of arid and 
desert lands, like those of coast lines, expand in area as the supply 



68 



COLLEGE PHYSIOGRAPHY 



continues and thus encroach upon neighbouring land. Ordinarily 
in desert countries this encroachment is of little importance, because 
the land is of so little value. But since oases are commonly caused 




Fig. 40. — Topographic maps of sand dunes in central Washington (upper) and in south- 
eastern California (lower). (After Moses Lake and Holtville Quadrangles, U. S. Geol. 
Survey.) 



by the water of streams descending from the mountains into the 
desert, and these streams are the source of the dune supply, it is not 
uncommonly the case that the sand encroaches upon these valuable, 
fertile spots, doing much damage. The encroachment of the dunes 



THE WORK OF WINDS 



69 



in semi-arid lands, in which there is often extensive irrigation, is an 
even more serious matter ; and already the problem of checking such 
encroachment is confronting settlers in western United States, as 
it has long confronted dwellers in older countries of the Old World. 

By the extension of sand dune areas, and by the slower drifting 
of sand and dust, great changes have been effected in the regions of 
ancient civilization in northern Africa and the desert and arid portions 
of Asia. A large part of the debris with which the ancient cities of 
Nineveh and Babylon are covered is wind-borne dust and sand, and 
the same is true of many other ruins of western Asia. In Central 




Fig. 41. — Cliff sculptured by the wind in southwestern United States. 



Asia there are hundreds of cities buried beneath wind-blown deposits, 
some of them beneath advancing sand dunes. These and other cases 
are possibly the result of a change in climate to a condition of greater 
aridity, thus permitting greater activity of wind action. 

Erosive Work in Dry Regions. — The work of winds in arid and 
desert lands is largely expended upon the transportation of loose, 
unconsolidated rock fragments, though it is by no means confined 
to this. As the material is drifted about, it is ground finer, and thus 
made ready for easier and more distant travel. Even the hard rock 
itself (Fig. 41) is attacked by the natural sand blast, and the frag- 
ments removed add to the supply for transportation. Pebbles and 
boulders over which the sand drifts are polished, grooved, and faceted 
by the abrasion of the wind-driven sand to a distinctive shape, the 



7o COLLEGE PHYSIOGRAPHY 

drikanter ; cliffs are attacked and slowly worn back, the weaker rocks 
being abraded faster than the resistant ones. 

Much of the fantastic sculpturing of rock exposed in desert lands 
(Fig. 41) is due partly to the erosive action of the natural sand blast. 
It is, however, difficult to assign an exact value to this agent, since 
weathering and rain wash are cooperating factors. Yet the fact 
that such rock forms abound in arid climates, but are rare in humid 
regions, makes it certain that they are normal products of the agents 
of denudation in the desert, of which the wind is certainly one of the 
most important. There seems no escape from the conclusion, there- 
fore, that the wind is a potent agent, even in the destruction of hard 
rocks, removing by its own force particles already loosened, and, by 
drifting sand against the rock surface, even scouring off the firmly 
adhering mineral grains. To the desert the wind as a geological agent 
is what running water is in humid lands ; but whether in gen ^ral 
denudation it works more or less rapidly than running water does in 
humid lands cannot be stated at present. Partly because of the fact 
that the humid lands are the main seat of human activities, while 
desert lands are sparsely settled and little known or studied, less is 
known about the physical geography of the desert, and the agents 
of change there are but partly understood, and possibly underes- 
timated. 

Transportation of Dust out of Deserts. — We have seen that the 
movement of sand and dust in the desert is almost incessant, and that 
it gives rise to important changes in and near the area of supply. 
Much is also borne out of the desert by the winds and allowed to settle 
on the surrounding regions. By this means the surface of the desert 
is being slowly lowered. 

During violent winds vast quantities of sand and dust are carried 
into the air. It has been estimated that during a desert storm as 
much as 126,000 tons of mineral matter may be present in every cubic 
mile of air. The heaviest of this settles as soon as the velocity of 
the wind decreases sufficiently, and most of the heavy part finds lodg- 
ment in the desert. But the finer dust particle will float in moder- 
ately moving air, and thus may not come to rest upon the earth until 
it has travelled hundreds of miles from its source. 

Vessels sailing off the west coast of Africa often experience a fall 
of dust, which sometimes settles in such amounts that it is necessary 
to remove it from the decks, and on the Mediterranean it is said to 
sometimes give the sails of vessels a reddish tint. Dust from the 
Sahara has fallen on the Canary and Cape Verde Islands, and, as 
we have already seen, in Italy and southern France. Similar deposits 
are known around the borders of other desert and arid lands. Such 
movement of finely comminuted rock material from arid regions is 
not only an effective cause for the lowering of the surface of such re- 
gions, but also of deposition of sediment upon neighbouring lands and 
seas. Few rivers drain desert lands, and many deserts are closed 



THE WORK OF WINDS 



7i 



basins, out of which no water runs. Were it not for the wind, there- 
fore, the disintegrated rock of desert lands would, in the main, accu- 
mulate there and fill the basins with a great depth of sandy sediment. 



U 



o 



n 



\ * 



K\ 



. f 


* ^ 


/ 


U| ^% y 


Hin-cliC 


7 i *, 




] / /V* 


4 { 


iS^i' \ - 




Fig. 42. — Contour map of a loess-filled basin in China. (After Willis and Sargent.) 

This tendency is partly counteracted by the action of the wind, which 
in desert lands is the prime agent, not only of denudation, but of 
transportation of rock fragments, as rivers are in humid countries. 



72 COLLEGE PHYSIOGRAPHY 

Formation of Loess. — On lands bordering some of the arid belts 
of the earth, there are deposits of a fine-grained loam called loess, 
with grains coarser than clay, but finer than sand. A similar deposit 
is found in some humid lands remote from arid regions, as, for in- 
stance, in the Rhine valley, whence the name originally came, and in 
the Mississippi valley. It is quite probable that deposits of more 
than one origin are here classified under a single name, based upon 
physical characteristics, rather than upon origin ; but certainly much 
of the loess and some of the most extensive deposits are of eolian 
origin. 

This is true, for instance, of the vast deposits of loess in parts of 
China (Fig. 42), where it occurs at elevations up to 5000 feet, and on 
hills as well as in valleys. It is also true of at least a large part of the 
extensive loess deposit in central and western United States. Richt- 
hofen, as a result of his studies in China, put forward the wind ex- 
planation for the remarkable deposit of loess in that country, a theory 
that has been generally accepted. According to this theory, fine- 
grained, wind-borne dust, driven from contiguous arid regions, has 
settled upon the grassy border lands, forming deep deposits, which 
by the action of rain wash has locally been thickened, especially in 
the valleys. 

Pumpelly proposed an extension of this theory to the loess deposits 
of the Mississippi valley, and this is now quite generally accepted, 
though it is also believed that rain wash and sedimentation in quietly 
and slowly moving waters is the explanation of certain parts of the 
deposit ; and some of the supply may have been derived from the 
sediments left by the retreating ice sheet of the Glacial Period. 

The wind theory receives support from a number of considerations, 
as follows. The most extensive loess deposits are on or near the border 
of arid lands and on the side toward which the prevailing winds 
blow from them. The material of the loess is of such texture as 
the dust which winds can transport. Both in China and in the United 
States and Alaska, the process is still in progress. For instance, dust 
from the plains farther west not infrequently falls in Kansas City ; 
and noticeable quantities of such dust accumulate on the window sills 
in the central part of Kansas during periods of strong west winds. 
In the loess are found the remains of plants and animals such as live 
on the land, and the casts of plant roots extend through the loess. 
There are other facts also pointing to wind origin, such as the ar- 
rangement of the component particles. Altogether, therefore, the 
wind theory for at least a large part of the loess seems well founded. 

Loess and Man. — In the Mississippi valley, the loess is not com- 
monly more than 50 feet in depth, but in China and in the more aridi 
parts of western United States it is from 1500 to 2000 feet in depth| 
in places. It makes a fertile soil and is often the seat of dense agri- 
cultural population, where the climate is humid, or where irrigation! 
is possible. In China the thick loess is, in places, much dissected byl 



THE WORK OF WINDS 



73 



drainage lines, and the valleys are bordered by steeply rising, often 
vertical walls, of the unconsolidated loess. These steep walls are 
the result of the development of vertical cracks, or joint planes in 
the loess; and the compact, fine-textured loam may remain in the 
vertical position for a long time. In some parts of China the inhabit- 
ants have excavated their houses in the loess slopes, and thousands 
of Chinese live in such excavations (Fig. 43). 




Fig. 43. — A vertical wall of loess in China with excavated dwellings. (G. F. Wright.) 



References to Literature 

W. P. Blake. On the Grooving and Polishing of Hard Rocks and Minerals 
by Dry Sand, Amer. Journ. Sci., 2d series, Vol. 20, 1855, pp. 178-181; 
Explorations and Surveys for a Railroad Route from the Mississippi to 
the Pacific, Vol. 5, pp. 92, 230; ibid., Appendix to Preliminary Geo- 
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H. J. L. Bradwell. The Sand Dunes of the Libyan Desert, Geog. Journ., 
Vol. 35, 1910, pp. 379-395- 

T. C. Chamberlin. Supplementary Hypothesis Respecting the Origin of the 
Loess of the Mississippi Valley, Journ. Geol., Vol. 5, 1897, pp. 795-802. 

V. Cornish. On the Formation of Sand Dunes, Geog. Journ., Vol. 9, 1897, 
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H. C. Cowles. The Plant Societies of Chicago and Vicinity, Geog. Soc. of 
Chicago, Bull. 2, 1901, pp. 56-65. 

W. Cross. Wind Erosion in the Plateau Country, Bull. Geol. Soc. Amer., 
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W. M. Davis. The Geographical Cycle in an Arid Climate, Geographical 
Essays, Boston, 1909, pp. 296-321. 

G. K. Gilbert. Wheeler's Geographical and Geological Surveys West of the 



74 COLLEGE PHYSIOGRAPHY 

iooth Meridian, Engineer Dept., U. S. Army, Vol. 3, Washington, 1875, 
pp. 82-84, Pis. VIII and IX ; Lake Basins Created by Wind Erosion, 
Journ. Geol., Vol. 3, 1895, pp. 47-49. 
Sven Hedin. Scientific Results of a journey in Central Asia, Stockholm, 
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A. S. Hitchcock. Controlling Sand Dunes in the United States and Europe, 

Nat. Geog. Mag., Vol. 15, 1904, pp. 43-47. 
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E. Huntington. The Pulse of Asia, Boston, 1907. 

C. R. Keyes. Eolian Origin of the Loess, Amer. Journ. Sci., Vol. 156, 1898, 

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D. T. McDougal. Desert Basins of the Colorado Delta, Bull. Amer. Geog. 

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P. Olsson-Seffer. Relation of Wind to Topography of Coastal Drift Sands, 
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S. Passarge. Die Kalahari, Versuch einer Physisch-geologischen Darstellung 
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Raphael Pumpelly. Relations of Secular Rock Disintegration to Loess, 
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144; Smithsonian Contributions to Knowledge, Vol. 15, 1867, pp. 1-143. 

Raphael Pumpelly, W. M. Davis, and E. Huntington. Explorations in Turkes- 
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F. von Richthofen. On the Mode of Origin of the Loess, Geol. Mag., new 

series, Decade II, Vol. 9, 1882, pp. 293-305 ; China, Ergebnisse Eigener 

Reisen und darauf Gegrundeten Studien, 4 vols., Berlin, 1883. 
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Geol. Mag., Decade III, Vol. 6, 1889, pp. 242-250, 289-295. 
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Vol. 5, 1894, pp. 207-212. 

B. Shimek. A Theory of the Loess, Proc. Iowa Acad. Sci., Vol. 3, 1896, pp. 

82-89; Papers on the Loess, Bull. Lab. Nat. Hist., Iowa State Univ., 
No. 5, 1904, pp. 298-381. 

G. H. Stone. Wind Action in Maine, Amer. Journ. Sci., 3d series, Vol. 31, 

1886, pp. 133-138. 

R. S. Tarr and Lawrence Martin. Glacial Deposits of the Continental Type 
in Alaska (including loess), Journ. Geol., Vol. 21, 1913, pp. 295-300. 

W. G. Tight. Bolson Plains of the Southwest, Amer. Geol., Vol. 36, 1905, pp. 
271-284. 

J. A. Udden. Erosion, Transportation, and Sedimentation Performed by the 
Atmosphere, Journ. Geol., Vol. 2, 1894, pp. 318-331; Dust and Sand 
Storms in the West, Pop. Sci. Monthly, Vol. 49, 1896, pp. 655-664; 
Loess as a Land Deposit, Bull. Geol. Soc. Amer., Vol. 9, 1898, pp. 6-9. 

J. Walther. Das Gesetz der Wiistenbildung, Berlin, 1900, 191 2 ; Die Denuda- 
tion in der Wiiste und ihre Geologische Bedeutung, Abhandl. Math.- 
phys. Classe Gesell. Wiss., Leipzig, Vol. 16, 1891, pp. 448-453. 

B. Willis. Research in China, Vol. 1, Carnegie Instn., Publ. 54, 1907, pp. 183- 
196, 242-254. 

J. B. Woodworth. Post-Glacial Eolian Action in Southern New England, 
Amer. Journ. Sci., 3d series, Vol. 47, 1894, pp. 63-71. 

G. F. Wright. Origin and Distribution of the Loess in -Northern China and 
Central Asia, Bull. Geol. Soc. Amer., Vol. 13, 1901, pp. 127-138. 

V. Zeigler. Factors Influencing the Rounding of Sand Grains, Journ. Geol., 
Vol. 19, 191 1, pp. 645-654. 



THE WORK OF WINDS 



75 



TOPOGRAPHIC MAPS 

In this and succeeding chapters all map references are to the standard 
quadrangles of the U. S. Geological Survey, unless otherwise specified. Other 
American maps will occasionally be referred to. No attempt will be made to 
cite the European and other foreign maps showing physiographic features, 
modern lists of which will be found, among other places, at the ends of the 
chapters in de Martonne's Traite de Geographic Physique, 1909; in Davis and 
Braun's Grundziige der Physiogeographie, 191 1 ; in Davis's Erkldrende Beschrei- 
bung der Landformen, 1912; in Davis's discussion of Large Scale Maps as 
Geographical Illustrations (Journ. Geol., Vol. 4, 1896, pp. 484-513); and in 
Martin, Bean, and Williams's Laboratory Manual of College Geography, 1913. 

Gannett has published an excellent list of 100 American topographic maps, 
classified as to physiographic features shown, — Topographic Maps of the 
United States showing Physiographic Types, U. S. Geol. Survey, 1907. Other 
good selections may be found in Davis, King, and Collie's Governmental Maps 
for Use in Schools, New York, 1894; in Gannett's Folios 1 and 2 and Hill's 
Folio 3 of the Topographic Atlas of the United States, U. S. Geol. Survey, 
1900; in the folios of the Geologic Atlas of the United States; in Jefferson's 
Exercises on Topographic Maps, 1906; in Salisbury and Atwood's The Inter- 
pretation of Topographic Maps, Prof. Paper 60, U. S. Geol. Survey, 1908; 
in Davis's Practical Exercises, 1908; in Emerson's Manual of Physical Geography, 
1909; in Salisbury and Trowbridge's 3 laboratory manuals, 191 2, 1913; in the 
author's Laboratory Manual of Physical Geography, New York, 1910 and 1913 ; 
and in several other American text-books and laboratory manuals of physical 
geography. 

Wind Erosion and Deserts 



Laramie, Wyo. 
Granite Range, Nev. 



Toole Valley, Utah 
Saypo, Mont. 



Ballarat, Cal. 
Coldwater, Kan. 



Sand Dunes 



Moses Lake, Wash. 
Monterey, Cal. 
Easthampton, N.Y. 



Holtville, Cal. 
Cherry Ridge, Mont. 
Syracuse, Kan. 



Yuma, Ariz. 
Sandy Hook, N.J. 
Wyndmere, N.D. 



Many U. S. Coast and Geodetic Survey maps, as Charts 119, 121, and 212, 
also show dunes. For maps showing the loess, see the atlas of Geographical 
and Geological Maps accompanying Willis's Research in China, 1906. 



CHAPTER IV 
THE WORK OF UNDERGROUND WATER 

Entrance and Movement of Underground Water 

Proportions of Run-off , Evaporation, and Percolation. — When 
rain falls upon the land, a part quickly runs off at the surface, a part 
is returned to the air by evaporation, and a part sinks or percolates 
into the ground. The latter is called underground or ground water. 
The proportion that pursues this latter course varies greatly, accord- 
ing to the rate and amount of the rainfall, the porosity of the ground, 
the dryness of the ground and air, the steepness of the slope, and the 
luxuriance of the vegetation which retards the run-off. It is, however, 
a very large percentage of the annual rainfall of most regions. Nat- 
urally less soaks into the ground where there are steep slopes for 
rapid run-off than from gentle slopes, on which the rain-water tends 
to stand ; less where vegetation interferes with run-off than where it 
is sparse or absent ; less into dense than into porous rocks ; less into 
dry rock than into that already wet ; and less where the rain is so 
heavy that it quickly forms tiny streams and rills than when the rain 
falls more slowly, and, therefore, runs off less readily. 

The Movement of Water Underground. — That portion of the 
rain-water that enters the soil and rock passes along the cavities, both 
large and small, and enters upon an underground journey of greater 
or less extent, according to circumstances. A very considerable 
portion has a short journey, for much of it is evaporated either 
directly into the atmosphere or into the air that occupies the cavities 
when water does not fill them. The rise of such vapour by upward 
diffusion from the ground is a familiar fact to campers who have slept 
upon the ground and, in the morning, found their rubber blankets 
dripping with moisture. At all times such vapour is rising from the 
damp ground, and it is ascending even when the surface layer is quite 
dry. 

Use of Underground Water by Plants. — Other portions of the 
underground water rise by capillary action, and still other portions 
are taken from the ground by the plant roots, and stored in the plant 
tissue, or given back to the air by evaporation from the leaves. This 
action of plants makes a heavy drain upon the moisture of the soil. 
It is in large measure the effort to obtain the necessary moisture that 
causes plants to send their roots over such large areas and to such 
depths. In arid climates the roots of a plant are often much more 

76 



THE WORK OF UNDERGROUND WATER 77 

extensive than the part of the plant above ground ; and in thin soils 
in humid lands the roots of trees often extend far from the tree, and 
the size to which the tree can grow is not uncommonly limited by the 
amount of moisture available within the reach of the widely spreading 
roots. Drain pipes are often split open and quite filled by a multi- 
tude of roots of a tree seeking this source of moisture. 

Water Absorbed by Rocks. — Still another termination of the 
underground journey of water is in chemical combination with the 
rock-forming minerals, by hydration, during the process of weathering. 
While much water is thus locked up in solid form, it is undoubtedly 
far less than the proportion whose journey is quickly ended by evap- 
oration and by the intervention of plants. 

Water Long Detained Underground. — Other portions of the 
ground water enter upon much longer journeys, slowly seeping 
through the soil, subsoil, and even the solid rock, remaining under- 
ground for days, months, and even years, before reappearing at the 
surface, perhaps at a point far distant from the place of its entrance. 
Some may remain in the rocks for indefinite periods, and there is 
undoubtedly underground water in the crust of the earth that was 
locked up there in early geological ages. Such water may not return 
to the surface until brought up in hot springs, or during volcanic 
eruptions, or when, by denudation, it is brought near the land surface. 

Source of Mine Water. — The presence of percolating water, even 
far below the surface, is often demonstrated by mines and other deep 
excavations, into which water seeps from the enclosing rock. It is 
one of the important and expensive tasks of mining to remove the 
water that enters from the surrounding rock, and if pumping is sus- 
pended for a time the water gathers in pools in the lower workings. 
Not all mines are wet, however, which proves that water is not per- 
colating through all rock layers. 

Porous Beds are Underground Reservoirs. — It is the more porous 
beds that offer the easiest path for underground water to follow, some- 
times the porosity between the rock grains, sometimes the openings 
resulting from the mechanical breaks or joint planes that traverse 
the rocks. Some such porous beds, notably sandstones, are reser- 
voirs of underground water, from which extensive supplies maybe 
obtained by boring to them, and tapping the slowly moving supply 
which entered the bed, perhaps scores of miles away at some point 
where it outcrops at the surface. So slow is the percolation, even 
through such porous beds, that the water rising in a well fifty miles 
from the point of entrance may have required fifty or a hundred 
years to make the journey. 

The Water Table 

Depth of Permanent Saturation. — The zone below which the soil 
or rock is saturated is known as the water level or the water table 



78 



COLLEGE PHYSIOGRAPHY 



(Fig. 44). The depth of this varies greatly from place to place, 
primarily according to the climate and the porosity. In humid 
climates it may lie at the very surface, as in swamps, while in arid 
regions.it may be hundreds of feet beneath the surface. The water 
table is also subject to change even in a given locality, rising during 
periods of rain and sinking during periods of drought, when it is 
lowered by the downward passage of water, by capillary rising, by 
evaporation, and by plant action. 

Relations to Topography and to Gravity. — The surface of the 
water table is by no means level. Speaking generally, it roughly 
follows the contour of the land, though it is also influenced by porosity, 
and sinks to a lower level in porous than in more impervious rocks. 
It is, however, farther beneath the surface on hilltops than in valley 
bottoms, and this difference in depth becomes even greater in periods 
of drought than in rainy periods. Indeed, if there could be rainfall 




Fig. 44. — Relation of the water table to topography. (After Veatch.) 



enough, the water table would coincide with the surface, and would, 
therefore, follow the topography exactly. That it does not do so 
normally is due to the fact that the water escapes faster than it is 
supplied, and this escape, while in part due to the causes stated above, 
is mainly the result of subterranean percolation. Underground, as 
well as at the surface, water tends to drain from high to low ground 
under the pull of gravity. In surface water this tendency finds im- 
mediate expression by the run-off ; but underground water is checked 
in its motion by the cavity walls, and it can obey the pull of gravity 
only by slow percolation. Thus it is that the water table surface on 
a hill is well above the water table level in a contiguous valley ; but, 
since the underground water is slowly obeying gravity by percolation, 
the water table of the hill sinks as the water percolates toward the 
valley. Consequently the water table is farther beneath the surface 
on the hill than in the valley, and in times of drought the level sinks 
still farther. The water table in the valley is in part supplied and 
kept near the surface by the percolation of underground water from 
the neighbouring higher ground, and the surplus is turned over to the 
stream to swell its volume. Thus the supply of underground water 
is an important factor in keeping streams fed and in conserving the 
rain-water and turning it over to the streams during periods between 
rains. 



THE WORK OF UNDERGROUND WATER 79 

Wells 

Relation to Water Table. — The presence of underground water 
is of great value to millions of persons as a source of water supply, 
especially those living in the country. By digging down below the 
water table a supply of water is insured, for the water seeps into the 
cavity from the water-charged subsoil or rock, rising in the well to 
the water table. As the water table rises and falls with the varia- 
tion in rainfall conditions, the surface of the well slowly responds, 
lagging behind a little because of the time taken for the water to seep 
through the ground. In periods of drought the water table may 
sink many feet and shallow wells go dry, especially upon the higher 
ground; and at times of unusual dryness the water may go out of 
even deeper wells, and over wide areas a water famine may result. 
Such a condition cannot take place in a uniformly damp climate, but 
in the variable climate of eastern and central United States it occa- 
sionally occurs. 

Depth of Wells. — The depth to which wells must be dug varies 
with the climate, being very shallow in humid regions, while in arid 
climates dug wells are quite impossible away from water courses. 
It also varies with the topography, it being necessary, in general, 
to dig deeper wells on hilltops and upper hillsides than in valley bot- 
toms and on lower hill slopes, because of the position of the water 
table described above. There is also a variation dependent upon 
porosity, as might be expected. At times, too, there are under- 
ground zones of more rapid percolation, as along joint planes, or 
along bedding planes, especially where a porous bed rests upon a 
more impervious one. Such positions are peculiarly favourable to 
large and steady supply; but even such supplies may be subject to 
exhaustion when the water table is lowered by drought. There is no 
known way of determining the existence of such underground water 
bodies without actual digging, notwithstanding the claims of the so- 
called " water witch " to be able to do so, the reported success being 
doubtless due to the fact that there is a water table, and that a well 
dug below it is certain to have water. 

Pollution of Wells. — Since the water supply of dug wells is rain- 
water that has entered the ground, upon an underground journey in 
which it has been interrupted, perhaps but a few score yards from 
the point of entrance into the ground, well water is subject to danger 
of pollution, which is further increased by the danger of surface wash 
finding its way into the well. Carelessness or ignorance in location 
of wells in relation to barns and other sources of possible pollution 
has been, and still is, the basis for a large amount of disease in 
country districts. Even though partially filtered through a few yards 
of soil, barnyard drainage is never a healthful beverage. Even a 
well uphill from the barnyard may be polluted if a porous layer, 
sloping downward from the barn, goes beneath the well. 



8o 



COLLEGE PHYSIOGRAPHY 




Fig. 45. — Emergence of a spring at S, 
where porous glacial deposits overlie 
impervious rock on the side of a valley. 



Springs 

Nature of Springs. — In certain situations underground water out- 
flows at the surface, seeping out in small quantities, or in places 
flowing out with considerable volume. Where the outflow of under- 
ground water is somewhat concen- 
trated it is called a spring. This 
concentration may be brought about 
by a variety of causes ; and, ac- 
cording to the degree of concentra- 
tion, the spring may be either of 
small or of large volume ; or it may 
be temporary or permanent, accord- 
ing as the supply is variable or 
constant. 

Causes of Springs. — Springs are 
common on hill slopes and in val- 
leys because the valley table is 
being lowered by the passage of 
underground water from higher to lower levels. A multitude of 
springs and less concentrated seepage supply streams with water 
as a result of this movement and outflow. During and immediately 
after wet spells many springs develop, which run dry during periods 
of drought. In the spring, when the water table is normally high, 
there are numerous, wet, boggy places on hill slopes which may quite 
dry up during the drier period of summer ; but there are many springs 
whose supply is constantly maintained, coming, as it does, from deeper 
sources. Such springs are commonly cold, even in the midst of sum- 
mer, for their water comes from deep enough to be beyond the influ- 
ence of the annual temperature change. 

Among the causes for so concentrating water as to lead to the devel- 
opment of springs, one of the 
most common is variation in 
porosity of layers. When, for 
example, water falls upon a 
porous layer beneath which 
is a more impervious one, it 
tends to flow along the junc- 
tion of the two layers, pro- 
vided either that the slope of 
the junction is downward or 
the pressure of the water table 
gives a sufficient head to force 
it along. If this junction of the two layers extends to the surface at a 
lower level than the entrance of the water, it will flow out at that point, 
very often as a spring or a series of springs. The junction of glacial 
drift with rock on a valley side is often the site of springs (Fig. 45). 




Fig. 46. 



A spring determined in position by a 
fault plane. 



THE WORK OF UNDERGROUND WATER 



81 



Joint planes and bedding planes in rocks frequently serve as the 
guiding paths of sufficient quantities of underground water to give 
rise to springs. Fault planes extending deep into the earth (Fig. 46) 
are paths along which underground water passes, rising to the surface 
when under sufficient head. The sites of faults are not uncommonly 
marked by the issue of copious springs, arranged in a line along the 
fault plane. Still another cause for springs is the issue of underground 
streams in limestone countries. Such springs are often of large 
volume, being veritable streams even at the point of issue. 



Artesian Wells 

Necessity of Deep Wells. — Springs can occur only where a zone 
of ready percolation extends to the surface ; but there are many 
such zones underground which do not rise to the surface. If, however, 
these are reached by a boring, the water will rise in the boring, and, 
if the pressure is sufficient, will flow out at the surface, or even rise 
like a fountain fifty feet or more in the air. Such a boring in which 
water rises is called an artesian well. By some usage an artesian well 
is one in which the water actually flows out at the surface, and this 
was the original meaning of the term ; but since there are all grada- 
tions from those that outflow to those in which the water rises only 
part way to the surface, the term artesian well is coming to be used 
for any bored well in which the water rises from an underground source. 
Where it does not actually outflow it is commonly pumped to the 
surface by windmills or gasoline engines. 

Artesian Water in Synclines. — The name artesian comes from the 
province of Artois in France, where such wells have long been known, 
but artesian wells are now 
widely distributed. In Artois 
the wells occur in a valley, 
beneath which the strata ex- 
tend, while rising on the valley 
sides. Such a system of down- 
folded strata is called a syn- 
cline. The water, entering the 
rock layers on those enclosing hills, sinks down into them, passing 
most easily along a porous layer and being prevented from rising in 
the valley bottom by an overlying bed of more impervious rock, I. 
The porous layer is, therefore, filled with water under considerable 
pressure. When tapped by a boring, the pressure drives the water 
up and causes it to outflow (Fig. 47) ; but the water of the artesian 
well does not rise quite as high as the head which supplies the 
pressure, because the effect of the pressure is partly lost by friction 
in passage through the rock pores and crevices. 

Artesian Water in Monoclines. — Such an arrangement of rock 
layers as that described above is of course neither common nor wide- 




Fig. 47. — Artesian well at A in a syncline. 



82 



COLLEGE PHYSIOGRAPHY 



spread ; but it has been found that even a single inclination of rock 
layers, a monocline, will give rise to conditions favouring artesian wells, 
and this is far more common. The necessary conditions are (a) a 
porous rock layer, outcropping in a region of rainfall, and dipping 
into the earth (Fig. 48), (b) an overlying bed sufficiently impervious 
to prevent the ready escape of the water in the porous bed, (c) no 
ready downward escape for the water, which means either saturated 
or impervious rock below. In such a set of conditions the water 
slowly percolates along the porous bed under the pressure of the column 
of water which fills the bed. If tapped by a well at a point lower than 
the point of entrance of the water, an artesian well will be formed in 
which the water rises to the height which the pressure determines. 




Fig. 48. — Artesian wells in a monocline and on a sand bar. 



Artesian Wells of United States. — Artesian wells are found in 
many parts of the United States. Some of them are very shallow 
wells in the deposits of glacial drift, obtaining their water from depths 
of from fifty to two or three hundred feet, from layers of sand. Such 
wells are usually of small volume, their water source is local, and if 
many wells are bored to it the water may be exhausted. Other wells 
go into the bed rock to depths commonly of from 50 to 1000 feet, 
and in some instances to depths of 4000 feet. 

There are certain regions where the conditions are favourable to the 
development of artesian wells over wide areas, where an extensive 
sheet of porous rock dips gently beneath the surface, receiving a large 
supply of water from a broad outcrop area. One of the most exten- 
sive areas is along the plains which skirt the Atlantic and Gulf coast 
south of New York, where there are great numbers of artesian wells. 
Some of these wells, as at Atlantic City (Fig. 48), go far below the 
sea level and bring up fresh water from the deep lying porous bed. 
There is another extensive artesian area in the Upper Mississippi 
valley, and another in the Great Plains of South Dakota, Nebraska, 
and Kansas, where the water supply enters a porous sandstone that 
outcrops farther west. 

Uses of Artesian Water. — Artesian wells, from some of which the 
water must be pumped, are an important source of water supply for 
municipal purposes, mainly for small towns and cities, though furnish- 
ing a partial supply for even large cities. Such wells are also used for 
factories and for homes ; and in the arid western United States some 
use is made of artesian water in irrigation. Even in the best artesian 
well areas there is a limit to the available supply, and therefore too 



THE WORK OF UNDERGROUND WATER 83 

many wells within a restricted area may lead to partial exhaustion 
of all. The water, though often charged with mineral matter in 
solution, is pure and usually excellent for drinking purposes; but 
the limited supply makes it impossible to utilize this source of pure 
water in large cities. 

Mineral Springs 

The Mineral Load of Water. — In the study of weathering it was 
pointed out that water percolating into the earth performs chemical 
work of solution and alteration of the rocks. It follows, therefore, 
that where such water rises to the surface again it will bring with it 
some of these soluble materials. As a matter of fact, all water es- 
caping from under ground is more or less charged with mineral matter 
in solution, although it entered the ground as pure rain-water. In 
the majority of cases the mineral in solution is in such small quantities 
that it would ordinarily escape detection ; but very often it produces 
such noticeable results that the fact of the presence of mineral has 
led to the common term of mineral springs. There are a multitude 
of different substances known in such springs, but here only a very 
few will be mentioned. 

Hard Water. — The " hardness " of water, so well known in con- 
trast to the " softness " of rain-water, is the result of the presence of 
carbonate of lime or magnesian carbonate of lime, or gypsum, or other 
mineral salts in solution in sufficient quantities to decompose soap 
and form insoluble compounds with its fatty acids, which settle as a 
whitish precipitate. Where carbonate of lime is even more abundant, 
it may be precipitated around the spring, forming a calcareous de- 
posit, this precipitation often being induced or aided by the growth 
of plants. Very extensive deposits of calcareous tufa are sometimes 
made around springs in limestone regions. Other springs bring to the 
air so much iron in solution that iron deposits are made around them. 

Medicinal Waters. — Some springs are sour and acid ; some alka- 
line ; there are sulphurous springs, brine springs, and springs charged 
with carbon dioxide, like a natural soda water ; and some of the springs 
are hot, others cold, even some down nearly to the freezing point. 
Many of these spring waters have medicinal properties, and hotels, 
sanitariums, and baths for invalids-and others who seek their beneficial 
properties are built where they outflow ; while the waters of others, 
like Vichy and Apollinaris, are bottled and sent to all parts of the 
world. Many of the best-known medicinal and mineral spring waters 
are found either in regions of recent volcanic activity or in regions 
of faulting, which gives opportunity for deep-seated waters to rise to 
the surface. 

Hot Springs 

Cause of Hot Springs. — Although the water of most springs is 
cool, having the temperature of the ground below the influence of 



84 COLLEGE PHYSIOGRAPHY t 

annual temperature change, there are many that are warm or hot, 
and some in which the temperature is at the boiling point. Such 
springs are most common in volcanic regions, and the source of heat 
is, with little doubt, to be ascribed to volcanic sources. Even in 
regions where the volcanic activity has died out, intruded lava doubt- 
less still exists beneath the surface, and, thus blanketed by overlying 
rock, it may require thousands of years before such heated rock will 
completely cool. Other possible sources of heat are (a) chemical 
change, (b) friction, as rocks are moved and ground against one 
another, (c) heat possibly inherent deep below the surface, and (d) 
radio-activity. 

In individual cases it is not usually possible to determine the exact 
source of the heat, and it becomes especially difficult in regions remote 
from recent volcanic activity. However, in such cases, the heated 
waters usually arise along fault planes, which may lead deep down into 
the earth and thus give opportunity for water to arise from the depths. 
Among medicinal hot springs are those of Carlsbad, Saratoga, and 
Hot Springs, Ark., the latter said to be salutary because of their 
radio-activity, although this last has been questioned. 

Hot Spring Deposits. — Heated water has a higher solvent power 
than cold water, and there are usually gases and solutions in such 
water that greatly aid it in its power of altering and dissolving mineral 
substances. Accordingly, the water of hot springs often brings a 
large quantity of dissolved mineral matter to the surface, and deposits 
about such springs are often extensive. This is illustrated around the 
hot springs of New Zealand and the Yellowstone National Park, 
where successive terraces are built up around the hot springs, forming 
extensive deposits of carbonate of lime or calcareous tufa (Fig. 49). 
Here a part of the colour for which these terraces are famous is due to 
the presence of minute plants, or algce, which grow in the hot water, 
and aid in the deposit of the mineral. The amount of dissolved min- 
eral matter may be inferred from the fact that the Excelsior cauldron 
in Yellowstone Park pours out 4400 gallons of boiling water per 
minute. 

Formation of Veins. — In hot spring waters are a great variety of 
mineral substances, even including salts of metallic substances. It 
is quite certain that mineral veins are even now being deposited by 
these hot waters along the cracks through which they are rising to the 
surface. Certainly in the past, the ascent of heated waters along fault 
planes and other zones of percolation has been responsible for the 
deposit of mineral veins at present being worked as a source of precious 
and other metals. As the water rises and cools, or as it loses some of 
its gases, or as solutions of various kinds are mingled, or by chemical 
change in the minerals of the enclosing rock, or for other reasons, some 
of the load of dissolved substances is deposited by the hot water in 
the passage through which it is rising, giving rise to veins. If among 
these deposits metallic salts are included, the deposit becomes a min- 



THE WORK OF UNDERGROUND WATER 



85 



eral vein. Not all mineral veins are of this origin, though many, 
including some of those of gold, silver, and copper, evidently are. 

Relation of Ore Deposits to Mountains and Volcanoes. — Since 
crevices are needed for a free flow of heated underground water, 
mineral veins deposited by such water are most commonly found in 
mountain regions where the rocks have been subjected to such strains 
as to lead to breaking ; and since heat is needed for the most efficient 
work of water in mineral vein formation, mineral veins are naturally 
most common in regions of former volcanic activity. It is 
in mountain regions, or regions of fracturing of the crust, that 




Fig. 49. — Terrace advancing over trees at Mammoth Hot Spring 
(Jackson, U. S. Geol. Survey.) 



Yellowstone Park. 



volcanoes develop, and, therefore, the two necessary conditions for 
the development of mineral veins occur commonly together. These 
facts have had local influence in determining the distribution of mining 
of the precious metals and others. 



Geysers 

Localities. — In a few localities, notably in Iceland, New Zealand, 
and the Yellowstone National Park (Fig. 50), certain hot springs 
have the habit of intermittent eruption, and are known as geysers. 
It is chiefly because of the large number and variety of these interest- 
ing phenomena in the last-named locality that an extensive tract of 
land has been set aside by Congress as a National Park, which is 
annually visited by thousands of people from all parts of the world, 
for this is with little doubt the most wonderful of all the geyser regions. 



86 COLLEGE PHYSIOGRAPHY 

This is a region of former volcanic activity, and the outflow of heated 
water seems to be one of the dying phases of the vulcanism. 

Geyser Basins. — In parts of the Park, notably within a few limited 
areas called " geyser basins," hot water issues from numerous 
vents, only a few of which, however, have the habit of intermittent 
eruption. Thus, while there are over 3000 known hot springs in 
the Yellowstone Park, there are only about 100 geysers. The hot 
springs vary greatly, some having but moderate flow and with tem- 
perature not very high, while others are at the boiling point, and 
some pour forth steady streams of hot water. In some of the hot 
springs steam bubbles rise from below, causing violent boiling at the 
surface, and even small explosions which, on a small scale, simulate 
geyser eruption. 

Geyser Eruptions. — Among the geysers themselves there are 
also notable differences. In some the column of steam and heated 
water thrown up during the eruption is small, rises to a height of but 
a few feet, and the eruption is over in a very few minutes; while 
in others, a vast quantity of hot water and steam is expelled, during 
a period of an hour or more, and rising to a height of over 200 feet 
(Fig. 50) ; and between these extremes there is almost every grada- 
tion. In some the period of eruption is so regular that the time of its 
occurrence may be accurately predicted, while in others the eruptive 
period is irregular ; and the interval between eruptions varies among 
different geysers from an hour or less to weeks and even months. 
Some of the geysers have erupted with regularity ever since the region 
was first discovered ; others have become less regular, or have be- 
come extinct, while, on the other hand, some new geysers have come 
into existence. 

Old Faithful. — Some of the best known of the Yellowstone Park 
geysers are the Old Faithful, Giant, Giantess, Castle, Beehive, Minute 
Man, and Lone Star, each erupting independently of the others and 
each with distinctive characteristics of its own. Among them one of 
the most remarkable is Old Faithful, which erupts with an interval 
of about an hour, sending a column of water and steam more than 
100 feet into the air for a period of five or six minutes. Then it re- 
lapses into quiet for nearly an hour during which one may sit on the 
edge of its crater and look into the pool of heated water with perfect 
safety. At the end of the appropriate interval there comes another 
great rush of steam and water, thrown into the air with a roar. It 
sends out 3000 barrels of water with each eruption. For at least half 
a century this process has been repeated about 8000 times a year. 

Geyser Deposits. — The heated geyser waters bear to the surface 
a variety of mineral substances in solution, among which the most 
important is silica, which is precipitated fn a loose porous deposit 
called silicious sinter. This deposit is most extensive in the immedi- 
ate neighbourhood of the orifice ; and consequently around most of 
the geysers a cone has been built, on the crest of which is a depres- 



THE WORK OF UNDERGROUND WATER 



87 



sion, or crater, in which lies a pool of hot water and from which 
the eruptions occur. During and immediately after an eruption the 
surface of the cone is wet with the flood of hot water, and by each 
eruption a small addition is made to the cone. Similar deposits are 
being built around 
many of the hot 
springs. 

These silicious de- 
posits usually have a 
varied form, due to 
irregularities of dep- 
osition and to the 
concretionary ten- 
dency, as a result 
of which spherical 
forms tend to grow 
as the silica is pre- 
cipitated out of the 
hot water. There is 
also much beautiful 
colour, partly from 
the effect of light in 
the clear hot mineral 
water, partly from 
the influence of mi- 
nute plants which 
live in the heated 
water and aid in the 
precipitation of the 
silica. When the sup- 
ply of silica ceases, 
through the closing 
up of an orifice, the 
silicious sinter loses 
its colour, and crum- 
bles under weather- 
ing to a chalky white 
powder. Here and 
there an extinct gey- 
ser cone is to be 
seen, offering a strik- 
ing contrast to the highly-coloured cones of certain of the active 
geysers. 

Cause of Eruptions. — While it is possible that more than one cause 
may operate to produce geyser eruptions, there is a single explanation 
that seems capable of explaining the phenomenon, and even of ac- 
counting for the different forms of eruption. This explanation as- 




Fig. 50. 



An eruption of Old Faithful geyser in Yellowstone 
Park. 



COLLEGE PHYSIOGRAPHY 



sumes, first, that there is a fairly long, narrow, irregular orifice ex- 
tending into the earth, that underground water finds its way into this 
orifice in fairly steady volume, and that it is heated in some part of 
its course. Under normal conditions this heated water will outflow 
as a hot spring, or, if the temperature is high enough, as a boiling 
spring. But among a multitude of such orifices some are so narrow and 
irregular, and the supply of heat is so great, that boiling is interfered 
with, and locally steam is generated down in the geyser tube, and 
the expansion of this steam lifts the column of water above and throws 
it into the air. This theory is supported by the fact that miniature 

geyser eruptions may,be pro- 
duced by applying heat to 
water in a long tube, imitat- 
ing the assumed conditions 
in the natural geyser. 

The accompanying dia- 
gram (Fig. 51) is intended to 
illustrate the theory of gey- 
ser formation. The vertical 
lines represent temperatures, 
the horizontal lines depths 
in the earth. At sea level, 
with a pressure of one at- 
mosphere, the boiling point 
of water is 212 F. Under 
two atmospheres the press- 
ure at sea level with a col- 
umn of water of 33.3 feet is 
250 ; under three atmos- 
pheres, or with a column of 
66.6 feet of water, it is 275 ; 
and under four atmospheres, with a column of water of 100 feet, the 
boiling point is 293 . Thus at a depth of 100 feet in a geyser tube, 
at sea level, a temperature of 293 is required to boil water. 

The boiling point curve of water down to a depth of 100 feet is 
shown by the line ed in Fig. 51. The line af is intended to represent 
the actual temperature of water in a geyser tube after an eruption, 
and while the water is nowhere hot enough to boil at that depth, it is, 
nevertheless, well above the boiling point of water at the surface and 
throughout most of the depth. About midway down a supply of heat 
is assumed, which raises the temperature of the water through a given 
area hig, and actually reaches the boiling point at i. 

In a broad, open tube, as soon as the boiling point is approached 
there will be convection, the heated water rising, and raising the 
temperature of the water column above ; but in the geyser tube 
convection is interfered with by the narrowness and irregularity. 
Therefore, locally, steam is formed in the tube. This lifts the column 



SURFACE 

1 atmos. a 



2 ATMOS. 
83.8 FT. b 



8 ATMOS. 
66. 6 FT. c 



4 ATMOS. 

^00 ft. a 

Fig. 51.- 



b °o c o °o 
0) t- m co 2 « 

CM CM CM CM 6cM 2 


• 




/ ./q 






/ 


y 
/n 


SUP 


>LY OF h 


EAT 


/ 




PRESSURE IN 
ATMOSPHERES 


BOILING 
POINT 


1 ATMOS. (SURFACE) 


212° 


2 ATMOS. (33.3 FT.! 


250° 


3 ATMOS. (66. 6 FT.) 


275° 


4 ATMOS. (100 FT.) 


293° 



Diagram to show temperature and press- 
ure conditions in a geyser. 



THE WORK OF UNDERGROUND WATER 89 

of water above, and as the pressure is relieved the boiling point at 
that depth becomes lower than the water temperature and more 
steam forms, giving rise to a powerful force which is able to throw 
the water above into the air, accompanied and followed by steam. If a 
stone or sod is dropped into the geyser tube, or if the water is soaped, 
making it less liquid, an eruption is made to come more quickly than 
normal, because convection is still further interfered with. 

This theory for geyser eruptions accords so well with the known 
facts, and so perfectly explains the phenomena, that it seems well 
established. It is not difficult to believe that differences in period 
of eruption and in mode of eruption among geysers may be due to 
variations in the form of the geyser tubes and to the amount and 
depth of the heat supply. It is a noteworthy fact that, although in 
each of the geyser basins there are a number of geysers, — often several 
are close together, — there is no evident sympathy between them. 
This fact indicates clearly that the phenomena of eruption are the 
outcome of conditions associated with the individual geyser. 

That the geysers are associated in groups, or basins, indicates a 
certain relationship as to cause, however. Throughout each basin, for 
example, there is probably the same general source of water and of 
heat. This we naturally assume to be the still uncooled, intruded 
lava, since geysers are found in regions of recent or present volcanic 
activity. One of the most peculiar facts is the limited number of 
geyser areas in the world and yet the presence of about a hundred in a 
single region. This may be due to some peculiarly favourable condi- 
tion of heat supply, or it may be the result of some peculiar condition 
giving rise to the form of tube necessary for geyser eruption. The 
latter seems the more probable, and there is some reason for believing 
that the development of the geyser tube is a result of deposit of 
mineral matter under favourable conditions. As silica is deposited 
on the walls of the tube a hot spring may be transformed to a geyser, 
and this, in turn, may ultimately become so clogged that eruption 
ceases, and even the outflow of water is cut off, leading to the extinc- 
tion of the geyser. 

The Formation of Caverns 

Solution Underground. — All underground water is engaged in the 
solution of mineral matter as it percolates through the soil, subsoil, 
or rock ; but the amount of solution varies greatly, according to the 
minerals which it encounters. In an extreme case, such as salt, the 
mineral is so readily dissolved that a common way of obtaining salt 
from layers deep below the surface is to let fresh water down to the 
salt, where it becomes a brine which is pumped to the surface and 
evaporated to procure the dissolved salt. Percolating water also 
dissolves salt from such layers, and this is a cause for salt springs, like 
those at Syracuse, N.Y., which early gave rise to a great salt-producing 



9° 



COLLEGE PHYSIOGRAPHY 



industry there. As the salt is dissolved, the surface of the ground 
settles irregularly, giving rise to local depressions in which ponds 
often develop. 

Other rocks, more common than salt beds, are also subjected to 
extensive solution, notably of gypsum and limestone. The latter is 
the most widespread of readily soluble rocks, being in fact one of the 
most common rocks of the earth's crust ; and the solution phenomena 
associated with the activity of underground water in limestone are, 
therefore, widespread. They produce notable results both under- 
ground and at the surface. 




FxG. 52. — Map showing sink holes in Florida. (Williston Quadrangle, U. S. Geol. survey.) 

The Dissolving of Caves in Limestone. — In its entrance into lime- 
stone rocks water behaves as all percolating water does, following 
both the microscopic crevices and the larger cracks. Pure rain-water 
can accomplish little solution of limestone, but water containing carbon 
dioxide dissolves it with comparative ease. Consequently the crevices 
and cracks through which the water enters are enlarged by solution. 
As the process continues, a labyrinth of cavities is developed under- 
ground, often attaining such large size as to deserve the name cave or 
cavern, and extending for miles. 

Sink Holes. — As a result of solution, the limestone is so honey- 
combed with cavities that water readily sinks into it, and during a 
rain the water disappears into the ground instead of running off in a 
multitude of rain-born rills. As the limestone is dissolved, the sur- 



THE WORK OF UNDERGROUND WATER 91 

face of the ground slowly settles, but it settles irregularly, and some- 
times by the collapse of caverns. The topography, therefore, con- 
sists of a series of swells and hollows, the latter normally being roughly 
circular or elliptical basins. Toward these basins, called sinks (Fig. 
52), the surface drainage flows, sinking beneath ground at the centre, 
which is sometimes an open cavity or pit, called a sink hole. At 
times, by the collapse of the walls, or by other means, these holes are 
closed, and then the surface drainage gathers in the basin to form a 
pond. 

Underground Drainage. — The water that enters the sink holes, 
and that percolates into the cracks of the limestone, rinds its way 
readily downward to a level that is determined either by the presence 
of an impervious layer, or by the influence of the surface rivers or 
other water to which the percolating water is tributary. From the 
cavern, drainage ultimately comes to the surface again, often as trib- 
utaries to large streams which flow even in a limestone country. 



Fig. 53. — Relations of caverns at two levels to sink holes above, springs on a valley side, 
and a natural bridge. (After Shaler.) 

These subterranean tributaries issue as springs in the river valleys 
and are often of large size. They are, on a much larger scale, analo- 
gous to the springs and seepage which contribute water to rivers in 
other regions, as underground water percolates from higher to lower 
water table areas. In limestone regions these springs are larger than 
in other regions, because the rock has been made more porous by solu- 
tion and the underground water gathers in subterranean courses, 
which are at times of such size as to warrant the name underground 
rivers. 

The level to which the underground tributary can work in cavern 
development is limited by the surface stream into which it flows, 
because its water must maintain a slope down to the surface stream. 
Consequently, the lowest part of the cavern is that nearest the sur- 
face stream, and its bottom will normally be nearly at the same level 
as the surface stream ; and the cavern bottom becomes higher away 
from the outlet. If the surface stream is engaged in lowering its 
valley, the cavern outlet must either be lowered also, or be abandoned, 
for the water can then more freely pass into the limestone beneath 
the cavern, and, in time, so enlarge cavities as to drain the water at 
a lower level. Thus it happens in limestone regions traversed by 
gorges which are still being deepened, that a series of cavern outlets 
are seen on the gorge walls (Fig. 53), from only the lower of which 



9 2 



COLLEGE PHYSIOGRAPHY 



a spring emerges, the upper ones being former outlets. And in caverns 
one often passes from one level to another, but only the lower levels 
are occupied by running water while the floors of the upper ones are 
often quite dry, because the water readily escapes to lower levels 
through crevices dissolved in the limestone. 

The drainage of a limestone country is, therefore, of a peculiar 
type. There may be some streams of good size, as in other regions, 

but the tributaries 
are springs, rather 
than small surface 
streams, and most 
of the surface 
drainage finds its 
way into the 
ground, then, af- 
ter a journey of 
greater or less 
length, emerges 
near the large 
streams, or lakes, 
or sea. In such 
a country one 
sees few surface 
streams; and these 
are chiefly large 
ones ; and instead 
of a succession of 
linear valleys there 
is an "undulating 
topography with 
numerous conical 
" knobs " and cir- 
cular sinks. Such 
a karst topography 
is seen in southern 




Fig. 54- 



Map of part of the Mammoth Cave. 
Hovey.) 



(After H. C. 



Indiana and Kentucky in the United States, in southern France, 
in Austria along the eastern side of the Adriatic Sea, and in many 
other places. Here and there in limestone regions surface valleys 
abruptly terminate and the streams in these plunge into the ground, 
emerging at another point, often miles from the place of disappear- 
ance. 

Cavern Systems. — The extent to which a system of caverns is 
excavated depends, in large part, upon the thickness of limestone and 
its extent. It is largely because of thick beds, lying in nearly hori- 
zontal position, that such an extensive labyrinth of caverns has been 
possible in Kentucky and southern Indiana, while in Virginia, where 
the limestone beds are thinner and tilted, the caverns are far more 



THE WORK OF UNDERGROUND WATER 93 

limited in extent. In Kentucky the limestone covers an area of about 
8000 square miles, with an average depth of about 175 feet, and it 
has been estimated that there are at least 100,000 miles of cavern 
tunnels in it. One may travel on the surface, it is said, as much as 
50 miles in certain directions without encountering running water, 
but there may be as many as 100 sink holes on a square mile of 
surface. 

Mammoth Cave and Other Caverns. — It is in this limestone area 
that Mammoth Cave, the greatest in the world, is situated; but in 
the same country there are said to be 500 other caverns. The num- 
ber of the tunnels and their extent are unknown, for they have never 
been fully explored, but there are certainly one or two hundred miles 
of connected galleries in the Mammoth Cave system (Fig. 54) . These 
galleries are often winding ; their height varies from a foot or two to 
over a hundred feet, and their width is similarly variable. There is 
often one gallery above another, and it is possible at points to go from 
one level to another. Here and there the cavern expands greatly, 
forming what is locally called a " dome," of which the highest is the 
Mammoth Dome, about 400 feet long, 150 feet wide, and from 80 to 
250 feet high. It is so large that a good sized church, steeple and 
all, could be built in it. 

Limited Value of Caverns. — Similar caverns are known in many 
regions ; in fact, they are commonly associated with limestone strata 
in all parts of the earth. Among the thousands known, most are of 
small size, and are of little more than local interest; but some are 
widely known and much resorted to by visitors. Among those best 
known in the United States besides the Mammoth Cave of. Kentucky 
are the Wyandotte Cave in Indiana, the Luray Cavern in Virginia, 
and Howe's Cave in New York. They have little direct value to 
man, though an attempt was once made to utilize the Mammoth 
Cave as a health resort for people with lung diseases, the idea being 
that the uniform temperature of the caverns (53°-56° F.) would be 
healthful ; but the attempt was a failure. In the early history of the 
human race, caves served as the home of primitive man, and many 
of our best records of this early state have been recovered from cave 
deposits. 

Cavern Deposits. — As a centre of scenic interest caverns attract 
large numbers of visitors and are, therefore, a resource of local im- 
portance. The interest is partly due to the marvellous labyrinth of 
natural underground chambers, and partly to the unique ornamenta- 
tion of these chambers by deposits of carbonate of lime brought into 
them by percolating water. As the water oozes from crevices in the 
cave roof it bears in solution carbonate of lime, dissolved in its pas- 
sage through the rock. By evaporation, oxidation, loss of gases, or 
change in pressure the water is forced to give up some of its dissolved 
load, and a pendant, icicle-like deposit slowly grows on the cave roof, 
forming a stalactite (Fig. 55). As the water drips to the cave floor 



94 



COLLEGE PHYSIOGRAPHY 



a similar deposit, called a stalagmite, is built upward ; and if the two 
deposits grow until they meet, a column is formed. 

These deposits are 
wonderfully varied in 
form as the water trickles 
over them and builds 
them up irregularly, giv- 
ing rise to fantastic 
shapes. There is also 
variation in colour as the 
nature of the deposit 
varies, or some mineral 
impurity is introduced. 
Some of the caverns, like 
Luray, are marvellously 
beautiful in their orna- 
mentation. The most 
highly ornamented cav- 
erns are those that have 
been formed longest, such 
as the upper galleries into which water has long been percolating 
from the roof. 

Enlargement of Caves. — In the lower galleries of a cavern system 
the drainage often gathers such volume as to form a broad, deep stream 
underground, like the so-called river Styx of Mammoth Cave, on which 
boats may go. Doubtless in places the running water deepens the 




55. — Stalactites, stalagmites, and columns in 
Kentucky cavern. 




Fig. 56. — Natural Bridge, Virginia. (Walcott, U. S. Geol. Survey.) 



THE WORK OF UNDERGROUND WATER 



95 



cavern by mechanical erosion, but most of the cavern excavation is 
the result of solution. There is a little cooperation of the atmos- 
pheric agents of weathering in enlarging the caverns ; but in the still 
air and uniform temperature, and with the general absence of life in 
caverns, such action is of minor importance compared with solution. 
Now and then parts of the roof and wall are given such instability that 




Fig. 57. — Modes of origin of natural bridges. (Cleland.) 

they fall to the cavern floor, and during such falls earth tremors, or 
slight earthquake shocks, extend through the neighbouring region. 
Masses of rock that have so fallen may be seen here and there in 
caverns. 

Natural Bridges 

Natural Bridge of Virginia. — In limestone countries, arches across 
valleys, called natural bridges, are sometimes formed like the Natural 



96 COLLEGE PHYSIOGRAPHY 

Bridge of Virginia (Fig. 56). The partial falling in of a cavern roof 
may leave such a natural arch, though few cases of this are known to 
exist. A more common cause for such arches is the disappearance of 
a surface stream into the joint planes of a limestone rock and its reap- 
pearance below. In time the water will excavate a valley beneath 
the arch, which then extends across the valley as a span or bridge. 
This is the origin of the Natural Bridge of Virginia, and of many other 
lesser examples. 

Other Causes of Natural Bridges. — It has recently been shown 
that there are numerous other ways in which natural bridges may 
be formed, as by the outflow of lava from beneath a solidified crust, 
by irregular wave erosion on lake and sea coasts, and by stream erosion. 
The remarkable natural bridges of Utah, in sandstone, one of which 
has a span of 275 feet, and stands 308 feet above the valley bottom, 
is the largest known natural bridge in the world. It is due to the 
lateral swinging of a stream against a cliff from two sides, undercut- 
ting it until a hole has been cut through into which the river then 
passed, thus forming a natural bridge (Fig. 57). 

Other Chemical and Mechanical Work of Underground 

Water 

A Prime Factor in Weathering, —r In the discussion of weathering, 
a phase of the work of underground water was considered in its rela- 
tion to rock crumbling. This phase involved solution, decay, and 
the mechanical action of frost, and it was separated from the other 
phases of underground water work because it was a part of a complex 
process whose combined result was that of rock disintegration. 

Deposition of Mineral Matter. — Much of the dissolved mineral 
matter derived by the underground water in its percolation is, as we 
have seen, brought to the surface and there either precipitated in 
deposits of mineral matter near the place of outflow, or contributed 
to the surface streams for transportation. But a part of the mineral 
load of percolating water is precipitated within the rock itself. Be- 
cause of oxidation, or loss of gases, or chemical change, or other causes 
the water may be forced to give up some of its dissolved load, even 
though it has only just obtained it. 

Cementation. — Such deposit naturally occurs most readily in 
the cavities of the rock, and this is the main reason why sediments 
are changed from the loose unconsolidated state to the condition of 
solid rock. The process is often to be seen in gravel banks where 
carbonate of lime or iron oxide has been deposited in sufficient quanti- 
ties to cement the pebbles together. In Bermuda and in Florida 
loose coral and shell sand is quickly transformed to solid rock by de- 
posit of carbonate of lime that the water has dissolved from some grains 
to deposit between others near by. It is by this process that sands 
are changed to sandstone, gravels to conglomerate, and shell and 



THE WORK OF UNDERGROUND WATER 97 

coral deposits to limestone. When buried deep in the earth and espe- 
cially when subjected to the percolation of heated waters, rocks may 
become so firmly cemented that most of the pores are rilled. Along 
larger cavities, such as joint planes and fault planes, veins of deposited 
mineral are found. 

Replacement and Petrifaction. — The underground water enters 
even the very interior of rocks and causes many changes by solution 
and deposit. One form of such action is the replacement of one mineral 
substance by another, as, for example, the replacement of the carbonate 
of lime in a fossil by sulphide of iron, or by silica. This is called 
petrifaction. One phase of petrifaction is the replacement of the 
woody tissue of a tree or other plant by silica, when exposed to the 
percolation of silica-bearing water. This gives rise to petrified wood, 
in which, though the original structure is perfectly preserved, the 
material is no longer wood, but silica. In Arizona and other parts 
of western United States there are places where there are numerous 
entire tree trunks thus changed to stone, the so-called petrified forests. 

Formation of Ore Deposits. — One of the most important results 
of the deposit of mineral substances by underground water is the 
formation of mineral veins. Some, as has been pointed out (p. 84), 
have been formed by the rise of heated water ; but other deposits, 
notably iron, have apparently, been made by descending waters, 
probably not heated. These have been formed by the removal of 
iron from soil and rock during the process of weathering and under- 
ground water percolation, and the concentrated deposit of some of 
the metal in favourable situations in the rock, or the removal of silica 
from a sedimentary rock made chiefly of iron and silica, leaving the 
iron as a very valuable ore deposit. This is thought to be the source 
of the great iron deposits of the Lake Superior region, upon which 
so much of the industrial development of the United States depends. 
Other mineral deposits are of similar origin. 

Mechanical Work of Underground Water. — Underground water, 
except by freezing, is not a powerful mechanical agent. Yet the 
subject would not have received complete treatment, if it were not 
recognized that underground water, by dissolving here and there, or 
by giving rise to planes of slipping, is responsible for much slipping 
of rock and soil on slopes. Weathering and other phases of under- 
ground water work are responsible for the development of conditions 
of such instability that, under the pull of gravity, large and small 
masses move from higher to lower levels. While this process oper- 
ates in a minute, invisible way, it also occasionally finds expression in 
a striking manner, when huge masses are abruptly dislodged and 
avalanched from steep slopes. Many landslides, and probably the 
great majority, are the result of the work of underground water, oper- 
ating for long periods of time in preparation for the great final down- 
fall, and, of course, cooperating with weathering and with gravity. 



9 8 COLLEGE PHYSIOGRAPHY 



References 'to Literature 

W. S. Blatchley. Indiana Caves and their Fauna, Indiana Dept. Geol. and 

Nat. Resources, 21st Ann. Rept. 1896, pp. 121-212. 
F. Carney. Springs as a Geographic Influence in Humid Climates, Pop. Sci. 

Monthly, Vol. 72, 1908, pp. 503-511. 
T. C. Chamberlin. The Requisite and Qualifying Conditions of Artesian 

Wells, 5th Ann. Rept. U. S. Geol. Survey, 1885, pp. 125-173. 
H. F. Cleland. North American Natural Bpidges, Bull. Geol. Soc. Amer., 

Vol. 21, 1910, pp. 313-338 ; Pop. Sci. Monthly, Vol. 78, 191 1, pp. 415-427. 
L. J. Cole. The Caverns and People of Northern Yucatan, Bull. Amer. Geog. 

Soc, Vol. 42, 1910, pp. 321-336. 

B. Cummings. The Great Natural Bridges of Utah, Nat. Geog. Mag., Vol. 

21, 1910, pp. 157-167. 
N. H. Darton. Geology and Underground Water Resources of the Central 

Great Plains, Prof. Paper 32, U. S. Geol. Survey, 1905, pp. 190-372. 
W. M. Davis. An Excursion in Bosnia, Hercegovina, and Dalmatia, Bull. 

Geog. Soc. Philadelphia, Vol. 3, 1901, pp. 21-50. 
M. L. Fuller. Summary of the Controlling Factors of Artesian Flow, Bull. 

319, U. S. Geol. Survey, 1908, pp. 1-44; Underground Waters of Eastern 

United States, Water Supply Paper 114, U. S. Geol. Survey, 1905, pp. 

1-285. 
J. C. Graham. Some Experiments with an Artificial Geyser, Amer. Journ. 

Sci., Vol. 145, 1903, pp. 54-60. 

F. A. Grooch and J. E. Whitfield. Analysis of Water of the Yellowstone 

National Park, Bull. 47, U. S. Geol. Survey, 1888, 84 pp. 

G. B. Hollister. A Curious Salt Pond in Kansas, Journ. Geog., Vol. 2, 1903, 

pp. 155-158. 

W. H. Holmes and A. C. Peale. Yellowstone National Park, Geology, 
Thermal Springs, Topography, 12th Ann. Rept., U. S. Geol. and Geog. Sur- 
vey of the Territories, Part 2, 1883, pp. 1-490, contains review of thermal 
springs and geysers of the world; A. C. Peale, Natural Mineral Waters 
of the United States, 14th Ann. Rept., U. S. Geol. Survey, Part 2, 1894, 
pp. 49-88. 

H. C. Hovey. Celebrated American Caverns, Cincinnati, 1896; H. C. Hovey 
and R. E. McCall, The Mammoth Cave of Kentucky, Louisville, 1897. 

T. A. Jaggar, Jr. Some Conditions Affecting Geyser Eruptions, Amer. 
Journ. Sci., Vol. 155, 1898, pp. 323-333. 

F. H. King. Principles and Conditions of Movements of Groundwater, 19th 
Ann. Rept., U. S. Geol. Survey, Part 2, 1899, pp. 59-294. 

W. von Knebel. Hohlenkunde, mit Berucksichtigung der Karstphanomene, 
Brunswick, 1906, 222 pp. 

W J McGee. The Potable Waters of Eastern United States, 14th Ann. 
Rept., U. S. Geol. Survey, Part 2, 1894, pp. 1-47. 

E. A. Martel. L'Eyolution Souterraine, Paris, 1908, 388 pp. 

Albrecht Penck. Uber das Karstphanomen, Vortrage des Vereines zur 
Verbreitung Naturwissenschaftlicher Kenntnisse in Wien, XLIV Jahrgang, 
Heft 1, 1903, 38 pp. 

N. S. Shaler. Aspects of the Earth, New York, 1889, pp. 98-142. 

C. S. Slichter. Theoretical Investigations of Motion of Ground Water, 19th 

Ann. Rept., U. S. Geol. Survey, Part 2, 1899, pp. 295-384; Water Supply 

Paper 67, U. S. Geol. Survey, 1902, 106 pp. 
J. E. Todd. Geology and Water Resources of a Portion of Southeastern 

South Dakota, Water Supply Paper 34, U. S. Geol. Survey, 1900, 34 pp. 
C. D. Walcott. The Natural Bridge of Virginia, Nat. Geog. Mag., Vol. 5, 

1893, PP- 59-07- 
L. F. Ward. Petrified Forests of Arizona, Smithsonian Instn., Ann. Rept. 

for 1899, pp. 289-307. 



THE WORK OF UNDERGROUND WATER 99 

W. H. Weed. Formation of Travertine and Siliceous Sinter by the Vegeta- 
tion of Hot Springs, 9th Ann. Rept., U. S. Geol. Survey, 1889, pp. 613- 
676; Geysers, U. S. Dept. of the Interior, Washington, 191 2, 32 pp. 



TOPOGRAPHIC MAPS 

Waterloo, 111. Princeton, Ky. Weingarten, Mo. 

Williston, Fla. Citra, Fla. Bloomington, Ind. 

Pikeville Special, Tenn. Williamsport, Md. Natural Bridge Special, Va. 

Administrative map of Yellowstone National Park. Petrified Forest, Ariz. 



CHAPTER V 

RIVERS AND RIVER VALLEYS 

General Considerations 

Nature of Rivers. — A river is a natural drainage line on the land. 
It is the means of disposal of the surplus water which falls as rain or 
snow, and its movement depends upon the pull of gravity, by which the 
liquid water, in being drawn toward the earth's centre, seeks the point 
nearest the centre that is most accessible ; that is, it flows down grade. 
Accordingly rivers will be found on any land surface where rain falls 
and where there is slope sufficient for it to run off. 

Source of River Water. — The water supply of a river comes partly 
from the direct rainfall, from the surface run-off either of rain or melted 
snow or ice, and partly from water that has first entered the ground 
and after a period of activity as underground water has emerged at the 
surface again in springs or by seepage. By a multitude of contri- 
butions from these two sources the river is kept supplied regularly or 
irregularly according to conditions, and is large or small according 
to the length and the volume of water contributed. In general fea- 
tures, and in origin, no distinction can be drawn between the small 
stream and the great river ; yet in individual characteristics there are 
vast differences among rivers. 

Work of Rivers. — Incidental to the run-off of the surplus water 
there are some results of the highest importance. The land is drained, 
and accompanying this drainage is the transportation of a vast quan- 
tity of rock material, some in solution, some in suspension, and some 
dragged along the river bed. By the movement of the water, and 
largely by the use of the transported rock material as scouring tools, 
valleys are cut in the land, along the lowest part of which the water 
runs in a narrow channel, thus concentrating the energy of the run- 
ning water. Finally the larger part of the rock material transported 
by the river ultimately finds a place of deposit. A river is, therefore, 
an agent of removal, not only of water, but of rock waste. Rivers are 
the most potent agencies for the sculpturing of the land surface and 
for the removal of the products of disintegration. 

Rivers and Man. — The relation of rivers to the occupation of the 
land by man is intimate and fundamental in importance. They help 
to make the valleys open routes of travel, often into and even across 
mountain ranges, as well as upon less rugged land surfaces; or at 
times, the valleys are so deep and narrow as to interfere with travel. 



RIVERS AND RIVER VALLEYS 101 

River deposits make fertile, level land and are often the seat of a dense 
agricultural population. The river water is useful for navigation, for 
watering desert lands by irrigation, as a water supply for many pur- 
poses, as a source of water power, as the home of valuable food fish, 
and for other purposes. In these and other ways rivers are closely 
linked with the past history of mankind, and with its present life and 
pursuits. Among the phenomena of physical geography rivers rank 
as one of the most important, and a study of them leads us along a 
number of lines of inquiry, the first of which is the manner in which the 
river performs its tasks. 

Rain Sculpturing 

Run-off. — The simplest lesson in river work is that which can be 
learned from the examination of the process of run-off on a bare slope 
during a heavy rain, a process which can be experimentally imitated 
by turning a spray upon a surface of loose earth. If the surface be 
originally smooth, though with a slope, the water will at first form a 
sheet, some of which sinks into the earth, while a general movement 
starts down the slope. Quickly, however, the sheet will break up 
into a multitude of little rills as the current locally removes some of the 
loose earth, forming depressions toward which more and more water 
drains as they grow deeper. As erosion continues, the tiny rills be- 
come sunk in steep-sided valleys a few inches deep and a few inches 
wide, and from the steep sides the earth slides into the water, broad- 
ening the valleys. 

Rill Work. — Here and there, where a pebble lies in the course of the 
rill, or where it crosses a layer a little harder than the rest, the current 
increases to a rapid, or the water tumbles in a small fall. One rill 
joins another, and the combined current of these two receives addi- 
tions from other rills and steadily grows in size ; and .with increased 
volume the valley becomes broader and deeper. The water that has 
cut these valleys has thereby imposed upon itself the task of trans- 
porting the material thus removed, and it is consequently clouded with 
sediment. If, perchance, the water reaches a more gentle slope, or 
a small pool of standing water, its current is so checked that some or 
all of the sediment load is deposited. 

Relation to River Work. — In miniature, this is an imitation in 
fundamentally important respects of river work in drainage, erosion, 
transportation, and deposition. Larger volumes of water, operating 
through long periods of time, but employing the same methods, have 
excavated similar valleys hundreds of miles in length, scores of miles 
in width, and thousands of feet in depth, even in the hardest rocks. 
Multitudes of such rivers, both large and small, have profoundly 
sculptured the surface of the lands, and removed from them thousands 
of feet of rock during the long ages of the geological past. And the 
rock waste which they have moved has been deposited in other places 
and has contributed the materials out of which new land has been made. 



102 



COLLEGE PHYSIOGRAPHY 



Bad Land Topography 

Humid Bad Lands. — The action of rain wash in sculpturing loose 
earth in the manner described above may commonly be seen in 
ploughed fields, and during heavy rains such fields are deeply gullied 
(Fig. 58). In places the continuation of the process has greatly 
sculptured deposits of clayey nature, giving rise to topographic forms 




Fig. 58. — Gully produced by headwater erosion in California. (Gilbert, U. S. Geol. Survey.) 



called bad lands. Local areas of bad land topography are found in 
humid climates, as in the Austrian Tyrol near Botzen, in the clay 
cliffs of a portion of the southern shore of Lake Ontario west of 
Oswego, and in some of the abandoned plantation land of Louisiana. 
Arid Bad Lands. — Typical bad lands, however, are characteristi- 
cally phenomena of arid climates, and it is here only that they occupy 
extensive areas. There are many such bad land tracts in the arid 
western part of the United States, but they are especially well de- 
veloped and extensive in western Nebraska, North and South Dakota, 
and Wyoming. Here there are scores of square miles in which rain 
sculpturing has so gullied the land that even travel across the surface 



RIVERS AND RIVER VALLEYS 103 

is difficult because of the innumerable, deep, steep-sided gullies, and 
the narrow, intermediate ridges. Since no vegetation can grow 
upon the changing slopes, and since the topography is so irregular, 
such regions are shunned by man and even by most kinds of animal 
life (Fig. S9). 

Earth Pillars. — In a bad land country sculpturing has often de- 
veloped not only gullies and ridges, but isolated columns, often fan- 
tastically shaped. One of the common types of form is the column, 
capped by a boulder or by a slab of harder rock, and called an earth 
pillar. As the clay is removed by rain wash, boulders in the clay, or 
portions of harder layers, such as cemented sandstone, resist the 




Fig. 59. — Bad lands in China produced through deforestation. (Willis.) 

process and protect the underlying clay from removal. Thus 
ultimately a narrow column is left standing, bearing on its crest the 
cap of hard rock to which its presence is due. Frequently groups 
of such earth pillars are found close together. 

Causes of Bad Lands. — For the development of bad land topog- 
raphy an easily removed rock, like unconsolidated clay, is necessary, 
and for the more perfect development an arid climate also. The 
aridity aids in the development apparently for four prime reasons : 
(1) the nature of the rainfall, which, though rare, commonly comes in 
the form of heavy showers, during which much work of rain sculptur- 
ing is possible ; (2) the general sparseness or absence of vegetation, 
thus permitting rapid run-off and effective erosion ; (3) the ineffi- 
ciency of weathering, as a result of which the forms produced by rain 
wash tend to remain as formed ; (4) the compactness of the soil in the 
warm, dry air, which tends to prevent removal during intervals be- 
tween rains. 



104 COLLEGE PHYSIOGRAPHY 

Sources of River Water 

Relation to Precipitation. — The ultimate source of river water is 
the rainfall and snowfall. Murray estimates that the total annual 
rainfall on the land is over 29 million cubic miles of water. Of this 
a fourth or a fifth goes to the sea by the rivers. This comes to the river 
in unregulated run-off during and immediately after a rainfall, or 
during periods of snow or ice. Rivers- also have a more or less per- 
fectly regulated supply from underground water. The first of these 
sources is notably variable, while the second is normally far more 
uniform. 

Floods and Low Water Stages. — The variability, of the water 
supply of rivers is illustrated in all regions by the fluctuations in vol- 
ume. After heavy rains or melting of snow, the run-off may become so 
great as to exceed the capacity of the river channel to hold it all, 
when the river rises in flood and overflows its banks and the country 
bordering them. In periods of drought, the supply may come wholly 
from underground, and the volume may sink so much in arid and 
desert lands that seepage and evaporation are able to dispose of it, 
leaving the stream bed quite dry. Among rivers there are great dif- 
ferences in respect to volume. Some receive a fairly steady supply, 
others are markedly irregular in volume (Fig. 60). 

Underground Supply. — Among the factors tending toward regula- 
tion of river volume the most widespread is the supply of water from 
underground ; but there are places, as in arid lands, where this source 
of supply is limited and even quite absent ; and there are times, even 
in humid lands, when smaller streams dry up entirely during periods 
of drought. Indeed, all the rills and minor rivulets, which are flooded 
with water during rains, quickly dry up by run-off, seepage, and 
evaporation almost as soon as the rain ceases. In large rivers, how- 
ever, with numerous tributaries draining thousands of square miles of 
country, there is almost certain to be a sufficient supply of water from 
underground to tide over any period during which no rain is falling 
in any part of the drainage basin. Thus the Mississippi, while it may 
rise in flood, can probably never so shrink in volume as even to ap- 
proach dryness. 

Supply from Lakes and Glaciers. — Lakes and swamps serve as 
regulators of river volume, storing water in times of flood, and supply- 
ing it in regulated volume both during rainy periods and droughts. 
To a lesser degree porous rocks, like beds of sand, likewise serve as 
storage reservoirs for water through seepage and consequently as. 
river regulators. Mountain snows and glaciers lock up water which is 
contributed to rivers at times not related to the period of precipitation. 
In general this supply is fairly regulated, increasing in the warm 
months, and sometimes even giving rise to floods when snow melting 
and heavy rains occur at the same time. The melting of mountain 
snows, and rains among mountains, are important reasons why many 




Fig. 60. — Variable conditions at Ithaca Falls, N.Y.: (upper), in spring flood; (middle), 
covered with winter ice; (lower), dry in summer with water diverted for use in a mill. 

105 



106 COLLEGE PHYSIOGRAPHY 

large rivers are kept supplied with water in their lower course during 
periods of drought. This is true, for example, of the Mississippi, 
many of whose tributaries rise among the Rocky Mountains, where 
they receive volume enough to carry them across the arid plains. It is 
true also of the Colorado River and the Nile, both of which are so well 
supplied with water that they are able to maintain a course across 
desert lands. 

Relation to Run-off. — Variability in river volume may arise either 
from a great increase in amount of water available, or from favourable 
conditions of run-off. The former results either from heavy rainfall 
or from rapidly melting snow or ice. The latter is more complex; 
for the rate and amount of run-off depend (a) upon supply, (b) upon 
the slope, (c) upon rock porosity, and (d) upon obstacles to run-off. 
A moderate rainfall, upon a gentle slope, of fairly porous rock, and 
covered by a forest will give rise to only slight and slow run-off. A 
heavy rainfall upon a surface otherwise similar will result not only 
in a greater run-off, but in the run-off of a larger proportion of the 
rainfall. 

Relation to Forests. — The forest is an important obstacle to rapid 
run-off, for the moss and undergrowth, and the litter of decaying 
vegetation, act like a sponge, absorbing large quantities of water; 
and they also introduce obstacles to the easy run-off of the surplus 
water. The removal of the forest is, therefore, an aid to ready run-off, 
and consequently tends toward greater variability in river volume. 
There seems little reason to doubt that the removal of the forest from 
parts of the United States has led to greater variability of river vol- 
ume, and to increase both in size and frequency of floods. 

Arid Land Variations. — The greatest variability in river volume is 
found in arid lands. There, during heavy rains, the water runs off 
readily from the barren slopes, and the streams rise quickly to tor- 
rential volume ; but they subside almost as quickly, and soon the river 
bed is quite dry. Such streams are called intermittent, and they may 
be without water for months, or even for years in some desert lands. 

Spring Floods. — Streams in frozen lands are also variable, for 
when it rains the water cannot sink into the frozen earth, and must all 
run off ; and, added to it, is a supply from the melting of the frost in 
the ground. Thus even a light drizzling rain causes the streams to 
swell rapidly ; but when the winter sets in they are frozen and cease 
to flow. Some of the winter and early spring floods of the United 
States are in large part due to the fact that the ground is still frozen 
so that none of the rain or melting snow water can enter it, thus giving 
rise to rapid and excessive run-off, during which one thing or another 
causes rivers to overflow their banks and inundate fields and towns. 
In regions of dense population these are particularly destructive, for 
example at Passaic and Paterson, N.J., in 1902, at Kansas City in 
1903, at St. Louis, etc. The flood damage to railways alone in United 
States from 1900 to 1908 is estimated at $85,000,000, and this was 



RIVERS AND RIVER VALLEYS 



107 



probably only a tenth of the total damage to property through floods 
in these eight years. 

The flood in Ohio in March, 19 13, during which Dayton and over 
200 other towns were inundated, resulted in the loss of over 400 lives 




Fig. 61. — Flooded street in an Ohio town in March, 1913. (U. S. Geol. Survey.) 



and damages to houses, bridges, railways, etc., amounting to over 
190 million dollars (Fig. 61). It was caused by heavy rainfall, from 
8 to 10 inches falling in parts of Ohio and Indiana in 5 days, sup- 
plying 560 billion cubic feet of water. There was no snow on the 
ground and it was not frozen, but it had been so saturated by heavy 
rains in January and 
the first part of March 
that practically the 
whole 8 to 10 inches 
had to be disposed of 
by run-off. 

Floods are also some- 
times due to the failure 
of dams, following heavy 
rainfall or melting of 
snow. This was the 
case at Johnstown, Pa., 
in 1889, when over 2000 
people were drowned, 
and in Wisconsin in 
191 1, when a large part 




Fig. 62. 



Sketch map of eroded area at Black River 
Falls, Wis. (After Pence.) 



io8 



COLLEGE PHYSIOGRAPHY 



of the city of Black River Falls was destroyed by erosion of a river 
bluff during a flood (Figs. 62, 63). This last was not a spring flood, 
however, occurring after heavy rainfall in October. 

Other floods are due to rivers rising over or breaking natural and 
artificial banks. Such floods usually occur when the rainfall is heavy 
or" snow melts rapidly, and either (a) the ground is saturated, or (b) 
frozen or without vegetation-cover, or (c) the volume of the river in its 




63. — The city of Black River Falls, Wis., before and after the flood in 1911. 



lower course is already great. Several of the greatest Mississippi 
river floods, which have come on the average about once in 6 years, 
occurred in 1882, 1897, 1903 and 191 2, the one reaching the highest 
stage and having the greatest duration being that of 191 2, when the 
cities of Vicksburg, New Orleans, and Memphis suffered most. 



RIVERS AND RIVER VALLEYS 109 

The rise in spring level in the Ohio, upper Mississippi, and Missouri 
rivers normally comes in different months, but in 191 2 the rise of the 
Ohio was late. Excessive rainfall in the Ohio and lower Mississippi 
basins was the chief cause of the 191 2 flood, but to this three things 
should be added : (a) the lateness of the Ohio rise in 191 2, (b) the Mis- 
souri flood, caused by melting snow, (c) the heavy local rains in the 
lower Mississippi basin. This 191 2 flood rendered about 30,000 people 
homeless in the region north of Louisiana. In that state 350,000 
people were affected by the flood, a third of them suffering severe 
losses. Flood warnings by the U. S. Weather Bureau are now pre- 
venting practically all loss of life during spring floods in the lower 
Mississippi. Careful estimates show that the destruction of property 
amounted to 27 million dollars, the loss of crops nearly 35 million 
dollars, damage to farm land nearly half a million dollars, and losses 
through suspension of business almost 16 million dollars, making a 
total loss of about 79 million dollars through this one flood. 

River Volume and Velocity 

Volume. — The volume of a river depends upon the water supply ; 
but the depth of the river depends upon both the amount supplied 
and the rate at which it can flow along the channel. 

There is much variability in river volume, as already explained. 
Some streams have always a small volume, some always a large vol- 
ume, but most have a variable volume, fluctuating with the supply. 
Were it not for the supply of underground water, the stream volume 
would vary far more than it does ; but, even with this aid toward 
regulation of volume, the great majority of streams experience wide 
fluctuations from time to time. Those whose source of supply is 
mainly from springs, or which emerge from lakes, are least subject to 
fluctuation in volume ; while those whose supply is in large part from 
run-off of rain-water or melting snow experience the greatest varia- 
tion. This is especially true on slopes denuded of vegetation or 
surfaces that are relatively impervious, as bare rock slopes or frozen 
ground. 

Illustrations of Variations of Volume. — Since under normal condi- 
tions from one-third to one-fourth of the rainfall is discharged by rivers, 
while the balance is either evaporated, or taken up by plants, or stored 
underground, it follows that as the rainfall varies, the volume of the 
streams must be subject to notable fluctuations. The Seine, for in- 
stance, which normally flows through Paris as a quiet, well-regulated 
stream, may rise 20 feet in periods of heavy rain or melting snow, and 
even flood the lower portions of the city, as it did in 1910. The 
Hoang Ho in China may rise 40 feet in times of flood and, breaking 
through its embankments, spread over the surrounding country in a 
devastating current 30 miles wide and from 10 to 20 feet deep in its 
deepest part. The Mississippi floods have already been described. 



no COLLEGE PHYSIOGRAPHY 

Periodic Variations of Volume. — Many rivers are subject to 
periodic variations in volume. This is true, for example, of rivers 
whose sources are among snow-covered mountains, like the Colorado, 
or the Ganges, which commence to rise when the snows of the moun- 
tains begin to melt in spring. Every April the Ganges begins to rise, 
and continues to do so until the surrounding plains are transformed to 
a lake 32 feet deep. The Nile is flooded every summer during the 
rainy season in the Abyssinian headwaters, and the river rises 23 or 
24 feet at Cairo, and floods the delta plain below that city, irrigating 
and fertilizing the soil and giving rise to a vast oasis in the desert 
region, on which a dense population has dwelt for thousands of years, 
where one of our earliest civilizations developed. 

Velocity. — The velocity of a river is primarily dependent upon the 
slope of its bed ; and, since the slope commonly decreases from head- 
waters to mouth, the velocity normally diminishes in the same direc- 
tion. In some parts of certain streams the slope is vertical, as at 
Niagara Falls, and there the velocity attains its maximum. Such 
conditions are, however, exceptional, and good-sized rivers rarely have 
a very rapid slope. The average slope of the larger rivers of the world 
is probably not over 2 feet per mile, and navigable rivers do not often 
attain a slope greater than 10 inches per mile. 

Illustrations of Variations of Slope. — Some, however, have a much 
lower slope, such as the Volga, with its average slope of but 2 inches 
per mile. The Colorado, which descends from a lofty mountain range, 
has an average slope of but 7.72 feet per mile, and for a large part of 
its course it rushes along with torrential velocity. The Amazon 
descends at the rate of f inch to the mile in the lower 500 miles of its 
course. But such an average does not represent the actual conditions, 
for the slope of the stream bed is gentle in places, and steep in others, 
so that there are stretches of lakelike water and stretches of torrential 
flow. 

Variations of Velocity with Volume. — Velocity of river water, how- 
ever, is not solely dependent upon slope, for, even without change of 
slope, the velocity becomes greater with increase in volume. Therefore 
whenever a river rises in flood its velocity is greatly increased as well 
as its volume. The rate of flow of a stream, accordingly, varies from 
time to time as the volume varies. When the volume dwindles, 
even a steeply sloping stream may have a fairly quiet flow ; but with 
the coming of a flood it is transformed to a raging torrent. A moderate 
river current is about i\ miles per hour, but torrents on steep slopes 
may attain a velocity of 18 or 20 miles per hour. 

Variations in Different Parts of Stream Course. — There is not a 
uniform velocity throughout a river at a given cross-section, any more 
than there is along a longitudinal section. The velocity is normally 
greatest in the centre and least along the margins, where the shallow- 
ness and friction diminish the rate of flow. It diminishes from near 
the surface to the bottom, where friction also tends to diminish the 



RIVERS AND RIVER VALLEYS in 

velocity. There is also a relation between velocity and the nature 
of the channel. A smooth, regular channel permits more rapid flow 
than an irregular channel ; and where water is forced from a broader 
to a narrower channel the velocity increases, as it also does where a 
tributary pours water into a channel which is not proportionately 
larger. By irregularities in the stream channel minor currents and 
eddies are set up which interfere with the general forward movement 
of the water, and may even give rise to local up-stream movements. 

These variations in volume and velocity of river water have an 
important bearing upon the work of rivers, both in transportation of 
sediment and in excavation of river valleys. 

The Mineral Load of Rivers 

Visible and Invisible Load. — All running water on the earth is 
carrying a load of mineral matter, though the load varies greatly 
from one stream to another, and from time to time even in the same 
streams. This mineral load is carried partly in solution, partly in 
fragmental form. The former is the chemical load, which is invisible, 
the latter, the mechanical, which is visible. 

Chemical Load. — By far the greater proportion of the chemical 
load of rivers is supplied by underground water, which, as we have 
seen, brings to the surface a great variety of mineral substances in 
solution. No matter what may be the nature of the rock through 
which the water percolates, it obtains a greater or less quantity of 
mineral matter, of one or of several kinds, and a large proportion of 
this the rivers transport to the sea. By analyses of river waters it is 
known that this dissolved load is great in total quantity. The 
Thames, for example, transports 548,000 tons of mineral in solution 
each year, which is equivalent to about 140 tons removed annually 
from every square mile of limestone in its drainage basin. If removed 
equally from all parts of the basin it would lower the surface 1 foot in 
about 13,000 years by solution alone. It is estimated by Reade that 
the mineral matter carried in solution in river water is the equivalent of 
about 100 tons for every square mile of land surface in the world. 
Most of this is doubtless derived from the more soluble rocks, such 
as limestone, but all rock through which water percolates is supplying 
some. Naturally, therefore, though the surface is being steadily 
lowered by the solvent action of water, the rate varies greatly with the 
kind of rock. 

Besides the chemical load contributed by the underground water, 
there is an addition to the supply obtained by the surface water itself. 
Every rill and every river may add to the chemical load as it flows 
over soil or rock. Ordinarily this contribution is slight, and its 
amount depends upon the composition of the water and the nature of 
the rocks. Water impure with organic acids will dissolve more than 
purer water; and river water is often charged with these or other 



ii2 COLLEGE PHYSIOGRAPHY 

substances that give it solvent power. That the solution of mineral 
substances is in progress in river beds is well illustrated in limestone 
regions, where the rock of the river bed is often etched into a series of. 
ridges and hollows by the irregular rate of solution, as in the Niagara 
River above the falls. 

Mechanical Load. — While the chemical load of rivers comes 
mainly from underground, the mechanical load is essentially a contri- 
bution from the surface. Some of it may fall to the river from steep 
slopes, where it has been dislodged by the pull of gravity upon 
weathered rocks; some is worn from the river bed by the attrition of 
the rock fragments against the stream bottom. But by far the greater 
portion of the mechanical load of rivers is washed into the stream by 
the multitude of rills and minor tributaries, especially those on steep 
slopes and in unconsolidated material, and particularly during heavy 
rains or rapid melting of snows. A source of sediment for some streams 
is the contribution of rock material from glaciers ; but this may be 
considered an exceptional source, while the others are normal to all 
rivers. 

Transportation of Mechanical Load. — The transportation of this 
mechanical load is accomplished partly by pushing or rolling the 
fragments along the river bed, partly by carrying them along bodily in 
suspension. It is only the lighter fragments that can be carried in 
suspension, ordinarily clay, though in swift Currents even sand may 
be thus carried. Even the finest clay particles are heavier than 
water, and will settle when the water is allowed to stand ; but in the 
river current there are eddies which serve to float the fine-grained 
sediment, as dust is floated in the air. It is not to be inferred that such 
particles are carried along uniformly, as the dissolved mineral sub- 
stances are, but rather that there is a constant tendency toward set-, 
tling to the bottom, so that, if a particle could be traced from a river 
source to a river mouth it might be found to descend to the bottom 
and rise in the river again many times on its way, and perhaps even 
to rest for a long time in a sand bar or other river deposit. 

Transportation by Dragging. — Near the bed of a heavily burdened 
stream the water may be so filled with sediment that the land itself is 
rapidly changing by the forward gliding of the water-filled sand ; and 
in all sediment-laden streams there is an important movement of the 
heavier particles by dragging. This transportation is accomplished 
by the push of the moving water, and since stones lose from one-half 
to one-third of their weight in water, it is possible for a rapid current 
to drag along even good-sized stones. The size of a stone that can 
be moved by a given current depends to a large degree upon its specific 
gravity and upon its shape, both of these factors depending primarily 
upon the area exposed to the force of the current. Rounded stones 
are more easily moved than flattish forms, partly because of the greater 
surface exposed to the current, and partly because they are more easily 
rolled along. 



RIVERS AND RIVER VALLEYS 113 

Muddiness is evidence that streams are carrying a load, but this 
muddiness may be checked (a) by forested slopes, (b) by gentle slopes, 
and (c) by the presence of prevailingly coarse, heavy sediment, which is 
dragged along the bottom of the stream instead of moving in suspen- 
sion and thereby clouding the stream. 

Relation to Velocity. — A current of half a mile an hour will carry 
coarse sand, while a current of two miles an hour will move angular 
stones the size of an egg. The transporting power of the water varies 
as the sixth power of its velocity, so that if the velocity of the current 
is doubled the power of transportation is increased 64 times. Con- 
sequently swift currents have an exceedingly high transporting power, 
some being able even to move boulders hundreds of pounds in weight, 
especially on steep slopes. In torrential waters one can often hear the 
stones bumping together as they are rolled along the bed. 

Velocity has much to do with transportation of the stream load 
near the headwaters of streams. Here there may be little water and 
coarse sediment, so that a steep slope and high velocity are necessary 
for the transportation of the stream load, the coarser part of which 
will move only by dragging. In the lower course, however, with the 
large volume of the stream, the bulk of the sediment, which is fine, 
needs less slope and. less velocity in order that it may be carried in 
suspension. 

Eddies and Ripples. — The movement of sediment along the stream 
bed is not a uniform process, for, owing to the irregularity of the bed, 
the velocity of the current varies from point to point. As a result 
there is a concentration of greater energy in some places than in others. 
A complex series of eddies is introduced, and their activity is often 
expressed by the excavation of deep holes which vary in position and 
depth as the velocity varies, or as the cause for the eddy changes. 
Another form of the concentration of the energy of moving water is the 
development of ripple marks on which the energy of the current is 
localized, moving the particles from the up-stream face of the ripple 
and rolling them into the depression on the down-stream side. As a 
result of the movement the ripple marks move down-stream, but the 
general ripple form is preserved. In a shallow, heavily burdened 
stream one can see the procession of ripple marks as they pass along, 
their positions being marked by the wavy water surface as it is thrown 
upward and downward in its passage over the hidden ripple marks. 
In fording such a stream one can feel the sand or gravel as it glides 
along ; and if the stream bed is abandoned, the wavy ripple marks, 
a foot or two high, are exposed to view. 

Wearing of Transported Material. — By this dragging of rock frag- 
ments, large and small, over one another, and over the stream bed 
there is constant attrition, by which the fragments are ground down 
and even the hardest rock in the stream bed cut away. It is by this 
attrition that a part of the finer sediment, which moves down-stream 
in suspension, is derived. This is also one reason why the size of the 



ii 4 COLLEGE PHYSIOGRAPHY 

fragments borne by a river normally decreases from source to mouth. 
But another reason is that the river current decreases in velocity 
toward the mouth and hence the size of particle that can be moved 
decreases. Nevertheless, if the coarser fragments of the upper part 
of a stream course were not ground down to size suitable for transpor- 
tation by the currents of decreasing velocity there would be an accu- 
mulation of coarser fragments which- are being steadily supplied for 
transportation. 

Variations in Transportation. — The quantity of sediment trans- 
ported by a river depends partly on the volume and velocity of the 
water, partly upon the amount of sediment contributed. All of 
these factors are variable in a given river, which may at one period be 
clear, limpid water and a little later be transformed to a rushing flood 
of discolored, sediment-laden water. They also vary from stream to 
stream. There is every gradation from the vertical fall of water, 
descending as rapidly as gravity can draw it down, to the stream of 
barely perceptible current; and from streams of large to those of 
small volume. There are some streams which are always compara- 
tively free from sediment load, notably those like Niagara, which 
issue from lakes, in whose quiet waters sediment has settled. Others 
vary in their sediment load from periods of heavy load to periods of 
light load or even absence of load. And still others are always heavily 
sediment-laden, as the Missouri and lower Mississippi are. 

Overburdened and Aggrading Streams. — Some streams are so 
heavily burdened with sediment that they cannot carry it all, and are 
overburdened, as the Platte River is. Such streams are forced to stead- 
ily lay down some of their burden in the stream bed, building it up. 
Such streams, of which the Platte is a typical instance, are said to 
be aggrading streams, in contrast to those which are cutting into their 
bed or are degrading. 

Amounts of Material Transported. — Heavily charged streams 
are efficient agents of transportation of the rock waste of the land. 
The Mississippi, for instance, pours into the Gulf of Mexico 
19,500,000,000,000 cubic feet of water each year, which carries with 
it about 812,500,000,000 pounds of rock fragments. This vast 
amount of sediment if collected would form a prism a mile square, 
and 268 feet high. If the rock material poured into the Gulf of 
Mexico by the Mississippi River each year were removed equally 
from all parts of the drainage area, the entire surface would be lowered 
about one foot in 6000 years. It is estimated that the rate of lowering 
of the drainage basin of the Hoang Ho is one foot in about 1464 
years ; of the Po, one foot in 729 years ; and of the Danube, one foot 
in 6846 years. 

Processes of River Erosion 

Corrasion and Corrosion. — River water is competent to remove 
unconsolidated material from its bed up to the size of fragments which 






i 



RIVERS AND RIVER VALLEYS 115 

can be dragged along during its periods of greatest velocity. River 
water is competent also to remove rock material in solution. There- 
fore even clear water can perform some work of degradation by 
mechanical means or corrasion in unconsolidated rock, by chemical 
means or corrosion in solid rock. The rate of corrosion is dependent 
upon the volume of water, the composition of the water, and the 
nature of the rock. Even in such a soluble rock as limestone, how- 
ever, the rate of excavation by the process of corrosion is exceedingly 
slow. 

Sediment furnishes River Tools. — The mechanical or corrasive 
work, while greatly influenced by the volume and velocity of the water 
and the nature of the rock, is primarily dependent upon the sediment 
load which the river current drags along its bed. Sediment therefore 
furnishes tools, used by running water in its work of excavation. If 
the stream has little sediment, its rate of work is necessarily slow in all 
but unconsolidated rocks ; and if, on the other hand, it has a heavy 
sediment burden, it may be forced to aggrade its bed and, therefore, 
be prevented from using its tools in deepening its channel. But 
streams with abundant sediment, though with no more than they 
can transport, are competent to degrade their beds even in the hardest 
rocks. For, as the sediment is dragged over the rock, it chips and 
grinds off particles and moves them down-stream. 

Agents of Stream Erosion. — The combined work of corrasion and 
corrosion, together with the accompanying transportation, is erosion. 
While the work is primarily that of solution and mechanical wear by 
the movement of sediment over the bed, there are some supplementary 
phenomena modifying the process of erosion. Some of these, such as 
the influence of the nature of the rock, the variation in volume and 
velocity, and the difference in chemical composition of the river water, 
have already been mentioned. Another is the effect of weathering 
upon the bed of a stream during intervals of low water when it is 
exposed to the air. And still another of much importance in regions 
of frost is the influence of ice. 

The Work of River Ice. — The formation of ice greatly modifies 
the volume of a stream, first by temporarily locking up some of the 
water, and then, on melting, by giving it back to the stream, often in 
considerable volumes. When the river freezes to the bottom, or when 
ground ice forms on the river bed, the same disruptive effect may be 
caused in the rock as results from frost action in the process of weather- 
ing. As the river increases in volume the ice may lift rock fragments 
from the bed and float them on down-stream, even carrying much larger 
fragments than the river unaided can transport. Now and then the 
ice blocks form a dam, ponding back the river water and causing serious 
floods above the dam, and, if it breaks, giving rise "to a temporary 
great increase in the river volume below the dam, by which, in a brief 
interval, much work of erosion may be performed. Such ice jams, or 
ice gorges, do much damage to life and property (Fig. 64), 



n6 



COLLEGE PHYSIOGRAPHY 



Rapid and Slow Erosion. — In its work of erosion, river water does 
not work uniformly either from the standpoint of time or of place; 
for at times the rate of work is far more rapid than at others, and there 
is also notable variation from point to point along the river bed. 
For weeks or months a stream may flow leisurely, with limpid current, 
accomplishing little or no work of erosion ; and then it may be trans- 
formed to a flood of rapidly rushing, sediment-laden water. Tempo- 
rarily the river then becomes a vigorous agent of erosion, in a day or two 
perhaps accomplishing more work than in the entire preceding year. 



&&&> 







Fig. 64. — River ice in a spring flood on the Susquehanna River. 
(Hoyt, U. S. Geol. Survey.) 



Even in streams that are always armed with cutting tools, and that 
always flow with considerable velocity, there are periods when an 
increased volume gives such added velocity that their rate of erosion 
is greatly increased. In the arid southwest the intermittent streams 
sometimes spread out in sheetfloods, a current perhaps a mile wide 
and 1 or 2 feet deep. 

Pot Hole Action. — Owing to the eddies in river currents and to the 
irregular rate at which the bed is worn down in rocks of different 
degrees of resistance, there is a tendency toward the greater concentra- 
tion of the energy of the flowing water at certain points, thus giving 
rise to local deepening. Once a depression is excavated, the added 
velocity which the increased slope induces tends toward still further 
deepening. The operation of this process is most typically illustrated 
in the development of pot holes, which abound in the rock floors of 
rapidly flowing streams. In their inception the pot holes may be due 
to irregular solution, to eddying currents, to the presence of joint 
planes in the rock, to the more rapid wearing away of weaker portions 



RIVERS AND RIVER VALLEYS 117 

of the rock floor, or to the falling of water in a waterfall. Whatever 
the cause for the first stage of the depression, once it is formed the ten- 
dency is for it to enlarge under the influence of the increased velocity 
of the water falling into it, the swirling of the current in the depression, 
and usually, also, by the grinding of rock fragments caught in the 
depression and whirled about in the pot hole eddy. 

By this action a hole a few inches or a few feet in depth, and varying 
similarly in width, is quickly excavated in the rock. It is usually 
enlarged below by the grinding action of the stones which the eddy 
whirls around, assuming the kettle shape from which the name pot 
hole is derived. There is a limit to the depth to which a pot hole may 
be excavated, for, as it grows deeper and the volume of water in it 
becomes greater, its eddy becomes slower and less effective ; or, before 
this stage is reached, the current may be deflected and pot hole work 
cease at that point, though perhaps beginning at another. 

Local Concentration. — Such local concentration of energy of the 
river water is an important factor in stream bed deepening, for, al- 
though the pot hole is only local, the process is operating at numerous 
points and is shifting from point to point, while every now and then 
the walls of contiguous pot holes give way, thus increasing the con- 
tinuous area of deepening. To concentrate upon a given portion of a 
stream bed, or at times even the whole volume of a stream with that 
maximum velocity which comes with vertical fall, necessarily means a 
great local work of excavation. And probably by such local concen- 
tration the average work of corrosion along a stream bed is greater 
than if the energy of the stream were equally applied to all 
parts of the channel. Not always are perfect pot holes produced 
by such concentration, but the process is similar, even though time or 
other factors do not permit the perfect form to develop. 

The Formation of Gorges and Canyons 

Youthful Steep-sided Valleys. — When a stream is steadily cutting 
into its bed, it sinks itself more and more deeply below the general 
surface of the land until finally it may be enclosed between lofty, 
steeply rising walls. Such a precipitously walled valley is called a 
gorge, gulch, or ravine, or, in western United States, in many cases, 
a canyon. There is no hard-and-fast line that can be drawn between 
a gorge and a canyon, for both are due to the rapid down-cutting of a 
stream bed, and both are narrow, steeply walled valleys sunk below the 
general surface of the country. A canyon may perhaps be considered 
to be a larger form of a gorge such as is characteristically excavated 
in a region of high plateaus, like Mexico and southwestern United 
States. But in common usage even this distinction is not followed. 

Vertical Deepening. — The primary cause of the gorge or canyon 
is the erosion by water as it wears away the rock along its line of flow. 
If no other process than this were at work, the gorge would have no 



n8 COLLEGE PHYSIOGRAPHY 

greater width than the stream, and there are gorges in which this 
condition is actually present for at least a part of their length or depth. 
There are, however, two other processes which tend to widen the gorge 
at the same time that the river erosion is deepening it. 

Lateral Cutting. — One of these is the lateral swinging of the stream, 
or eddies deflected toward the side of-the stream, by which some of the 
erosive energy of the running water is employed in lateral excavation. 
Where the rocks are weak, as in unconsolidated beds, the lateral erosion 
is an important aid in broadening valleys ; it is also effective in con- 
solidated rocks, but to a far less degree. Valleys are broadened more, 
on the whole, by undercutting than through the aid of weak strata. 
In some cases, where the rocks are held firmly together, the effect of 
the lateral erosion has merely resulted in causing overhanging cliffs, 
or in giving the gorge a diagonal or curving form, so that from its 
bottom one looks upward to a rock roof, and cannot see the sky. 
Naturally, such a condition, cannot be enduring, for gravity and 
weathering will in time destroy such an unstable valley wall. Con- 
sequently, when this condition is present, we may be sure that the work 
of excavation has been both recent and rapid, and that the gorge is 
young. 

Gorges Broadened by Weathering. — The second process, by which 
the narrow gorge due to vertical erosion is broadened, is by weathering. 
A steep rock slope exposed to the air is normally a place of rapid 
weathering ; and if there is a stream at its base to remove the 
weathered products, the wasting back of such cliff should proceed 
apace. When, therefore, a gorge is found in which the walls are still 
precipitous, it may be confidently inferred that the gorge is young 
in the geological sense ; for, if it had long been exposed, it would be 
broadened and rounded by the operation of the agencies of weathering. 

Both lateral swinging and weathering are at work at all times upon 
the walls of a gorge, from the moment the stream first sinks its channel 
below the surface. An indication of the truth of this statement is 
commonly to be observed in gorges, for they are usually broader at the 
top, where weathering has been longest at work, than they are at the 
bottom, where there has been little time for the operation of weather- 
ing. And gorges in weak, easily weathered rocks have a more flaring 
form than those in the more durable rocks. The fact that gorges are 
no broader than they are testifies not only to the youthfulness of this 
land form, but also to the fact that the vertical erosion along the bed 
of a vigorous stream is a more rapid process than weathering, even 
on steep slopes. 

Gorges Indicate Youth. — The development of gorges is possible 
only where there is opportunity for a rapid flow of water, and such 
opportunities are most commonly found among mountains and 
plateaus where there is a sufficient elevation to give the slope for the 
necessary velocity. It is possible also only where these processes have 
been begun recently in the geological sense ; consequently it is among 



RIVERS AND RIVER VALLEYS 119 

mountains and plateaus of recent development that gorges and 
canyons are most abundant, though gorges may develop in other 
situations where a favourable condition of slope has been recently 
introduced. 

The process of gorge excavation is that of river work in general, as 
outlined in the preceding pages. By corrosion and corrasion, by varia- 
tion in volume and sediment load, and by concentration of energy 
in pot hole work and in waterfalls, the rock in the river bed is worn 
away. Usually the processes are still in operation, and may be ob- 
served, though even such rapid work is slow from the standpoint of the 
human time measure. It is only in the geological sense that it may be 
considered rapid. Doubtless careful measurements would show 
changes from year to year, but casual observation, even during a life- 
time, might fail to note any change. 

The Limitation by Baselevel. — Even the gorge deepening must 
reach an end, for, when the flow becomes so gentle that the sediment 
load is no longer dragged over the bed as a tool of erosion, deepening 
must cease. At no point in its course can the stream lower its valley 
appreciably below the level of its mouth ; and this level is, for the 
stream, its baselevel. The ocean surface is the great baselevel, but 
individual streams may have temporary baselevels well above sea level, 
such, for example, as a lake. At the mouth of a stream the bed may 
be very slightly below baselevel, as in the Rhone where it enters Lake 
Geneva. Nevertheless the baselevel absolutely controls the depth of 
cutting by streams. Another temporary baselevel is a main stream, 
below which no part of the tributary valley may be cut so long 
as the level of the stream mouth is maintained. A temporary base- 
level may even exist below sea level, where a stream is tributary to a 
part of the land that lies below sea level, like the depression in which 
the Dead Sea lies. 

The Grade of Streams. — While at its mouth a river may lower its 
valley bottom to the baselevel, this cannot be done for any great 




Fig. 65. — Relation of grade to baselevel. 

distance above the mouth, since the water must have a slope over which 
to flow and transport its sediment load. The lowest slope over which 
a river can transport its sediment load may be called its grade, or 
gradient. This grade is a curve, flattest near the mouth where the 
volume of the stream is greatest, but increasing in steepness toward the 
headwaters where the volume is least (Fig. 65). 
By its erosive work a stream is tending toward the attainment of a 



120 COLLEGE PHYSIOGRAPHY 

perfect grade, and the development of a gorge valley is an early step 
in this process, in which the stream is so far from flowing with a perfect 
grade that it can cut rapidly along its bed. As the graded condition 
is more nearly approached, the rate of erosive work diminishes and 
ultimately practically ceases. But the grade is not to be considered a 
definite curve that, once established, -is forever fixed ; for, since it is 
the lowest slope over which the river volume can transport the sedi- 
ment load, if the volume increases without change in the sediment 
load, the grade can be lowered ; or, if the sediment load increases 
without change in volume, the grade must be increased. In the first 
case the grade is rectified by degradation ; in the second, by aggra- 
dation. It commonly happens that a grade established under one 
set of conditions must later be altered to meet newly developed condi- 
tions. 

Widening of Graded Streams. — As the perfect grade is approached, 
the rate of down-cutting along the river bed becomes so slow that 
finally the rate of weathering is in excess of the rate of down-cutting. 
And ultimately, when further down-cutting is at an end, weathering is 
the main factor in valley formation. During these stages the valley 
sides waste away, the valley broadens, the slopes lose their steepness, 
and the gorge form is destroyed. This process, however, is a slow one, 
and is attained only after the lapse of sufficient time for the operation 
of the agencies of weathering. Many times as long a period is re- 
quired for the rounding of the valley slopes as is required for the 
formation of the gorge. In this process the river is still an important 
agent, for upon it falls the task of removing the rock materials which 
weathering supplies, and which come to it from the multitude of rain- 
born rills and other stream courses that develop upon the valley sides. 
Up to a certain point the river works directly also by its lateral swing- 
ing and consequent removal of material by lateral erosion even after 
vertical erosion has ceased. 

Grand Canyon of the Colorado 

The Most Wonderful Work of Nature. — Of all the gorges and can- 
yons of the world, and perhaps of all works of nature the most won- 
derful example, is the Grand Canyon of the Colorado. The Colorado 
River, which has carved this canyon (Fig. 66), is made by the union 
of two tributaries, the Grand and Green. It has a total length of 
about 2000 miles, and drains an area of about 225,000 square 
miles. Having its source among the lofty ranges of the Rocky 
Mountains, this river has an abundant supply of water and plenty of 
sediment for cutting tools. On its way to the sea the river must 
cross a lofty plateau, in places over 8000 feet high, and so recently 
elevated that the river has not yet been able to cut down to grade. 
Accordingly the river has still a steep slope (7.72 feet per mile) which 
insures high velocity. 



RIVERS AND RIVER VALLEYS 



121 



Factors Favouring Canyon Cutting. — The Colorado River has all 

the favourable factors for pronounced canyon cutting : (a) large vol- 
ume, (b) high velocity, (c) an abundance but not a superabundance of 
sediment, (d) a great thickness of rock to cut into before grade can be 
reached. Still a fifth factor of importance is the aridity of the 




Fig. 66. — The Grand Canyon of the Colorado, with deltas of tributary streams. 
(Hillers, U. S. Geol. Survey.) 



122 



COLLEGE PHYSIOGRAPHY 



climate throughout the canyon section, as a result of which the 
canyon form tends to be preserved because of the slowness of the 
operation of the agencies of weathering. 

Canyon cutting is necessary, as may be estimated in the case of a 
stream flowing 200 miles across a plateau 2000 feet high. If it was 
on the surface of the plateau at one side and had a grade as steep as 
the Colorado, nearly 8 feet per mile, it would necessarily incise a 
canyon 1600 feet deep by the time it had crossed to the other side of 
the plateau. 

Relationship to Rock Strata. — In crossing the plateau the river 
has entrenched itself in a canyon for a distance of between 200 and 
300 miles and to a depth varying from a few hundred feet to 6000 feet. 
As the river crosses different sets of rock strata the form of its canyon 
varies, and different names are given to the several parts. Where 




Fig. 67. — Cliffs and slopes due to resistant and weak strata at the Colorado Canyon. 

(Holmes.) 



the rocks are massive, the canyon is narrow and the walls rise pre- 
cipitously with little variation ; but where the beds vary in texture, 
denudation has etched them out at varying rates, and there the 
canyon walls are wonderfully sculptured (Fig. 67). 

The sculpturing of the canyon walls is along both horizontal and 
vertical planes. The horizontal sculpturing is due to the varying 
degree of resistance to weathering of the more or less horizontal strata, 
giving rise to a series of steps in the canyon walls with precipices 
where resistant strata outcrop, and more gentle slopes where the 
weaker beds lie. Since these strata are variously coloured, they give 
rise to a gorgeous, though gaudy, colour effect, as well as to marvellous 
sculpturing. Vertically the canyon walls are gashed by a multitude 
of ravines, gorges, and canyons, where running water has cut into the 
plateau as the rain-water has hurried down the canyon sides to the 
river; and between these are pillars, minarets, ridges, and table- 
topped spurs. 



RIVERS AND RIVER VALLEYS 123 

Evidences of Origin. — As one looks down upon the maze of sculp- 
tured rock forms from the plateau edge it seems hardly possible that 
running water and weathering could have performed such a vast 
work ; but, on descending to the canyon bottom, the result seems less 
difficult of conception. The steep cliffs are seen to be ragged through 
weathering, and here and there one sees places from which masses 
of rock have only recently fallen. In the bottom of the lateral can- 
yons are seen huge rock fragments, which the floods of occasional rains 
are moving down the steep slopes of the canyon bed to the main river. 
And the river itself, seemingly a silvery thread when viewed from the 
plateau top, is found to be a rushing torrent, discolored with the heavy 
burden of sediment which it is hurrying on toward the sea. All 
the necessary processes are plainly visible ; and all that one needs 
supply to attain even such a grand result is the element of time for the 
effective operation of these processes. Of that there has surely been 
sufficient. 

The Canyon as a Barrier. — The great gash which the Colorado 
River has cut in the plateau forms an almost insuperable barrier to 
travel. Even though at the top it is ten or twelve miles wide in places, 
thus giving an average slope of no great steepness, it is so sculptured, 
and there are so many precipices where resistant rocks outcrop, that 
only with the greatest difficulty is it possible to get to the bottom at 
certain favourable spots. Trails to the canyon bottom are maintained 
at the points tourists visit, but elsewhere the bed of the canyon is 
quite inaccessible. No road crosses this canyon, and a person living 
on one side would need to make a journey of two or three hundred 
miles to reach a spot only eight miles away on the opposite side of the 
canyon. The Colorado Canyon is one of the most perfect barriers to 
travel in the world (PL VIII). 

While the Colorado Canyon is the grandest of the type, it is, in real- 
ity, but one of a class. There are hundreds of similar, though smaller, 
canyons in western United States, and in other regions of high plateaus, 
while gorges, the smaller forms of the same valley type, occur by the 
thousands in plateaus and mountains, as well as elsewhere where 
favourable conditions exist. 

The Formation of Waterfalls 

Rapids, Waterfalls, and Cataracts. — Where the bed of a stream 
steepens abruptly, the current quickens, giving rise to a rapid, or, if 
the slope becomes vertical, to a waterfall. There is no distinct line that 
can be drawn between a waterfall and a rapid, for they grade into one 
another, and what are really rapids under the terms of the preceding 
sentence are sometimes called waterfalls. The term cascade is often 
applied to a small waterfall, and cataract to a large one like Niagara. 

Weak and Resistant Rock Layers. — Waterfalls and rapids normally 
develop in a stream which is cutting into its bed, because, in its exca- 



I2 4 



COLLEGE PHYSIOGRAPHY 




RIVERS AND RIVER VALLEYS 



125 



vation, it finds opportunity for more rapid work in some places than in 
others. Among the conditions which give rise to this opportunity by 
far the most common is the difference in resistance of the strata, and 
especially where the stream is cutting into strata that are horizontal 
or approximately so. If we assume, what is common, a series of 
strata of different kinds, lying approximately horizontal, and a 
stream flowing over and sinking its bed into them, the layers will be 
cut into one by one. If one layer of rock is fairly resistant, and the 
one next below is less hard, the stream will find it easier to cut away 
the lower than the upper one. Since tne stream must have a slope, 
while the strata are horizontal, this weaker bed will be first reached 
in the lower course of the stream, while higher up the course the stream 
will still be flowing on the resistant upper layer and cutting into it. 




Fig. 69. — Abandoned waterfall in Washington. For location see Fig. 337. 
(Quincy Quadrangle, U. S. Geol. Survey.) 



But since the stream cuts more rapidly in the weak layer, which it has 
discovered, than in the resistant layer, it will cause an abrupt increase 
in the slope of the bed at the point where it leaves the ledge which is 
holding it back. There either a waterfall or a rapid will develop, 
according to the extent of the difference of the stream excavation in 
the two layers. If the difference in resistance to stream erosion by the 
weak and resistant layers is great, there may be a high waterfall ; and, 
if there is also a great volume of water, there may be a veritable 
cataract. If, on the other hand, the difference between the layers 
is but slight, or if the layers are thin, there will be only a small fall, or 
perhaps only a rapid. 

Relationship to Pot Holes. — Once an increase in slope of the 
stream bed is introduced, its further development is made easier by 



126 



COLLEGE PHYSIOGRAPHY 



reason of the increased velocity of the water at the point of fall. Here 
pot hole action commonly aids matters, for the falling water excavates 
a hole at the base of the fall, and the vertical distance through which 
the water falls is considerably increased by this action. The formation 
of the pot hole also aids in causing the waterfall to recede, for, as the 
swirling waters excavate beneath the resistant layer, it is undermined, 
and its front recedes. But although the fall recedes, it is not destroyed, 
for the cause remains ; therefore the fall gradually migrates up-stream. 
Forms of Waterfalls. — Rapids and falls are common phenomena 
in the beds of streams which are cutting downward into the rocks. 
There are often a succession of these as the water leaps from one ledge 
of hard rock after another. They are sometimes perfectly straight 
across the face, but most commonly their outline is irregular, for the 
water, in cutting through the resistant layer to which they are due, 
does so irregularly, finding a joint plane here or there, or for some 
other reason concentrating the current at one part of the fall more 
than at another (Fig. 69). 

Waterfalls only in Youthful Streams. — Since waterfalls depend for 
their existence upon an increase in the slope of the stream bed they 
cannot exist in graded rivers; for there the river bed is everywhere 
reduced to the lowest slope down which the sediment load can be 
transported. Nor can any new falls be developed in such a stream, 

since it is no longer 
cutting into its bed. 
Therefore waterfalls are 
phenomena of the early 
stages of valley devel- 
opment, being really 
features of youth, like 
the gorges and canyons 
with which they are so 
commonly associated. 

Niagara as a Type 
of Waterfall 

Relationship to 
Niagara Limestone. — 

Among waterfalls de- 
pendent upon irregular 
excavation in essenti- 
ally horizontal strata, 
Niagara is easily the 
most noted and grand- 
est. The cataract of Niagara is precipitated over a thick bed of 
limestone, called the Niagara limestone (or Lockport dolomite), which 
at the falls has a thickness of from 60 to 80 feet, and beneath which 




7o. — Relation of Niagara to limestone and shale. 
(Modified from Gilbert's diagram.) 



L 



RIVERS AND RIVER VALLEYS 



127 



are strata of shale and sandy layers. The vast volume of water 
tumbles vertically through a distance of nearly 160 feet, thus becom- 
ing an engine of enormous power, by means of which the shales and 
sandstones are readily ground away, thus undermining the thick 




Fig. 71. — The two cataracts at Niagara. American Fall in the foreground. 



limestone. Every now and then a block of this limestone falls away 
and thus the cataract recedes (Fig. 70). 

Method of Recession. — The process of fall recession at Niagara 
is primarily the result of pot hole action, for beneath the cataract a 
deep hole is excavated in which the water swirls about in a great eddy, 
carrying with it the limestone blocks that tumble from the crest of the 
fall, and with them grinding away the rock from beneath the lime- 



128 



COLLEGE PHYSIOGRAPHY 



stone. Supplementary to this major cause are other processes as 
follows : (a) solution, by which some of the rock material is removed 
by the falling water ; (b) frost and ice action, by which pieces of rock 
are loosened and removed ; and (c) contraction and expansion of air 
under the varying impulse of the falling water. 

The Two Cataracts. — Niagara Falls consist of two cataracts be- 
cause the current of the river is split by an island in its course, — Goat 



iurveyed in 1842 by State of NewYork 
" >» 1875 by Lake Survey 
>j " 1886 by U.S. Geological Survey 

»» ■>•> 1890 by State or NewYork 

»» 1905 by U.S.G.S.andStateof N.Y. 
»» »»I9(I by Internat. Waterways Comm. 



Scale 
ioo 20u 300 400 500 6oofeet 
j 1 1— j. 



TABLE ROCK HOUSE! 




Successive crests of Niagara. (After Gilbert and Taylor.) 



Island. The larger volume, 95 per cent, flows on the Canadian side of 
Goat Island, giving rise to the Horseshoe or Canadian Fall, while the 
lesser volume gives rise to the much smaller American Fall (Fig. 71). 
Slow Recession of American Fall. — The volume of water that 
passes over the American Fall is so slight, and is distributed over so 
broad an area, that no pot hole is developed at its base ; and the lime- 
stone blocks that have fallen from the crest of the cataract have not 



RIVERS AND RIVER VALLEYS 



129 




been ground up and removed, but remain like a talus of huge blocks. 
Upon these the falling water dashes, and over and between them it 
rushes. Because of the small volume of water and the protection 
to the base afforded by the limestone blocks, the American Fall is 
receding very slowly, 
— by measurements 
between 1827 and 
1905 the rate is de- 
termined as less than 
3 inches per year. 

Rapid Recession 
of Canadian Fall. — 
In theCanadian Fall, 
on the other hand, 
a vast volume of 
water is discharged, 
it being estimated 
that the average 
depth of water at 
the crest of the falls 
is four feet, while at 
the central part the 
depth is twenty feet. 
It not only disposes 
of the falling lime- 
stone, but, as already 
stated, undercuts the 
base of the falls. 
Measurements be- 
tween 1842 and 191 1 
show that the rate 
of recession of this 
fall is about 5 feet 

a year. At the point of greatest depth of water it is most rapid. 
Owing to this increase in rate of recession toward the centre of the 
cataract, the Canadian Fall has a horseshoe shape, and is often called 
the Horseshoe Fall. The maximum average rate at which the fall is 
receding at the apex of the horseshoe is the rate at which the fall as 
a whole is receding, for the lesser rate on the side of the horseshoe 
results merely in broadening the gorge which the receding of the fall 
is causing (Figs. 72, 73). 




Fig. 73. — Comparison of a camera lucida sketch of the 
Horseshoe Fall in 1827 with a photograph from the same 
point in 1905. (After Gilbert.) 



History of Niagara Falls 

The Two Plains near the Niagara. — Niagara River came into being 
at the close of the Glacial Period, when the retreating ice sheet un- 
covered the region between Lakes Erie and Ontario, and the outflow 



130 COLLEGE PHYSIOGRAPHY 

of the former lake sought a way to the latter. The surface of the 
country between these two lakes consists of two plains, one in which 
Lake Erie lies, the other bordering Lake Ontario. The two plains 
are separated by a cliff, or escarpment, about 200 feet high, determined 
by the outcrop of the Niagara limestone which underlies the Erie 
plain in a nearly horizontal sheet with a southward dip of about 35 
feet per mile. At the escarpment the limestone is but 20 feet thick. 
As the limestone dips into the earth, more and more of it is left, so that 
it is 60 to 80 feet thick at the falls, and a short distance farther south 
its total thickness is about 150 feet. 

The Original Cataract. — The great volume of water that issued 
from the glacial predecessor of Lake Erie flowed in a broad course on 
the surface of the upper plain, and, on reaching the escarpment, 
abruptly fell to the level of the Ontario plain. It was one of several 
such falls, the others being to the east (Fig. 194). Thus the original 
Niagara, just south of Lewiston and Queenston, was determined by an 
irregularity in the surface of the land, in consequence of which the 
stream bed had an abrupt increase in slope. Such a waterfall has 
been given the name consequent fall. At once the falling water began 
excavating and undermining the limestone to which the escarpment 
was due, and as it did so the fall began to recede. This recession has 
continued through the whole length of the Niagara gorge, which is 
about seven miles long. 

Proofs of Seven Mile Recession. — The proof that Niagara Falls has 
receded seven miles is of several kinds, as follows : (1) the cataract is 
at present receding at a rapid rate by processes which might easily 
have been in progress for the necessary length of time ; (2) the gorge 
is* a youthful topographic form - , such as a recession of the falls would 
normally produce ; (3) banks of the old river which flowed on the sur- 
face of the plain before the gorge was cut are still plainly visible, 
and river gravels with river shells are present there also ; (4) at Foster 
Flats, two-thirds of the way down the gorge, there is an abandoned fall 
of the same type as the American Fall of to-day, having been aban- 
doned by the more rapid recession of a larger cataract, like the present 
Horseshoe Fall, which then existed on the American side. 

The Time Factor. — Since it seems established that Niagara Falls 
have retreated throughout the seven miles between the escarpment and 
the present cataract, and since the rate of present recession is known, 
it might seem a simple task to determine the length of time required 
for this recession, and, therefore, to fix the date of the disappearance of 
the ice of the Glacial Period from this part of North America. As a 
matter of fact, numerous calculations have been made, some of them 
as low as 3000 to 12,000 years, while others have reached so high a 
figure as from 50,000 to 100,000 years. The reason for this difference 
is the entrance into the problem of a series of factors of unknown 
value, such, for instance, as (a) the increase in thickness of the lime- 
stone toward the south, and (b), what is more important, the variation 



Plate II 




NIAGARA FALLS AND GORGE 

rhe waterfalls separate the broad, shallow river above Goat Island from the deep, narrow gorge be- 
low. Contour interval 10 feet. (From Topographic Map of the Niagara Gorge, by the United 
States Geological Survey and the Canadian Geological Survey.) 



RIVERS AND RIVER VALLEYS 131 

in river volume during the closing stages of the Glacial Period, when 
for a time the upper Great Lakes found other outflows than that 
through Lake Erie. Five stages of variation in the volume of Niagara 
are indicated in Figs. 75 and 198. 

A third important cause for variation is the fact that, as the cataract 
receded, it found at the whirlpool a juried gorge of an earlier stream 
filled with glacial drift. This unconsolidated material it quickly 
swept out, and therefore, for a time, the rate of recession was far more 
rapid than normal. Until the full value of each of these three and 
other variable factors is known, any attempt at a final accurate state- 
ment of the time required to excavate the Niagara gorge can possess 
little value. The estimate of 35,000 years, made by Lyell in 1841 
and verified by Taylor in 1913 upon the basis of very careful calcula- 
tions, is probably essentially correct. 

The Future of Niagara. — At the present relative rate of recession 
of the Canadian and American Falls it can be a matter of but a short 
period before the water of the American Fall is diverted from its 
present course. There are, in fact, indications that this is even now in 
progress ; and man, by diverting water for power purposes, is aiding in 
the process of extinction of the American Fall. If the present condi- 
tions persist, as there is every reason to believe they will, the American 
Fall will become extinct, as that at Foster Flats has become, and the 
combined waters will form a huge cataract which will continue to 
recede southward. There will come a limit to its recession, however, 
for as the limestone dips into the ground, the difference in resistance 
to which the fall is due will finally disappear. The cataract will then 
change to a rapid, and perhaps this will be worn down to an even 
grade. In any event, there can be no recession of Niagara southward 
till it taps Lake Erie, as some have suggested. 

Niagara as an Example or a Young River 

Before the Glacial Period there was no valley along the present 
course of the Niagara. It, therefore, furnishes a good illustration of 
the work which such a river can perform in a short time and of the 
resulting valley. 

The Broad, Shallow, Upper River. — One of the most notable 
features of the valley is its abrupt and absolute contrast of form above 
and below the cataract. Above, for fifteen miles, it is a broad stream 
with moderate current, excepting at one point where there is a rapid, 
and again near the crest of the falls where there is another rapid. In 
this course the stream flows almost on the surface of the plain, having 
cut away little more than the surface cover of loose glacial deposit. 
The reason why, notwithstanding its great volume, this large river 
has not entrenched itself more deeply in the plain is primarily the fact 
that it bears almost no sediment for use as cutting tools ; for the waters 
have been filtered of sediment in Lake Erie, and no large muddy stream 



I 3 2 



COLLEGE PHYSIOGRAPHY 



contributes sediment along its course. Where the river crosses a bed 
of limestone, below Buffalo, and again where it crosses the Niagara 
limestone, just above the crest of the falls, solution has removed 
some of the rock in the stream bed, roughening it and increasing its 
slope so that slight rapids have developed (Fig. 74). 

The Deep, Narrow Gorge. — Then comes the mighty plunge of the 
huge volume of water, which, thus concentrated, is able to remove 
the shale and sandstone and undermine the limestone. Thence, 
for about seven miles, the water flows rapidly through a gorge or small 
canyon, whose depth, including the part below river level, ranges from 
390 feet near the falls to 490 feet at the escarpment. Its width is 




Fig. 74. —The broad, shallow Niagara River above the falls and the narrow gorge below. 



from 725 to 1900 feet (PI. II). Below the gorge the current is again 
moderate, and steamboats from Lake Ontario can ascend it toLewiston. 
In this part the river has excavated its bed down to the temporary base- 
level of Lake Ontario, below which it cannot cut so long as the lake 
exists. 

The rapids in the gorge are due to the fact that, with its slight sedi- 
ment load, the river has not yet been able to cut down to grade. These 
rapids, together with variations in depth of the water and in width of 
the gorge (Fig. 75) are associated with the variations in volume, as the 
outlets of the upper Great Lakes changed during the recession of the 
ice in the closing stages of the Glacial Period mentioned above. 

The Whirlpool. — At one point the gorge bends abruptly at ap- 
proximately right angles, and in the elbow of the bend is the only part 
of the gorge which is not rock-walled. This is the site of the buried 
gorge discovered as the cataract receded ; and, since the drift filling 
at its end has been removed more easily than the rock wall, this elbow 
has been extended some distance into the buried gorge. Into this 
the current of the river is directed, and in it the water swirls around in 
a great eddy, called the Whirlpool. Above the Whirlpool are the most 



RIVERS AND RIVER VALLEYS 



*33 



notable rapids in the river, the Whirlpool Rapids, the exact origin of 
which is not yet determined, though there is some reason for believing 
them to be due to the presence of large limestone boulders which 
dropped into their present place from a waterfall which existed here 




Fig. 75. — Map showing the several divisions of Niagara Gorge. (Taylor.) 



before the last advance of the ice when the buried gorge was being 
formed. If this be the correct interpretation, then the buried gorge 
which passes out at the end of the whirlpool elbow extended along the 
site of the present gorge up to the railway bridges, where it terminated 



i 3 4 COLLEGE PHYSIOGRAPHY 

in a waterfall that had receded this far when the glacier advanced, 
extinguished the stream, and filled its gorge with drift. Such an 
explanation would account not only for the rapids, but also for the 
narrowness of the gorge at this point, for in this part the Niagara would 
have had the task merely of sweeping out the glacial drift in a pre- 
existing gorge of a much smaller stream (Fig. 75). 

Other Types of Waterfalls 

Falls of Niagara Type Commonest. — The fact that horizontal, or 
nearly horizontal, strata occupy a large proportion of the land is one 
reason why falls of the type described in preceding pages are the most 
common. And another reason is that such falls persist for a long 
time by recession up-stream, as is so well illustrated in Niagara. There 
are, however, other causes for waterfalls. 

Consequent Waterfalls. — One of these is the presence of a sharp 
descent on a land surface over which water has only recently been led 
to flow, — falls consequent upon topography. In the beginning Niag- 
ara was a fall of this type, and there are many others in glaciated 
regions, for the glacier made changes in the topography ; it destroyed 
or altered many preexisting valleys ; and it turned many streams out 
of their former courses. Waterfalls therefore abound in regions of 
recent occupation by glaciers, and many of them are of the consequent 
type, while others have been normally developed, as deflected streams 
have cut gorges along their courses, and thereby discovered irregulari- 
ties in rock resistance. Some of these falls are more specifically re- 
ferred to in the chapters on glacial action. 

Influence of Joint Planes. — As streams cut into their beds, any 
difference in resistance of the rock will give rise to differential cutting. 
Besides the influence of horizontal beds, there are two other conditions 
that are noteworthy in fall and rapid development, one the influence 
of joint planes, the other the influence of inclined beds. It is not 
uncommonly the case that joint planes may be so spaced as to aid in 
vertical excavation by running water. At Taughannock Falls, in 
central New York, for example, the shale rock is so thoroughly 
cleaved by joint planes that, although the strata are horizontal, 
great slabs fall away from the cliff face where loosened along the 
closely set, vertical joint planes. As a result of this vertical cleavage 
a waterfall 220 feet high has been developed by a small stream which 
is cutting a gorge in the steep hill slope of Lake Cayuga valley 
(Fig. 76). 

Waterfalls in Lava. — Joint planes aid in the development of water- 
falls in many lava rocks, and, by the irregular erosion of the water 
along the planes, give rise to the irregular outline of such falls. Thisj 
influence is well illustrated in the Victoria Falls of South Africa and! 
in the Shoshone Falls of Idaho. In both cases the falls are developed 
in the course of gorge cutting in the lava, in the first case by the Zam- 



RIVERS AND RIVER VALLEYS 



135 







136 



COLLEGE PHYSIOGRAPHY 



bezi River, in the second 
by the Snake River ; and 
in each case there is an 
extensive gorge, or can- 
yon, below the fall (Figs. 

77, 78). 

Falls in Dipping 
Rocks. — Difference in 
resistance of layers of 
rock that are inclined 
will give rise to falls or 
rapids as the water wears 
them away differently. 
But such falls differ 
materially from those 
in horizontal strata. 
Among the noteworthy 
differences is the manner 
of recession. The fall 
in horizontal strata re- 
cedes up-stream and may 
maintain or even in- 
crease its height as it 
recedes ; but the fall 
developed in vertical 

strata, as it is worn away, loses height and recedes vertically. 

Between these extremes there is every gradation as the inclination 




KlG. 



77. — Map of the canyon below Victoria Falls. 
(After Encyclopaedia Britannica.) 




FlG. 78. — Victoria Falls, South Africa 



RIVERS AND RIVER VALLEYS 



137 



of beds varies. It follows, therefore, that a fall in inclined strata 
has a shorter life than one in horizontal strata. 

Yellowstone Falls. — Of the falls due to vertical differences in rock 
resistance to river erosion the Lower Yellowstone Falls may be taken 
as a typical example. Here a vertical mass of more resistant lava 
crosses the weaker massive beds in which the great varicoloured canyon 
of the Yellowstone below the falls is excavated. The work of excava- 
tion is held back by the resistant layer and the water falls in a massive, 




7Q 



View down the canyon of the Yellowstone River from the brink of the lower 
falls. (Killers, U. S. Geol. Survey.) 



beautiful sheet into the canyon which it has excavated in the weaker 
lava. Gradually, however, the resistant layer will be cut away, and, 
as it is worn down, the crest of the fall will steadily sink (Fig. 79). 

Law of Waterfall Formation. — The manner of development of a 
waterfall in a stream may be stated as a law, as follows : when a stream, 
in seeking its grade, discovers sufficient difference in the resistance of the 
rocks in its bed, a waterfall (or rapid) will develop. In both vertical 
and horizontal strata the fall will persist so long as the water, in 
excavating, discovers sufficient differences in resistance to cause an 
abrupt descent in the bed. When grade is reached, no matter what 
differences may exist in the rock of the river bed, no fall or rapid can 
exist. 



i 3 8 COLLEGE PHYSIOGRAPHY 

Law of Waterfall Extinction. — The disappearance of waterfalls may 
therefore also be stated as a law, as follows: when the stream grade 
intersects the jail producing layer, the waterfall will disappear. Since 
the grade will intersect a vertical fall-producing layer directly beneath 
the point at which the fall started, much less cutting, and therefore 
much less time, will be required to cause its disappearance than in the 
case of a fall in horizontal strata which must retreat far up-stream 
before the grade intersects it. 

Importance of Waterfalls 

Advantages and Disadvantages to Man. — From one standpoint 
waterfalls are a disadvantage, from another an advantage, to man. 
Niagara illustrates both cases well. It is the greatest single obstacle 
to travel up and ^down the Great Lakes-St. Lawrence waterway, and 
to surmount it expensive canals have been constructed. On the other 
hand, the force of its falling water has come to be a source of enormous 
power for use in manufacturing, estimated as 4 million horse power. 
Formerly such power was of use only locally, but now it may be trans- 
formed to electric power and transmitted by wire far and wide. Thus 
Niagara power is used not only at Niagara Falls, but at Buffalo, and 
even in central New York. 

Water Power. — The value of water power is illustrated in scores 
of manufacturing towns and cities in the United States, in New Eng- 
land, for example, and at Rochester, at Minneapolis, at Keokuk, 
Iowa, on the Mississippi, in the Fox River valley of Wisconsin, and 
many other places. Such power has been a factor of great signifi- 
cance in the industrial development of many nations ; and now, by 
the aid of electric transmission, it seems about to enter upon a new and 
even greater usefulness. 

Canals around Waterfalls. — As an obstacle to navigation, water- 
falls are of importance only in the larger rivers, as at Minneapolis 
on the Mississippi, Louisville on the Ohio, the cataracts of the Nile, 
and the falls of the Kongo. When the demands of transportation are 
sufficient, the effect of the obstacle can be readily minimized by human 
effort, as the Canadians have done in their canal building around the 
rapids in the St. Lawrence, the English in railroad building along the 
Nile, and the people of the United States by canal at Louisville, by 
rail at Minneapolis, and by the same means in other places. 

References tp Literature 

R. M. Brown. Bull. Amer. Geog. Soc, Vol. 34, 1902, pp. 371-383; ibid., 
Vol. 35, 1903, pp. 8-16; ibid., Vol. 39, 1907, pp. 147-158; ibid., Vol. 44, 
1912, pp. 645-657; ibid., Vol. 45, 1913, pp. 500-509. 

V. Cornish. Progressive Waves in Rivers, Geog. Journ., Vol. 29, 1907, pp. 

23-31- 
N. H. Darton. Bad Lands of South Dakota, Nat. Geog. Mag., Vol. 10, 1899. 

PP- 339-343- 



RIVERS AND RIVER VALLEYS 139 

A. P. Davis. The New Inland Sea, Nat. Geog. Mag., Vol. 18, 1907, pp. 37-49- 
W. M. Davis. An Excursion to the Grand Canyon of the Colorado, Bull. 
Mus. Comp. Zool., Vol. 38, 1901, pp. 108-201 ; An Excursion to the Pla- 
teau Province of Utah and Arizona, ibid., Vol. 42, 1903, pp. 1-50. 

C. E. Dutton. The Physical Geology of the Grand Canyon District, 2d Ann. 

Rept., U. S. Geol. Survey, 1882, pp. 47-166; Monograph 2, U. S. Geol. 

Survey, 1882, 264 pp., and atlas. c 
H. C. Frankenfield, Bull. Y, U. S. Weather Bureau, 1913. 
A. Geikie. Text-book of Geology, 4th edition, Vol. 1, 1903, p. 589. 
G. S. Gibbs. The Breaking Up of the Yukon, Nat. Geog. Mag., Vol. 17, 

1906, pp. 268-272. 
G. K. Gilbert. Niagara Falls and their History, Nat. Geog. Monographs, 

New York, 1896, pp. 203-236; Rate of Recession of Niagara Falls, Bull, 

306, U. S. Geol. Survey, 1907, 31 pp. 
L. C. Glenn. Denudation and Erosion in the Southern Appalachian Region 

and the Monongahela Basin, Prof. Paper 72, U. S. Geol. Survey, 191 1, 

171pp. 
A. W. Grabau. Guide to the Geology and Paleontology of Niagara Falls 

and Vicinity, Bull. 45, N. Y. State Museum, 1901, 284 pp. 
J. W. Gregory-. Constructive Waterfalls, Scottish Geog. Mag., Vol. 27, 191 1, 

PP- 537-546. 
L. E. Hicks. Some Elements of Land Sculpture, Bull. Geol. Soc. Amer., 

Vol. 4, 1893, pp. 133-146. 
W. G.Hoyt. Effects of Ice on Stream Flow, Water Supply Paper 337, U. S. Geol. 

Survey, 1913, 77 pp. 
A. A. Humphreys and H. L. Abbott. Report upon the Physics and Hydraulics 

of the Mississippi River, Prof. Papers 4 and 13, Corps of Topographic 

Engineers, Philadelphia, 1861, and Washington, 1876. 

D. W. Johnson. A Geological Excursion in the Grand Canyon District, 

Proc. Boston Soc. Nat. Hist., Vol. 34, 1909, pp. 135-161. 
G. W. Lamplugh. The Gorge and Basin of the Zambezi below the Victoria 

Falls, Rhodesia, Geog. Journ., Vol. 31, 1908, pp. 133-152, 287-303. 
H. G. Lyons. On the Nile Flood and its Variations, Geog. Journ., Vol. 26, 

1905, pp. 249-272, 395-421. 
W J McGee. Sheetflood Erosion, Bull. Geol. Soc. Amer., Vol. 8, 1897, pp. 

87-112. 
D. W. Mead. Notes on Hydrology, Chicago, 1904, 202 pp. 
A. J. C. Molyneux. The Physical History of the Victoria Falls, Geog. Journ., 

Vol. 25, 1905, pp. 40-55. 
C. C. O'Hara. The Badland Formations of the Black Hills Region, Bull. 

9, South Dakota School of Mines, 1910, 152 pp. 
J. W. Powell. Exploration of the Colorado River of the West, Washington, 

1875, 2 9 J PP- 5 Canyons of the Colorado, Meadville, Pa., 1895. 
T. Mellard Reade. Addresses, Liverpool Geol. Society, 1876 and 1884. — 

See Geikie's Textbook of Geology, 4th edition, Vol. 1, 1903, p. 489. 
I. C. Russell. Rivers of North America, New York, 1898, 327 pp. 
N. S. Shaler. Aspects of the Earth, New York, 1900, pp. 143-196. 
J. W. Spencer. The Falls of Niagara, Dept. of Mines, Canada, 1907, 482 pp. 
R. S. Tarr. Physical Geography of New York State, Chapters V, VIII, IX, 

New York, 1902, pp. 155-192, 240-299; Prof. Paper 64, U. S. Geol. 

Survey, 1909, pp. 122-123. 

F. B. Taylor. Origin of the Gorge of the Whirlpool Rapids at Niagara, Bull. 

Geol. Soc. Amer., Vol. 9, 1898, pp. 59-84; Folio 190, U. S. Geol. Survey, 
I 9 I 3- 
Water Supply Papers. For floods, see U. S. Geol. Survey, Nos. 88, 1903; 96, 
1904 ; 234, 1909 ; and 334, 1913 ; for volume and load of rivers in United 
States, see other Water Supply Papers, as in Nos. 44, 93, and 289. 

G. M. Wheeler. Ascent of the Colorado River in 1871, Report upon U. S, 

Geog. Surveys, Eng. Dept., U. S. Army, Vol. 1, 1889, pp. 156-171. 



140 



COLLEGE PHYSIOGRAPHY 



Rock Springs, Wyo. 



Bright Angel, Ariz. 
Kaibab, Ariz. 
Boise, Idaho. 



Glassboro, N.J. 
Niagara Gorge, N.Y. 
Fargo, N.D. 



TOPOGRAPHIC MAPS 

Bad Lands 
Fort McKinney, Wyo. 

Canyons 

Shinumo, Ariz. 
Higbee, Colo. 
Bisuka, Idaho. 

Immature Drainage 
Barnegat, N.J. 

Waterfalls 
Quincy, Wash. 

Young Valleys 
Marshall, Ark. 



Scotts Bluff, Neb. 



Vishnu, Ariz. 
Abajo, Utah. 



Norfolk Special, Va. 



Great Falls, Mont. 



Hartford, Conn. 



CHAPTER VI 

RIVER DEPOSITS 

River Bed Deposits 

Stream Beds. — In the course of transportation of its sediment load 
it commonly happens that a river deposits a part of its burden in its 
bed, though usually only temporarily and in spots. For this reason 
even the bed of a stream which is rapidly cutting a gorge will, if exposed 
to view, be found to consist not solely of bare rock, but in part of loose 
rock fragments which, for one reason or another, the stream had found 
it necessary to leave on its bed. 

Deposits in Slowly-moving Water. — Among the various causes 
for such deposit the most important is change in velocity. A stream 
in flood may be sweeping along a quantity of sediment or rock frag- 
ments of a size which cannot be moved when the flood subsides. As 
the velocity diminishes with the subsidence of the flood some of this 
load must be left along the stream bed. As has been pointed out, a 
stream current is irregular, with some places of rapid flow, and others 
of quieter water ; and the sediment which can be moved in the more 
rapid stretches may come to rest in the quieter pools. Or a tributary 
stream with steep slope may bring fragments of a size which the 
main stream cannot move onward. This finds excellent illustration in 
the Grand Canyon of the Colorado, where some of the tributary streams 
have brought so many large fragments that the river is ponded back 
by them in lake-like expanses, while the current over the obstruction 
quickens to a violent rapid, — the greatest obstacle to navigation of 
the river by a small boat (Fig. 66). 

Where streams flow through unconsolidated deposits of glacial drift, 
in which there are also large fragments of resistant rock, it is often 
the case that the smaller materials are removed, while the larger remain 
in the stream bed because they cannot be moved by the current. In 
such cases the stream bed may become a mass of boulders, which, 
however, the current is slowly wearing down. 

Sand Bars. — River bed deposits of the kind described above are of 
no great importance, for they are of limited extent and very local as 
well as temporary. But in streams that are heavily burdened, and 
especially in those that are so overburdened that they are aggrading 
their beds, this class of deposit assumes considerable importance. 
In the Platte River, for example, sediment is steadily being deposited, 

141 



142 COLLEGE PHYSIOGRAPHY 

and the bed is steadily rising. The deposit is not made in a uniform 
sheet, but more at some stages than at others, and more in some parts 
of the bed than at others. At one spot, where the current is slack, 
deposit may begin and a sand bar commence to develop, while on 
either side of it the current flows in a channel which its velocity is able 
to maintain. But as deposit proceeds the size and form of the sand 
bars change, and the channelways shift in position and vary both in 
size and in the volume of water. Therefore the river flows not in a 
single channel but in a multitude of anastomosing channels, which, 
together with the sand bars that separate them, are forever varying. 
Such a stream, split into many branching and reuniting channels, is 
called a braided stream (Fig. 80). 




Fig. .80. — Braided stream courses in Alaska. 

Obstacles to Navigation. — The formation of sand bars in a naviga- 
ble stream is a menace to navigation, for their form and position are 
almost constantly changing. This is well illustrated in the lower 
courses of all large rivers, and is well described in Mark Twain's 
" Life on the Mississippi." Although not written as a treatise on 
geography, this gives an excellent word picture of the subject under 
consideration, as well as of other features of the navigable Mississippi. 
The stranding of a tree, or a slight variation in the current, may in a 
few hours cause a sand bar to develop where before there was naviga- 
ble water. The heavy sediment load which the Mississippi bears and 
the consequent deposit which it is making offer the most serious 
obstacle to the utilization of this great waterway as a highway of 
commerce. The difficulty exists all the way from the river mouth 
to the mouth of the Missouri, which is the contributor of the bulk of 
its sediment load. 



RIVER DEPOSITS 



H3 



Floodplains 

River Banks. — A river channel is bounded by banks, between which 
the river water commonly runs. Sometimes these are so steep and 
high and so near together that, as in some gorges, the river is always 
confined between them ; but usually the banks that confine the river 
at ordinary stages of water are so low that at times of flood the water 
overflows them and spreads beyond them. Even in most gorges, as a 
result of lateral swinging, the gorge bottom. is broader than the normal 
river channel, so that at times of flood the water spreads beyond the 
channel bank on one or both sides. 

Deposits in Shallow Water. — During such an overflow the stream 
is apt to be most heavily sediment-laden ; but, as the water spreads 
out beyond the banks, the current there is checked, because the stream 
is shallower than in the channel. Accordingly it may not be able 
to carry all the sediment which it bore before leaving the swifter 
channel portion. Then some must be deposited. The same process 
may be seen during a heavy rain, or the rapid melting of snow, when a 
swift stream in a gutter overflows the sidewalk and leaves a layer of 
sediment which its shallower current could not transport. 

The Nature of Floodplains. — As a result of this process little strips 
of plain are built up even in a gorge, usually first on one side of the 
stream, then on the other side, as the stream swings away from the 
gorge wall, or, where the stream flows in the middle of the gorge, on 
both sides. Such a plain is a floodplain, for it is made by the floods ; 
and it is a plain, because its surface cannot rise above the level to which 
the floods rise. In a 
gorge it may be a very 
rough, and a very small 
plain, and it may be 
made of coarse gravel. 

Where streams flow 
in more open valleys 
and the valley walls are 
farther apart, the flood- 
plain strips are wider ; 
and if the slope of the 
stream be not too great, 
they are composed of 
fine gravel, or sand, or 
even of clay. But the most extensive floodplains are those that 
develop along streams which, for one reason or another, are aggrading 
their beds. Such a stream, like the lower Mississippi, at ordinary 
stage of water flows in a well-defined channel, bordered by low banks ; 
but when floods come, and the channel can no longer hold the volume 
that is supplied, the water rises over the banks and spreads out in a 
sheet over the bordering lands. Here by the slackness of the current 




- Floodplain of the Missouri River, showing where 
sand bar deposition is still adding to it. 



144 COLLEGE PHYSIOGRAPHY 

and by the interference with its motion caused by the vegetation, some 
of the sediment is of necessity deposited. Thus with each flood the 
level of the flooded land is raised, and gradually a plain is built on 
either side of the river. The sediment may accumulate to a depth of 
scores or even hundreds of feet, but all the time the form of the plain 
is maintained. Its extent is limited by the distance to which the floods 
can reach on either side of the river, and very often this is the wall of 
the valley which the stream is aggrading. In that case the floodplain 
terminates against the base of a bluff, or of a hillslope which is gradu- 
ally being submerged by the rising floodplain (Fig. 81 and PL III). 

Excessive Stream Load. — The condition under which floodplains 
develop is the presence of more sediment in time of flood than can be 
carried over the flooded lands. The majority of streams fulfil this 
condition, but to form extensive floodplains there must be frequent 
overflow and an abundance of sediment. The former depends upon 
variation in run-off, and is normal to most large streams in their lower 
course where the slope is so gentle that the channel is not competent 
to dispose of a great addition to the volume. The sediment is also 
normal to most large streams, for into some, if not all, of its tributaries 
there is almost certain to be a notable inwash of sediment with the 
run-off. For these reasons floodplains are typical phenomena of the 
lower courses of large streams. 

Causes for Deposition. — The aggrading condition, under which the 
largest floodplains are developed, may result from several causes, as a 
result of which a river, after having excavated a valley, may proceed 
to aggrade it by floodplain deposit. One of the most common causes 
for the change from a condition of cutting to one of building up is the 
increase of the sediment supply. This may result from a change in 
rainfall conditions among some of the tributaries, on account of which 
the run-off sweeps along a larger burden of sediment, thus overburden- 
ing the lower, gently sloping course of the river. Or man, by stripping 
off the forest, may aid in giving the river a heavier burden of sediment 
than it can transport to the sea. But more common than either of 
these is the increase in surface from which sediment can be washed 
as the valleys of a river system are developed, cutting into land with a 
multitude of slopes down which sediment may be washed. Thus, 
floodplains are to be expected in the normal development of a valley 
system by the increase in sediment supply without a corresponding 
increase in run-off. If the sediment supply remains the same, while 
the rainfall diminishes, an overburdened condition may result, just 
as well as it may by increasing the sediment without a corresponding 
increase in the volume. 

Effect of Change of Slope. — Another cause for an overburdened 
condition of a river that formerly was able to dispose of its sediment 
load, and even deepen its valley, is a change in slope. If a stream is 
graded to a certain volume of water and sediment, and then, by change 
in the level of the land, its slope is decreased, sediment deposit must 



Plate III 




WILLIAMS ENGBAVIN6 CO.. 



MISSISSIPPI RIVER 



The floodplain within the trench or gorge of the Mississippi at La Crosse, Wisconsin, with bayous and 
floodplain lakes. Contour interval, on the bottom land 5 feet, and on the bluffs 20 feet, with 
steep cliffs hachured. The disadvantage of two contour intervals in the same map is shown 
south of La Crosse where the bluff at the edge of the low terrace on which the city is located 
erroneously appears to be nearly as high as Grandfather Bluff. (From Charts 172 and 173, 
Mississippi River Commission, Engineer Corps, U. S. Army.) 



RIVER DEPOSITS 



H5 



take place. A similar result is reached by the building of a delta at a 
river mouth. For this disturbs the equilibrium of the stream by add- 
ing a level tract at its lower end, over which the water must flow with 
its sediment load. It cannot flow over a level surface, and must, 
therefore, grade up its bed so as to introduce a slope. This grading 
would pond back the waters of the river above the delta if it did not 
also extend up-stream. So, as a .delta grows, a process of grading is 
introduced, not only on the delta, but far up-stream from it, for the 
river will undertake to adjust its grade to the new conditions. In .this 




Fig. 82.- 



Map of floodplain deposits in California. Contour interval 5 feet. 
Quadrangle, U. S. Geol. Survey.) 



(Palermo 



adjustment aggrading takes place, and with it an extension of flood- 
plain development. 

Deposits Greatest near River Mouths. — Floodplains are broadest 
and most extensive in the lower portions of large streams which have a 
heavy burden of sediment. Being in such situations, they are com- 
monly made of fine-grained sediment, such as clay, since that is the 
size of particle which a current in that part of a large river can trans- 
port in suspension, for that which is dragged along the bed cannot be 
carried out on the floodplain in large quantities. Because of the 
levelness of the land, the fine texture of the soil, the fertility of the soil, 
the fertilizing of it by frequent overflow, and the dampness of the 
ground, large floodplains are favourable to agriculture, and many flood- 
plains, especially in Asia, are the seat of a dense agricultural population. 



i 4 6 COLLEGE PHYSIOGRAPHY 

Natural Levees. — The menace of the floods is a serious obstacle to 
successful occupation of such floodpiains, though it is lessened to some 
extent by the presence of a low ridge on either side of the channel, 
called the natural levee. This levee may slope 5 to 10 feet per mile, as 
in the Mississippi, where it is 15 times as steep as the slope of the 
floodplain. 

This levee serves as a partial protection against floods, and because 
of its height the soil is better drained than in the backwater swamp 
outside (Fig. 328). The natural levee of the lower Mississippi was 
early settled and is even now more fully occupied than the floodplain 
beyond. It was upon the natural levee that New Orleans was built, 
and both above and below this city the river is bordered on each side 
by a succession of plantations on the natural levee. How much of an 
embankment it is, may be seen by the fact that it prevents the entrance 
of some of the streams that flow toward the Mississippi, but take 
independent courses to the sea along the low floodplain beyond the 
natural levee. The Yazoo River is deflected for about 200 miles 
before it finally enters the Mississippi where the latter swings over 
against its valley wall at Vicksburg. 

Cause of Natural Levees. — The cause for the natural levee is, 
first, the fact that coarser and more sediment can be deposited near 
the channel than on the more remote parts of the floodplain, and, 
secondly, that floods more often rise upon it. Only the greatest floods 
overspread the entire floodplain, whereas moderate floods are able to 
bring sediment to the natural levee. 

Artificial Levees. — Upon the natural levee artificial embank- 
ments, or levees, are built to still further check the floods by confining 
the river in its channel. By such artificial levees large tracts of flood- 
plain are rendered habitable that otherwise would be too frequently 
flooded for habitation. This is true in Holland, where dikes are built 
along the lower Rhine, and in Italy, where the Po is confined between 
the embankments, as well as along many other streams in both the 
new and old world. 



River Meandering 

River Courses. — On a large floodplain and on many small ones the 
river course is tortuous and even shifting. This is the result of lateral 
cutting, similar in character to that through which a stream broadens 
a gorge by lateral erosion. If a stream channel could be made perfectly 
straight with an absolutely perfect cross-section and be occupied by a 
current subject to no influence tending to divert it, the current would 
flow with greatest velocity along the middle and with least velocity 
along the banks. Such a channel might preserve a straight course. 
Manifestly, ideal conditions of this kind cannot exist in nature, for 
there are a number of ways in which streams normally depart from 
them. The stream channel is not straight ; the cross-section is not 



RIVER DEPOSITS 



147 




i 4 8 COLLEGE PHYSIOGRAPHY 

perfect, but, on the contrary, is very irregular as a result of deposit, 
or irregular erosion ; and there is thought to be an ever-present tend- 
ency toward deflection from a straight course by the effect of the 
earth's rotation, through which moving bodies are deflected toward 
the right in the northern hemisphere and toward the left in the southern. 

Lateral Erosion. — As a result of the actual conditions in the flow- 
ing water the current, instead of flowing with greatest velocity along 
the centre of the stream, is every here and there deflected from the 
centre toward the side. Lateral erosion, therefore, results. The 
effect of this lateral erosion becomes especially noticeable on large 
floodplains because (a) of the great volume, which gives the power 
for lateral cutting, (b) the slowness of the current, which makes deflec- 
tion from the straight line more easy than in swiftly flowing water, and 
(c) the lowness and softness of the banks which enclose floodplain 
streams. 

Cutting and Depositing. — If a stream with a straight course were 
established on a floodplain, deflection would at once commence. This 
deflection would consist of cutting at a certain point where the current 
was turned against the bank ; but the cutting could not continue far 
if there was not corresponding deposit on the opposite side, for, with- 
out that, the breadth of the channel would be increased and the current, 
therefore, diminished. As a matter of fact, filling on one bank goes 
hand in hand with cutting on the other. This gives rise to a steep 
bank on the side of cutting and a gently-sloping sand or silt bank on the 
other, on which vegetation may not be able to encroach as fast as 
deposit builds it out into the river. On the side of deposit the water 
is shallow, on the side of cutting it is deep. 

Cause of Meanders. — From the point of cutting the current is 
deflected, not abruptly, but along a curve, so that the cut face is a 
curve concave toward the river, while the built side is convex. Deflec- 
tion from the curved face of cutting swings the current over against 
the opposite bank lower down, and there another curve of cutting, 
with opposed curve of filling, is begun. This process continues until 
the river swings over its floodplain in a great series of curves, or 
meanders, a term derived from the small river Meander in Asia Minor, 
which flows in meandering course over its floodplain and delta 
(Fig. 253). 

Development of Meanders. — The perfection of the meander form 
depends upon the length of time during which a given curve or series 
of curves have been developing, the regularity of the deflecting force, 
and the uniformity of the floodplain deposits. In its most perfect 
form the meander is a horseshoe-shaped curve, or the curve of an ox- 
bow. But for one reason or another this curve may not be reached, 
or may have been passed, so that there is, in reality, great variety in the 
form of a meandering river from one curve to another. The size of the 
curve is limited partly by the volume, partly by the slope of the river. 
A large- volume stream develops large meanders, but the same sized 



RIVER DEPOSITS 



149 




Fig. 84. — Shifting of meanders in the Mississippi from 1881 (light lines) to 1907 (heavy 
lines) . Lakes Lee and Chicot are ox-bow lakes. (After Mississippi River Commission.) 



iSo 



COLLEGE PHYSIOGRAPHY 



stream would have even larger meanders with a gentler slope. On a 
small meadow brook a meander may be no more than 10 or 20 
feet across, while some of the meanders of the lower Mississippi River 
are 6 miles across and 16 miles around their circumference. 

Cut-offs. — The meander curve, in its normal development, while 
it grows outward to a certain limit on the cutting face, is constricted 
on the opposite side by the cutting action of the river. This con- 
striction is caused by attack on two sides, on the upper side by the 
current deflected from the meander next above, on the lower side by the 
deflected current from the meander itself. With the growth of the 
meander this constriction becomes so reduced in width that finally 

the current breaks across the 
neck and the meander curve is 
abandoned, while the river flows 
temporarily along a straighter 
course at this point, giving rise 
to an ox-bow cut-ojf. When the 
ox-bow curve is abandoned its 
ends are sealed by deposit of 
sediment, and a circular lake, 
called an ox-bow lake, is formed 
in the floodplain. Slowly such a 
lake is filled by sediment as the 
river floods spread over the flood- 
plain, and finally it is extermi- 
nated. On floodplains caused by 
meandering rivers all stages in 
the formation and extinction of 
85). Some abandoned courses 
along the lower Mississippi are called bayous. 

Shifting of Meander Belts. — By its meandering a river may swing 
back and forth across its floodplain, thus temporarily laying aside 
sediment, later to pick it up again and move it a step down-stream. 
The extent of swinging is, however, not limited to the width of the 
meander, for the belt of meandering also shifts back and forth across 
the plain. Thus, while the larger meanders of the Mississippi are 
about six miles across, along the longest diameter, the area over which 
the river swings is several times that distance. In considering a 
meandering river, therefore, there is both the swinging of the river in 
the individual meanders, and the swinging first one way, then the 
other, of the meander belt. 

Effects on Man. — Such constant shifting of a river course leads to 
constant change of great significance to those who dwell upon the 
floodplain of a meandering river. A farm may slowly be eaten away ; 
the boundary of a state or of a county may be changed ; and a town 
may be destroyed as the river cuts away its site, or it may be left far 
from the river on which it was built when the river swings away from it, 




Fig. 85. — Ox-bow lake in the Connecticut 
valley in Massachusetts. As late as 1833 
the Connecticut River still went around 
the meander. 

ox-bow lakes are found (Figs. 8, 



RIVER DEPOSITS 



iSi 



or leaves it abruptly by taking a new course along a cut-off. The 
Mississippi and Missouri rivers have given numerous illustrations of 
the abandonment and destruction of towns and farms by the meander- 
ing river. So powerful and persistent is the action of a great meander- 
ing river that man is quite helpless in his efforts to confine it and 
prevent it from continuing its meandering. 

River Towns and Meanders. — The meander belt of a rloodplain 
is commonly bordered by a bluff, against which, from time to time, the 
river swings. This bluff is usually the old valley side, but it has been 




Sr.Genev/eve 



Fig. 86. — Three stages in meander development of the Mississippi River at Kaskaskia. 

(After Emerson.) 



trimmed by the cutting of the fiver as the meander belt swings over 
to it. Upon such bluffs towns may be more safely built, as Vicksburg 
is on the Mississippi. But even these sites are unreliable, for a bluff 
town may be a river port to-day, and to-morrow the river may be 
several miles away. General Grant in his campaign against Vicks- 
burg undertook to isolate that city by leading the river by an artificial 
cut-off across the neck of the meander that swings against the bluff 
there ; but the river was not yet ready for the change, and the plan 
failed, although the stream cut off the neck of the meander about 
13 years later, leaving Vicksburg on the bayou. The site of the town 
of Kaskaskia (Fig. 86), once the capital of Illinois, has been com- 
pletely destroyed by a meandering stream. 



152 



COLLEGE PHYSIOGRAPHY 



Deltas 

Deposits at River Mouths. — Some of the sediment that a stream 
transports finds its way ultimately to the mouth. If the stream is 
tributary to another, the master stream must dispose of the sediment, 
though sometimes the task is too great and a deposit is formed at and 
below the junction. If the stream terminates on the land, the sedi- 




Various forms of deltas, the Nile, Yukon, Hoang Ho, and Orinoco. 



ment is necessarily deposited, thus giving rise to deposits whose 
characteristics are considered later. The other possibility is that the 
stream mouth is in a lake or in the ocean. In that case, the abrupt 
checking of the river current necessitates the deposit of the sediment, 
unless the water there is in sufficient motion to carry it away. This is 
not commonly the case, and therefore an accumulation of sediment is 
formed at the river mouth, often going to form a delta (Fig. 87). 



RIVER DEPOSITS 



153 



Causes of Deltas. — While such deposits are found at the mouths 
of many rivers, they are by no means universal ; they are, in fact, 
present at the mouths of only a small proportion of the rivers of the 
world. The conditions favouring delta formation are (1) a supply of 
sediment, (2) a checking of the current carrying the sediment so that 
it may settle, (3) sufficient stability, of the sea bottom to permit the 
deposit to rise to the level of the sea, and (4) a sufficient length of time 
for the deposit to accumulate at the river mouth. The salinity of 
ocean water is said to also aid in deposition. 




Fig. 



Four stages in accumulation at the mouth of Columbia River. (Putnam.) 



Streams without Deltas. — It is because of the absence of some one 
of these conditions that deltas are not formed at the mouths of all 
rivers. Some streams carry so little sediment that a delta has not 
been formed at their mouths, especially in those cases where currents 
exist in the body of standing water, or where the river has not long 
entered it at the present point. Niagara River is an illustration of this 
condition, for it has little sediment and has only in a recent stage 
discharged into the lake at the present point. Many streams, espe- 
cially those entering the open ocean, have their sediment distributed 
far and wide by the waves and currents. But an even more important 
cause for the absence of deltas is the recent subsidence of the land so 
that river mouths are submerged. A sinking sea bottom will soon 
lower a delta below sea level, and then even a moderate subsidence will 
suffice to exceed the rate of sediment accumulation. Many coasts, 



i 5 4 COLLEGE PHYSIOGRAPHY 

like those of northeastern America and northwestern Europe, have in 
recent geological time suffered subsidence, and, speaking generally, 
the streams of these coasts have not yet had time to build deltas in 
the new position of their mouths. 

Effect of Quiet Water. — While deltas are not absent on open 
coasts, they are most numerous and more perfectly developed at the 
mouths of streams which enter lakes and enclosed or partially enclosed 
seas. This is not because such rivers have more sediment, nor can it 
be due, except in small degree, to the lesser depth and the greater 
stability of the bottom of such seas. The main reason is apparently 
the fact that in such seas the waves and currents are less effective in 
removal of sediment, and therefore the sediment load is concentrated 
in a deposit at the river mouth. 

The Steep Delta Front. — Since the delta is due to the checking of 
the current which brings the sediment, its front lies beneath the sea 

'2 \ feet above sea level . . T °P 3 f b «*s End f j etties 

/iZmnes from end of jett.e s J Present land surface \ 

Land surface of 1813 (100 years"ago)~ 

Horizontal scale 
1 2 3 4 5 Mile 



Vertical scale 
O 100 200 300 400 500 Feet 
i 1 1 1 1 1 




Fig. 89. — Cross section of Mississippi delta. (Shaw.) 

at the point where the sediment can be carried no farther. This front 
is steeply sloping, and as the delta grows outward it advances, always 
maintaining its steep slope at the point where the abundant sediment 
settles. The layers that are deposited here have an inclination sea- 
ward, and they lie upon more nearly horizontal layers made of the 
finer sediment that had reached this part of the sea bed before the 
delta advanced over it. 

The Flat Delta Surface. — Back of the steep front deposit continues 
until the delta surface is built up to sea level, and then it is raised 
higher still by the work of the waves which push back the sediment 
before them and throw some of it up in beaches or bars. Then, as 
the river rises in flood, it overflows the delta land and raises it higher 
still by floodplain deposit. Thus, nearly horizontal layers are de- 
posited on the inclined delta layers as well as below them. The struc- 
ture of a delta is illustrated in the diagram (Fig. 89). The form is 
that of a flat-topped plain at or above the level of the body of water 
in which it is built, and extending out beneath shoal water to a steep 
front, which abruptly descends to the level of the bottom of the lake 
orsea. The full form of the delta cannot ordinarily be seen, though 
it is known by soundings ; and in some cases where lakes have formerly 
stood, perfect fossil deltas may be seen, with their typical steep fronts 
and flat tops. Artificial deltas, with all the characteristics described, 



RIVER DEPOSITS 155 

may easily be made by causing running water to carry sediment from 
a sand pile into a small body of standing water ; and perfect deltas 
of small size may often be seen, after a freshet, in the little roadside 
pools. 

Relation to Floodplains. — The Nile illustrates typically the build- 
ing of a delta. The sediment-laden water is checked by the Mediter- 
ranean, and the deposit is there building the land outward. As the 
plain is formed the slope of the river, both over the delta and above 



LAKE rl< SS >'" 

* POA<rcmRrR#m, 







30 



• •• <^ 

GULF OF MEXICO ^ *o° p 



Fig. go. — The Mississippi delta. (After U. S. Coast and Geodetic Survey.) 

it, must be increased so as to permit the water to flow to the sea, other- 
wise it would be covered by a sheet of water, as it is when the volume 
is so increased during its annual floods that the river channels cannot 
carry it all. Therefore, both the river bed and the delta plain are 
raised, and the latter is transformed to habitable land, sloping toward 
the sea. Near the sea this new land is still swampy because not yet 
built up and given a slope by sediment deposit. As the delta is formed, 
the river above the delta must also correct its grade, as already stated, 
and during this aggradation floodplains develop. Indeed, the delta 
itself is ultimately transformed to a floodplain. One does not com- 



156 



COLLEGE PHYSIOGRAPHY 



monly think of the lower Mississippi from the mouth of the Ohio to 
the Gulf as a delta ; yet this is what it is in fact, though graded up by 
floodplain deposit. 

Distributaries. — Even with the grading that occurs on deltas, the 
delta plain of a large river is often so level that even the water of 
ordinary stages cannot find escape through a single channel. In 
consequence of this fact the river splits and flows over the level, lower 




LEGEND 



Depth 
more than 35 feet 

















Depth 
30-35 feet 

Depth 
25-30 feet 




Depth 
20-25 feet 
















Depth 
15-20 feet 






r-Ir^rri: 




Depth 
Jess than 15 feet 




^^^ 




500 


Mud lump 
Feet 

O 



Fig. 91. — Map of one of the mouths of the Mississippi, showing location of mud-lumps. 

(After Shaw.) 



portion in two or more channels, or distributaries, while in the time of 
flood the whole delta is inundated. It is common to consider the river 
delta as merely that portion below the point of splitting of the channel, 
and it was on this basis that the term delta originated from the resem- 
blance of the Nile below Cairo to the Greek letter delta A. 

Man may prevent deposition of sediment in distributaries by build- 
ing jetties and quickening the current, so that the river scours rather 
than deposits. This was done on the Mississippi by the engineer, Eads. 

All Deltas not Triangular. — There is much difference in the number 
and position of the distributaries of deltas, and there is much difference 



RIVER DEPOSITS 




Fig. 02. — The delta of the Danube. 



also in the delta form and composition. When allowed to develop 
freely, a form approaching that of the Nile delta is common, both in 
large streams and in small. But where the growth is interfered with, 
there may be wide departure from that form. A delta, for instance, 



1 58 COLLEGE PHYSIOGRAPHY 

may be formed in a valley enclosed by mountain walls, and its lateral 
boundaries are therefore determined by it. This is illustrated in the 
delta of the Mekong River in Siam ; and it is frequently illustrated in 
the deltas of inlet streams of lakes enclosed between valley walls, as 
at Ithaca, N.Y. 

Mud-lump Development on Mississippi Delta. — It was pointed 
out nearly 50 years ago by Sir Charles Lyell, and later amplified by 
Hilgard, that the Mississippi delta is exceptional in its bird's foot 
terminus (Fig. 90). This is due to the rising of low mud-lumps, which 
have temporarily closed the mouths of several of the passes. The 
lumps (Fig. 91) are masses of tenacious, difficultly eroded clay. 
They are domed up at times of high water when sediment load of 
coarser clay is laid down faster than the creep of the finer and more 
fluid clay below can compensate for, or else where the seaward flow 
of semi-fluid clay is opposed by resistant foreset beds. Marsh gases 
escape from those mud-lumps, but the rise is not thought to be due to 
the gas. They form a serious obstacle to keeping the channels open, 
and to them is ascribed the peculiar form of the delta terminus. 

Effect of Waves and Currents. — Another condition interfering with 
the perfect development of deltas is the effect of waves and currents. 
In Lake Cayuga, in central New York, the deltas are so modified by 
waves and currents that they are pointed on the outer end, and some- 
times the points are turned away from the prevailing direction from 
which the waves come. The Rio Grande, which pours much sediment 
into the Gulf of Mexico, has succeeded merely in projecting the coast 
slightly as a rounded point ; and much of its sediment load has found 
a resting place in the sand bars which sweep northward along the Texas 
coast. Great rivers are capable of building deltas against the waves, 
however, as in the case of the Mississippi. Even here the resistant 
sediment of the mud-lumps may affect the case, though in the 
Ganges-Brahmaputra delta the deposit is made in spite of strong tidal 
currents which aid the waves in carrying sediment away. 

Steeply Sloping Deltas. — The delta surface slope also varies under 
different conditions. In large deltas, and even in small ones made of 
fine-textured sediment, the delta surface is a plain with almost imper- 
ceptible slope. But in deltas made of coarser sediment the slope 
must be graded up more steeply, otherwise the coarse fragments could 
not be carried across it. Some deltas in lakes between steeply 
rising mountain walls are made of large cobblestones and boulders, 
and the slope of such deltas must be steep. There is every gradation 
from deltas of such slope to those made of fine silt, in which the eye 
can detect no slope whatsoever. 

Growth of Deltas. — The outward growth of deltas ordinarily pro- 
ceeds at a fairly rapid rate. The Mississippi delta, for example, is 
normally advancing at the annual rate of 340 feet a year. The delta 
of a stream from Hidden Glacier in Alaska was built forward 1600 feet 
between 1899 and 19 10, but this was a heavily loaded glacial stream. 



RIVER DEPOSITS 



159 



Temporarily and locally, as between distributaries, delta growth may 
be exceedingly rapid. Thus the Mississippi delta advanced about 2000 
feet in Garden Island Bay in the spring of 191 2. What even the slow, 
normal growth of deltas means in the course of time may be inferred 
from the fact that Pisa, in the Middle Ages an important seaport, is 
now back from the sea on the Arno ; the ruins of Ostia, the ancient sea- 
port of Rome, lies three miles inland as a result of the outward growth 
of the delta of the Tiber, while Adria, a seaport at the head of the 
Adriatic 1800 years ago, is now 
fourteen miles inland ; and scores 
of similar cases are known (Fig. 

93)- 

Rapid as such deposit is, 

however, the total time required 
for the growth of a great delta 
is to be reckoned in many thou- 
sands of years. It is over 200 
miles from the sea to the head 
of the Mississippi delta, and the 
total area is over 12,000 square 
miles, while the delta formed by 
the Ganges and Brahmaputra 
has an area of over 50,000 square 
miles. Since deltas may have 
a depth of several hundred feet, 
the vast amount of sediment 
that the rivers have poured into 
the sea is readily appreciated. 
The material represents rock 
fragments worn from the land 
surface. By its deposit at the 
river mouth, this sediment gives 
rise to the formation of new 
land built out of the waste of old 
land. The deltas are real additions to the land area, for the materials 
of which they are made have been obtained in the process of lowering 
of the land, not of its destruction. 

Man's Use of Deltas. — As in the case of rloodplains (p. 151) and 
other features of river valleys, deltas are much used by man, when the 
river has raised the delta enough so that it is not swampy. The 
deltas of the Ganges and Brahmaputra in northeastern India, of the 
Yangtse and Hoang Ho in eastern China, of the Po in Italy, the Nile 
in Egypt, and the Rhine in Holland and Belgium are among the most 
densely settled parts of the earth. The flat topography, the fertile 
soil, and the position where river valley meets the sea are responsible 
for this. Deltas in lakes are also often used as sites of towns, as at 
Interlaken, Switzerland, and towns on Lake Como (Fig. 94) . Farms 
and towns are often found on deltas in the fiords of Norway and Alaska. 




- Growth of 
Cubit's Gap. 



Mississippi delta 
(Putnam.) 



160 COLLEGE PHYSIOGRAPHY 

Floods on Deltas. — On the other hand, when there is a dense agri- 
cultural population on a delta, especial care is needed for protection 
against the floods. Those of the Nile come with such regularity that 
there is no danger from them ; but the people of Holland have effec- 
tively shut out the Rhine and have even reclaimed a part of the delta 
which is beneath sea level. The delta of the Hoang Ho or Yellow 
River, on the contrary, though occupied for thousands of years by a 
dense agricultural population, is subject to such floods, of varying 
violence, that even the patience and labour of the Chinese have not 
sufficed to control it. Every now and then the river breaks through 
its embankments and rushes in a devastating flood over the densely 
settled neighbouring land. The river course has changed many times 
since records began to be kept by the Chinese over 4300 years ago, 
and some of these changes have shifted the position of the mouth 
several hundred miles. There have been 5 shifts from the Gulf of 
Pechili to the Yellow Sea and back, the river flowing into the former 
during 2 periods before the present, for a total of 3420 years and into 
the latter during 2 periods for a total of 792 years. A single flood, like 
that of 1887, has drowned a million people, besides destroying hun- 
dreds of villages, and causing famine by which the loss of life was 
greatly extended. It is no wonder that the Hoang Ho has been called 
" China's sorrow." 

The vast destructiveness of the Hoang Ho is due in large part to the 
effort to confine a river which is aggrading its bed. The result of this 
is that the river bed is built higher and higher, and the surface of the 
water becomes higher than the surrounding land, as the Po has come 
to be in Italy. If then a part of the embankment gives way, or if a 
great flood rises above the embankment, the water naturally sweeps 
over the lower land from which it has been excluded. Similar, though 
less disastrous, floods occur along other rivers on deltas. 

Deposits on the Bed or the Sea 

Sediment Carried beyond Deltas. — Rivers also carry to the sea 
sediment which does not come to rest in deltas. Some of this is drifted 
away from the coast by currents and settles to the sea floor. Some of 
it is driven along the coast and built into beaches and bars. Some 
settles in bays and other indentations along the coast. Thus sedi- 
mentary strata are being formed on the sea floor out of sediment de- 
rived in part from the waste of the land by river action. Added to 
these deposits are the remains of marine organisms. The shells or 
other hard parts of these organisms are composed of mineral sub- 
stances which were dissolved by underground water, transported to 
the sea t>y the rivers, and then extracted from the sea water by plants 
and animals and, upon their death, contributed to the sediments 
accumulating on the sea floor. 

These sedimentary strata may, by uplift, be transformed to dry 



RIVER DEPOSITS 



161 




l62 



COLLEGE PHYSIOGRAPHY 



land, and this has been the origin of much of the rock of the continents. 
This subject will be followed no further at present than to point out 
that this is one important phase of river work in the process of devel- 
oping the physical features of the earth's surface. 

Alluvial Fans 

Deposition through Change of Slope. — When the bed of a stream 
decreases in slope, the velocity of the stream is lessened, and, therefore, 
its transporting power is decreased. Such a change in slope is common 
where streams descend from mountains to plains or plateaus, and also 




'iG. 95. — Alluvial fan in the Alps. 



where tributary streams descend from steeper courses into a valley of 
moderate slope. At such points it frequently happens that the stream 
can no longer transport its sediment load, but is forced to deposit 
some of it. Such a deposit, called an alluvial fan , spreads out, fan- 
shaped, at the point where the stream emerges from its steeper portion. 
It is fan-shaped because it is being aggraded, and when one part is 
built up, the stream shifts to another course and grades that up. 
Thus, in time, the entire surface is reached by the shifting stream. 
In many cases the fan-building stream splits into a series of dis- 
tributaries and then several beds are being aggraded at the same 
time; and, during periods of flood, numerous distributaries may 
develop. 



RIVER DEPOSITS 163 

Variations in Size. — The size of the alluvial fan varies greatly, 
from little deposits but a few square inches or square feet in area, where 
rain-born rills descend a steep clay bank, up to extensive fans of large 
streams, which may have a radius of thirty or forty miles. They vary 
in depth, too, some reaching a depth of several hundred feet near their 
apexes. 

Variations in Material. — Ordinarily alluvial fans are made of coarse 
material such as sand and gravel, since they are made by the deposit 
of sediment brought by rapid streams ; but they may be composed of 
fine silt, especially at their peripheries. 

According to the coarseness of the sediment and to the volume of 
water, there is much variation in the surface slope of the alluvial fans. 
Those made of fine sediment are flatter than those of coarse rock frag- 
ments, and those made by small streams are steeper than those built 
by large streams. On some large alluvial fans there is an almost 
imperceptible slope, while others are quite steep. There is, in fact, 
every gradation from alluvial fans to the cone-shaped deposits at the 
base of cliffs where running water adds to the talus deposits enough 
material to cause a cone of dejection to rise above the talus toward 
some point in the cliff face. 

Distinction from Deltas. — Some of the steeper alluvial fans may be 
called cone deltas, and this name is, in fact, sometimes applied to allu- 
vial fans as a whole. They are, however, neither true cones nor 
deltas. Yet there is a certain semblance to deltas, and enough perhaps 
to class them as deltas on the land. Like deltas they are due to deposit 
of sediment through a decrease in carrying power of the water ; they 
have the general delta form ; they are areas of aggradation ; and they 
are crossed by distributaries. They differ from deltas in place of 
accumulation, and in the uniform frontal slope in place of the steep, 
submerged delta front. Many small deltas of steep slope, like those 
in lakes among mountains, closely simulate alluvial fans in the part 
above water, and these parts are sometimes called alluvial fans. In a 
sense they are alluvial fans built up on deltas. 

Decrease in Volume of Streams on Fans. — One factor in the growth 
of alluvial fans is the decrease in water supply for transporting the 
sediment. For, once a fan is started, a porous bed is provided, into 
which the water readily sinks. On many such fans no stream appears 
except at flood stage, for the water sinks into the gravel at or near the 
apex, and if it reappears at all, comes out in one or more springs at the 
periphery of the fan. Manifestly, such complete loss of water necessi- 
tates complete abandonment of sediment load ; and from this extreme 
there is every gradation to those in which only a part of the water 
finds its way across the alluvial fan during ordinary stages. At all 
times there is a notable loss of water by seepage into the alluvial fan, 
and consequently a loss in transporting power. Added to this is the 
diminution in velocity induced by shifting of the stream into two or 
more smaller distributaries (Fig. 96). 



164 



COLLEGE PHYSIOGRAPHY 



Large Fans in Arid Lands. — Alluvial fans occur in all climates, 
but they are best developed in arid and desert regions, where they 
sometimes assume great size. In such countries stream after stream 
builds a fan on emergence from a steeper course, and the mountain 
bases are fringed with a succession of fan-shaped alluvial plains slop- 
ing away from the mountain, forming a looping fringe of evenly graded 
surface over which stream channels extend, and the size of which is 
roughly proportional to the size of the streams which built it. Very 
often these alluvial fans coalesce, or they may completely cross a 
valley and unite with those from the opposite valley slope. Such fan 
deposits, in all variety of size and slope, form a characteristic feature 
of arid land topography, carrying graded slopes of unconsolidated 
sediment up to and even up on the mountain slopes (Fig. 97). 

There are apparently 
three prime reasons why 
alluvial fans attain the 
height of development 
in arid regions : (1) the 
nature of the rainfall, 
which is intermittent, 
and, when it comes, is 
often violent enough to 
cause rapid run-off ; (2) 
the bare slopes, which, 
having little vegetation, 
permit a rapid run-off 
and an abundant sedi- 
ment supply to the 
streams to which the 
occasional rains give rise ; (3) the rapid evaporation, as a result of 
which the volume of the fan-building stream is diminished, thus add- 
ing another factor to its inefficiency, in addition to change of slope 
and loss of water through seepage. 

Uses by Man. — The evenly graded surfaces of large alluvial fans 
are frequently excellent farm land ; and in arid regions they are very 
often irrigated. Many oases in the deserts are irrigated alluvial 
fans, forming garden spots in the midst of the desert. Irrigation is 
favoured in such situations, first, by the fact that there is often a steady 
supply of water at the apex of the fan, and, secondly, because the 
grade of the fan is favourable for the construction of the irrigation 
canals. 

In the valleys of humid regions small alluvial fans of tributary 
streams are often chosen as town or village sites, because they are 
above the valley bottom and therefore offer drier sites. Such towns 
are, however, menaced by the danger of the shifting of the stream 
course and by the destructiveness of the torrents that sometimes flow 
over the fans. In Switzerland, and elsewhere, the alluvial fan streams 




Fig. 96. — Alluvial fans in Armeni 



RIVER DEPOSITS 



i65 



are straightened and transformed to canals where they flow through 
the towns ; but even with such careful regulation they are often the 
cause of much damage. 

Desert Valley Filling 

The Sources of Valley Filling. — The growth of alluvial fans and 
the less definite forms of deposit caused by the rain wash are sources 
of valley rilling. The effect of these processes may be seen even in 
humid regions, where alluvial fans are common in the valleys of hilly 
and mountainous sections, and where the hill base is often fringed by 




Fig. 97. — Coalescing alluvialfans in central Arizona. Contour interval 50 feet. (Desert 
Well Quadrangle, U. S. Geol. Survey.) 



a deposit of rain-washed sediment, even when no definite stream de- 
scends to the valley. 

Filling of Arid Valleys. — In arid countries, and especially in deserts, 
the influence of such deposits upon the topography is much more 
clearly seen, for there the rainfall is ordinarily so slight that the streams 
are unable to move out of the valleys all that is brought into them 
by the tributary streams and the rain wash. Consequently a portion, 
and often a very large portion, of the sediment derived from the waste 
of the bordering hills or mountains finds a resting place in the valleys, 
with the result that they are slowly filled by the inwash of sediment. 

Basins without Outlets. — Naturally, in interior basins, out of 
which no water flows, all the sediment that finds its way in remains 
there, excepting that portion that is transported out of the basin 
by the wind. Thus the valleys of interior basins are often deeply 
rilled with such sediment. In such arid lands, however, a part of 






i66 COLLEGE PHYSIOGRAPHY 



the deposit, and a considerable part of its final arrangement of it, 
is caused by the action of the wind. 

Description of an Arid Valley. — The great valley of California, es- 
pecially in its more arid southern portion, illustrates this desert 
valley filling, as do also the smaller intermontane valleys of southern 
California, Arizona, and Nevada. In such a valley the enclosing 
walls are angular mountain slopes, showing evidence of the operation 
of arid land denudation, and transected here and there by valleys 
leading back into the mountains. At the mouth of each of these 
valleys is an alluvial fan, whose size is roughly proportional to the 
size of the mountain valley which it terminates, and whose slope is 
roughly proportional to the slope of the valley down which the sedi- 
ment has been brought. Near the mountain base the alluvial fans 
have their steepest slope and their coarsest sediment, perhaps even 
in large part boulders and large cobblestones. Between the fans is 
an alluvial slope rising up on the mountain base, or, if the mountain 
is precipitous, a talus. In any event, the valley bottom rises toward 
the mountains with a slope increasing as the mountain base is neared ; 
and this rise, due to deposit of rock waste, contrasts strikingly with 
the more ragged outline of the mountain. 

Away from the mountain the .slope of the alluvial fans and other 
deposits decreases, and if the valley is broad, becomes almost im- 
perceptible. At the same time the sediment decreases in coarseness. 
If the climate is very arid, wind work begins to manifest itself here, 
and areas of dunes may be present, or, if not, the evidence of wind 
action is readily seen in the tufts of vegetation that grow upon the 
sand mounds. Over the flatter portion of the valley bottom wind 
work is apparently more effective as a distributor of sediment than 
running water. 

Such deposits may accumulate to depths of many hundreds of 
feet. The slopes leading away from the mountain walls may be of 
unequal extent on the two sides, as they are in the southern part of 
the great valley of California, where the streams from the east, com- 
ing from the Sierra Nevada, are much larger than those coming from 
the lower Coast Ranges on the west. The drainage of the main 
valley is along the axis between the fans of the two sets of streams ; 
and, by the growth of these fans, the feeble drainage may even be 
ponded to form a lake or a marshy tract, as in Tulare Lake formed by 
the alluvial fan of King River. 

River Terraces 

The Nature of Terraces. — If, for any reason, a valley that has 
been partly filled by sediment is excavated again, the alluvial filling 
is, during the process, carved into a series of terraces with flat top and 
steep face. These may rise one above the other, and occur on both 
sides of the stream, sometimes in long parallel strips, but more com- 



RIVER DEPOSITS 



167 



monly in strips of variable length and width as well as at different 
levels. Two terraces, each ten or fifteen feet high, may merge into a 
single one, twenty or thirty feet high, or a single terrace strip may 
split into two or three terraces either up or down stream. 




Fig. 98. — Three stages in the making of terraces. 



Terrace Cutting. — Such terraces are the remnants left in the as-yet- 
incomplete removal of the valley filling. As the stream is cutting 
down in the alluvium, it meanders, as all streams tend to do. Cut- 
ting laterally for a while at one side of the valley filling, it excavates 
the alluvium, giving it a steep face toward the stream. If, then, the 
stream swings away from this bank to the other side, it scours out a 
level floor as it swings. Then, standing for a while at this new posi- 
tion, and cutting down into the bed, it makes a new terrace face. Thus 
as it swings back and forth, but cutting into the valley filling all the 
time, a succession of terraces is produced (Fig. 98). 

Terrace Preservation. — Ultimately all of the filling would be re- 
moved, and the terraces are to be considered as uncut remnants, 
representing stages in the removal. By this process alone terraces 
may be formed, but many, if not most, 
terraces depend upon still another fac- 
tor, namely, unequal resistance to the 
lateral swinging of the terrace-making 
stream. The presence of massive re- 
sistant deposits in the valley filling or 
of boulders may check the lateral swing 
of the river ; but, more important still, 
if the swinging stream discovers rock 
spurs buried beneath the fill, there is 
an effective cause for the checking of 
the lateral erosion. Such spurs are not 
uncommon in valleys filled with allu- 
vium. By checking the lateral swinging 
of the river such rock spurs act as a 
defence for those terraces below that were formed at an earlier stage 
before the discovery of the rock defence, and also for the terrace be- 
ing formed- while the stream is swinging against the rock spur. Such 
terraces have been called rock-defended terraces, and they are found 




Fig. 99. 



Terraces in a valley of the 
Andes in Peru. 



168 COLLEGE PHYSIOGRAPHY 

to be common. Were it not for such defence the swinging stream 
would prove much more destructive of terraces previously formed, and 
the terraces would be much less numerous, extensive, and perfect. 

Cause for Terracing. — That a stream should find power to exca- 
vate a filling previously made in its valley is not difficult to explain. 
A river, aggrading its valley under a certain condition of rainfall, 
may, if the rainfall increases, commence removing the filling. 'Such 
climatic change is known to have occurred in past times. 

Or if an aggrading river suffers a diminution in its sediment load, 
it may be able to excavate deposits previously laid down. Such a 
change is not uncommon ; as, for instance, the change from streams 
heavily loaded with sediment that issued from the front of the con- 
tinental glacier during the Glacial Period, and flowed, down a valley 
now occupied by streams with a much smaller burden of sediment. 
Many of the terraced river valleys have been formed by the excava- 
tion of deposits resulting from glacial action of one kind Or another. 
Even without change, either in rainfall or in sediment load, a river may 
excavate valley filling if its slope is increased by uplift of the land. 

Finally, a lake, acting as a temporary baselevel to a stream, may 
be filled, and then, the stream having a new and lower baselevel, it 
may proceed to remove the deposit that it laid down in the lake. It 
is under one or the other of these conditions that the excavating of 
valley alluvium, with accompanying terrace formation, may suc- 
ceed a period of valley filling. 



References to Literature 

J. Barrell. Relation between Climate and Terrestrial Deposits, Journ. 

Geol., Vol. 16, 1908, pp. 159-190, 255-295, 363-384; Criteria for the 

Recognition of Ancient Delta Deposits, Bull. Geol. Soc. Amer., Vol. 23, 191 2, 

PP- 337-440. 
R. M. Brown. The Protection of the Alluvial Basin of the Mississippi, Pop. 

Sci. Monthly, Vol. 69, 1906, pp. 248-256. 
J. E. Carmen. The Mississippi Valley between Savanna and Davenport, Bull. 

13, Illinois Geol. Survey, 1909, 96 pp. 
L. J. Cole. The Delta of the St. Clair River, Geol. Survey of Mich., Vol. 

9, Part 1, 1903, 28 pp. 
H. Credner. Die Delten, Petermanns Geog. Mitteilungen, Erganzungsheft 56, 

1878, 74 pp. 
W. M. Davis. The Fresh-water Tertiary Formations of the Rocky Mountain 

Region, Proc. Amer. Acad. Arts & Sci., Vol. 35, 1900, pp. 345-373; De- 
velopment of River Meanders, Geol. Mag., Decade 4, Vol. 10, 1903, pp. 

145-148; River Terraces in New England, Geographical Essays, Boston, 

1909, pp. 514-586. 
R. E. Dodge. The Geographical Development of Alluvial River Terraces, 

Proc. Bost. Soc. Nat. Hist., Vol. 26, 1894 , pp. 257-273. 
F. V. Emerson. The Geographic Story of Kaskaskia, Journ. Geog., Vol. 

8, 1910, pp. 193-201 ; Life along a Graded River, Bull. Amer. Geog. Soc, 

Vol. 44, 1912, pp. 674-681, 761-768. 



RIVER DEPOSITS 



169 



E. F. Fisher. Terraces of the West River, Proc. Bost. Soc. Nat. Hist., Vol. 33 

1906, pp. 9-42. 
G. K. Gilbert. The Transportation of Debris by Running Water, Prof. Paper 86 

U. S. Geol. Survey (in press). 
A. W. Grabau. Early Paleozoic Delta Deposits of North America, Bull. Geol 

Soc. Amer., Vol. 24, 1913, pp. 399-528. 
E. W. Hilgard. The Exceptional Nature and Genesis of the Mississippi 

Delta, Science, N. S., Vol. 24, 1906, pp.^861-866 ; Amer. Journ. Sci., 3d 

series, Vol. 1, 1871, pp. 238-246, 356-368, 425-435. 
M. S. Jefferson. Limiting Width of Meander Belts, Nat. Geog. Mag., Vol 

13, 1902, pp. 373-384- 
L. C. Johnson. The Nita Crevasse, Bull. Geol. Soc. Amer., Vol. 2, 1891, pp 

20-25. 
Sir Charles Lyell. Mud-lumps off the Mouths of the (Mississippi) River 

Principles of Geology, Vol. 1, 1867, pp. 447-454. 

D. T. McDougal. The Delta of the Rio Colorado, Bull. Amer. Geog. Soc. 

Vol. 38, 1906, pp. 1-16. 
Lawrence Martin. The Copper River Delta, Alaskan Glacier Studies, Wash 

ington, 1914, pp. 458-466. 
J. Menauer. Die Laufanderungen des Gelbes Flusses in Historischen Zeit 

Nurnberg, 191 2. 
A. Norlind. Die Geographische Entwicklung des Rheindeltas bis um das Jahr 

1500, Lund, 1912, 272 pp. 

E. W. Shaw. The Mud Lumps at the Mouths of the Mississippi, Prof. Paper 

85B, U. S. Geol. Survey, 1913, pp. 11-27. 
A. L. Smith. Delta Experiments, Bull. Amer. Geog. Soc, Vol. 41, 1909, pp. 

729-742. 
R. S. Tarr and O. D. von Engeln. Representation of Land Forms in the 

Physiography Laboratory, Journ. Geog., Vol. 7, 1908, pp. 73-85. 
W. S. Tower. The Development of Cut-off Meanders, Bull. Amer. Geog. 

Soc, Vol. 36, 1904, pp. 589-599- 

A. C. Trowbridge. The Terrestrial Deposits of Owens Valley, California, 

Journ. Geol., Vol. 19, 191 1, pp. 706-747. 

B. Willis. Conditions of Sedimentary Deposition, Journ. Geol., Vol. 1, 1893, 

PP- 476-520. 



TOPOGRAPHIC MAPS 

Alluvial Fans 



Desert Well, Ariz. 
Needles Special, Ariz. 
Camelsback, Ariz. 



Amargosa, Cal. 
Cucamonga, Cal. 
Parker, Ariz. 



Livingston, Mont. 
Sierraville, Cal. 



Jefferson City, Mo. 



Bluffs 
Lexington, Neb. 



Elk Pt., S.D. 



Gothenburg, Neb. 
Maxwell, Cal. 



Braided Courses 

Lexington, Neb. 
North Platte, Neb. 



Disaster, Nev. 
Kearney, Neb. 



East Delta, La. 
Cucamonga, Cal. 



Deltas and Distributaries 

West Delta, La. 
Donaldsonville, La. 



Plattsburg, N.Y. 



170 



COLLEGE PHYSIOGRAPHY 



Flood plains 

Map of Alluvial Valley of the Mississippi, 8 sheets. 

Mississippi River Commission, 1 : 63,360. Charts 14, 16, etc., 

Mississippi River Commission, 1 : 20,000. Charts 8, 22, 35, 36, 38, 39, 52, 
etc. 

Missouri River Commission, 1 : 63,360, Sheets I, LXXI, LXIV ; also Sheets 
XIV and XXIII, editions of i878-i87oand 1890, compared for floodplain and 
river changes at Kansas City and Omaha. For latter see also U. S. Geol. 
Survey map of Omaha and vicinity, 1 : 62,500, 1898 edition. 

Map of Salinas Valley, Cal., 1 : 31,680, Sheets 1 to 3. 

Map of Sacramento Valley, Cal., 1 : 31,680, Sheets 
Kearney, Neb. Nebraska City, Neb. 

Butler, Mo. Donaldsonville, La. 

Lacon, 111. Ottawa, 111. 

St. Louis, Mo. Jefferson City, Mo. 

Lake Providence, La. Browns Valley, Cal. 

Jonestown, Miss. Bouldin, Cal. 



A to Q. 

Minneapolis, Minn, 
Marshall, Ark. 
Isleton, Cal. 
Palermo, Cal. 
Nocalaus, Cal. 
Chico Landing, Cal. 



Millikin, La. 
Elk Pt., S.D. 
Fort Payne, Ala. 
Maynardville, Tenn. 



Meanders 

Butler, Mo. 
Junction City, Kan. 
Estillville, Ky. 
St. Louis, Mo. 



Jefferson City, Mo. 
Coahoma, Miss. 
Marshall, Mo. 
Ypsilanti, Mich. 



Donaldsonville, La. 



Natural Levees and Crevasses 

East Delta, La. Maxwell, Cal. 



River-made Plains and Graded Rivers 



Shasta, Cal. 
Marysville, Cal. 



Amargosa, Cal. 
Maxwell, Cal. 



Marshall, Mo. 
Elk Pt., S.D. 



Cohoes, N.Y. 



Terraces 
Springfield, Mass. 



Lacon, 111. 



CHAPTER VII 
THE RIVER VALLEY CYCLE 

Davis's Scheme of the Cycle 

One of the great contributions to physiography is the statement, 
exposition, and persistent teaching, by Professor W. M. Davis, of 
the idea that land forms pass through a cycle of development. This 
has not only led to a clearer understanding of the physiographic 
processes, but has been a powerful factor in the rational interpretation 
of the physiographic features of the lands. While applicable to. other 
land forms, this idea is of most fundamental importance in an inter- 
pretation of river valleys. The geographical cycle has been defined as 
" the period of time during which an uplifted land mass undergoes 
its transformations by the processes of land sculpture, ending in a 
low featureless plain." 

The Earliest Stage Of Youth 

An Uplifted Sea Bottom. — For the statement of the cycle of devel- 
opment of river valleys, the clearest exposition can be made by con- 
sidering first the simplest case, and at appropriate points indicating 
the variations from this. For this purpose we will assume a new land 
surface, elevated from the sea to no great height, sloping toward the 
sea, and in this finished state subjected to rainfall. This is no purely 
ideal case, but one that, with unimportant variations, has occurred 
repeatedly during the earth's history. 

Effect of Slopes. — The rain that falls on this new land surface will 
run down the slopes toward the sea, and, where there is slope enough, 
will quickly find some parts lower than others toward which the run- 
off tends to concentrate. Along these lines there is excavation, and 
with excavation still greater tendency for the water to concentrate 
there from the neighbouring higher portions. And as the water flows 
toward these channels, tributary channels are cut. 

Consequent Streams. — Elsewhere the surface may be too level 
for run-off, and there swampy tracts are developed by the standing 
water. Portions of the plain may contain original depressions where 
ponds or lakes accumulate. On such youthful flat divides water is 
removed chiefly by evaporation. 

The course of any particular channel is determined by the natural 
irregularities of the surface, and it may assume a very sinuous route 

171 



172 



COLLEGE PHYSIOGRAPHY 



to the sea, passing through swamps, through lakes, around low eleva- 
tions, and even falling in a rapid or low waterfall as it descends some 

unusually steep slope. 
This course is consequent 
upon the natural features 
of the surface, and the 
stream may be called a 
consequent stream, as the 
falls are also consequent 
(Fig. 100). 

Illustrations from 
Florida and Dakota. — 
A near approach to this 
ideal condition is found 
in the southern part of 
the Florida peninsula, a 
recently uplifted sea bottom, with consequent drainage of exceed- 
ingly immature type. A recently drained lake bed, like that in the 
valley of the Red River of the North, in North Dakota and Manitoba, 
gives rise to a similar condition of immature drainage. Coastal 




Fig. 100. — Youthful drainage with lakes, few tribu- 
taries, flat-topped divides, and the stream high above 
baselevel. 




101 . — A young stream in Florida. 



plains and former lake beds in many other parts of the world have 
a drainage in this or in only a slightly more advanced stage of 
development. 



Young Valleys 

Earliest Cutting near Stream Mouths. — As the run-off from the 
new land proceeds, the channelways are deepened. Each main stream 
that enters the sea can cut its bed no lower than this baselevel, and 
each tributary is temporarily limited in its downcutting by the tem- 
porary baselevel of the stream to which it is tributary. Partly be- 
cause the volume of water in the stream is greatest at its mouth, and 



THE RIVER VALLEY CYCLE 



173 



partly because this is the point where it can cut deepest, the lower 
portion of the stream is the part where the valley develops earliest. 
Accordingly at, and just above, the mouth of the main streams, valley 
cutting proceeds apace, as it does also at the mouths of tributaries 




Fig. 102. — The broadening of a valley through lateral swinging by the stream. 



that enter a cut channel. From these points the valley development 
extends up-stream farther and farther, as the downcutting in the lower 
portion gives the stream opportunity for excavation. 



i74 



COLLEGE PHYSIOGRAPHY 



Variations in Depth. — Where such rapid downcutting is in prog- 
ress, a gorge form of valley necessarily results. Whether it be a 
gorge of but a few feet in depth, or one of several hundred feet, will 
depend upon the elevation of the land surface above baselevel. The 
rate of the gorge formation will vary with the volume and velocity 
of water, and the nature of the rock to be cut away. But whether 
the rock be weak or resistant, the volume and velocity great or small, 
or the elevation high or low, the result of the excavation will be a gorge 
form of valley. If the rock be unconsolidated, lateral erosion will 
give rise to a broader form than if the rock is hard ; and weathering 
will also be more effective in broadening the gorge. 

Broadening near Stream Mouths. — Once the lower portion of a 
stream has reached baselevel, the gorge form wastes away slowly 
under the attack of rain wash and weathering. As the stream cuts 
down to grade, the lower valley may broaden considerably while 
the gorge valley is developing farther up-stream and in the headwaters. 

Waterfalls and their Extinction. — In this process of downcutting, 
waterfalls of normal development are caused if the stream at any 
point discovers sufficient irregularity or resistance in the rocks of its 
bed ; and if these irregularities are due to horizontal strata, the water- 
falls slowly retreat up-stream. The consequent waterfalls developed 
in the earliest stages will be of short life, unless they are the result of 
some irregularity in rock structure which will aid in preserving them. 
Even though far from the river mouth, they will be removed by the 




103 



Two stages in waterfall development ; the cataract at W retreating up- 
stream while smaller falls are formed in two tributaries. 



more rapid cutting of the stream, resulting from the added velocity 
over the steep slope in which a gorge is being cut. 

Lakes and their Destruction. — Each lake will serve as a temporary 
baselevel below which the stream above cannot cut its bed. It is, 
therefore, an obstacle in the way of the valley development, and will 
continue to be an obstacle until removed. The removal of the lake 
is possible in one of two ways, or by a combination of the two : 
(1) by cutting down at the outlet, (2) by filling with sediment. The 



THE RIVER VALLEY CYCLE 



i7S 



latter will be undertaken at once by the stream if it has a sediment 
load, for the lake will filter this out as .it checks the current that 
enters it. If the lake is not too large, the process of extinction by 
filling will be brief, as geological time goes, provided the stream can 
bring to it the necessary sediment. Lacking the sediment, as the 
Florida streams do, the destruction of the lake will require a far 
longer time and may have to wait for the still slower process of filling 
by organic remains, or until the deepening of the lower valley proceeds 
far enough up-stream to tap the lake, as the Niagara River, for instance, 
may in time be expected to do in the case of Lake Erie. In any event 
the lake is but a temporary phenomenon, a feature of youth in a 
valley and subject to extinction by one means or another as the river 
develops its valley toward maturity of form. 

The Narrowing of Divides by Headwater Erosion. — In the earliest 
stages of drainage there are areas of poorly drained land and even of 
swamps. The divides are flat-topped and broad, and the distance 
between well-defined channels is great. This condition finds illus- 
tration in the broad plains between streams in the valley of the Red 
River of the North, and 
in the plains and swamps of 
southern Florida. As the 
tributaries of the main stream 
eat back, and as secondary 
tributaries develop from 
these, and still others from 
these, the flat-topped di- 
vides are narrowed, and 
more and more of the sur- 
face has slopes down which 
the run-off can flow. This 
extension of the tributaries 

is accomplished by gnawing back at the upper portion, a process 
that may be called headwater erosion. 

Stream Systems like Trees. — The stream system is then passing 
out of the stage of youth ; individual parts of it may already have 
passed out of this stage to that of maturity, while the smaller head- 
water tributaries are still developing the characteristics of youth. 
The condition is somewhat like that of a tree which may be broad and 
mature with gnarled trunk, while at the same time it sends out a 
multitude of fresh young twigs from each of its branches with the 
return of every growing season. Just so, the developing river sends 
out an extension of its younger headwater tributaries with the return 
of every heavy rain ; and, as the twigs of the tree harden and mature 
with succeeding seasons, so the headwater valleys broaden, deepen, 
and mature as time goes by. 

The Characteristics of Youth. — The young stream valley, whether 
developed on a simple plain, like the one considered, or on a much more 




Fig. 104. — Narrowing of divides by stream de- 
velopment. Since the line BB is longer than A A, 
the slopes to be drained become greater and the 
stream load greater as maturity is approached. 



176 



COLLEGE PHYSIOGRAPHY 



complex surface, whether developed on lower ground or on high, has 
certain characteristics, so that, when one or more of these character- 




Fig. 105. — The narrow gorge or canyon of a youthful stream. This stage of youth is, 
of course, more advanced than that shown in Fig. 10 1. 

istics are found in a river valley, one may with safety class it as a 
young valley. The characteristics of youth in river valleys are the 



THE RIVER VALLEY CYCLE 177 

presence of : (1) the gorge form, (2) waterfalls, (3) lakes, (4) poorly 
developed divides. A single one of these features is indicative of 
youth ; and a young river valley may possess but one (Fig. 105) ; or 
it may have all four of the typical characteristics of youth. 

Development of Overlapping Spurs. — In early or middle youth 
the gorges or canyons are usually widened sufficiently and enough 
curves are developed so that lateral spurs begin to be prominent. 
These are formed because of undercutting on one bank of the stream, 
which is therefore steep, and a slipping off on the opposite wall of the 
valley, which accordingly slopes more gently. These lateral spurs 
usually project alternately from opposite sides of the stream course, 
so that it is impossible to see a long distance down the valley. They 
are overlapping spurs (Fig. 335). As the valley broadens in late 
youth, the overlapping diminishes. Later the spurs may be partly 
buried beneath rloodplain deposits. 

Stream Junctions. — The entrance of tributaries into master 
streams obeys two general laws. First, the tributary and the main 
river usually form an acute angle pointing down-stream ; secondly, 
there is no discordance in the level of main and side valleys. Both 
of these conditions are attained during the youth of the stream valley. 

The first principle needs no amplification. The relationship is a 
natural one in streams consequent on slopes of the land. In original 
consequent drainage there are few exceptions. When the angle of 
junction points up-stream, however, it is usually because of some later 
adjustment. The latter are called barbed tributaries, and certain 
streams in the mountains (Fig. 370) illustrate this. 

Except at a very early and rare stage in mountain drainage, the levels 
of main and side valleys are accordant (Fig. 151). No matter what 
difference there may be in the volume of tributaries, they always seem 
to be able to keep pace with their master streams in downcutting. 
One reason for this is that, although the volume of the tributary is 
less, its slope is steeper. Even in the Colorado River in the Grand 
Canyon the tributaries, which are without water part of the year, cut 
their gorges fully as fast as the main canyon and enter with accordant 
grade, as do practically all streams throughout the world in regions 
which have never been occupied by glaciers. More than a century 
ago this law was stated by Playfair as follows : 

Every river appears to consist of a main trunk, fed from a variety of 
branches, each running in a valley proportioned to its size, and all of them 
together forming a system of vallies, communicating with one another, and 
having such a nice adjustment of their declivities that none of them join the 
principal valley either on too high or too low a level ; a circumstance which 
would be infinitely improbable if each of these vallies were not the work of the 
stream that flows in it. 

Youth not Measured in Years. — In the use of the terms youth and 
maturity it is intended to convey the idea of stage rather than age in 
years. Here again comparison may be made with plants. An oak 



i 7 8 



COLLEGE PHYSIOGRAPHY 



tree requires many years to reach a stage of maturity, while a young 
annual plant might reach the same stage in its development in a few 
days or weeks. Yet the stage of youth or maturity could be easily 




Fig. 106. — A young valley in Italy near Naples, with houses on the gorge walls. 



recognized in each case by well-defined characteristics. The case 
is similar with river valleys. They have notably different charac- 



THE RIVER VALLEY CYCLE 179 

teristics, and they require very different periods of time in which to 
develop. One valley may reach maturity in a fraction of the time 
required for another to reach that stage of development. 

Variations with Height, Rock, and Volume. — If, for example, a 
stream has thousands of feet of excavation before reaching its grade, 
a longer time will be required for its youthful work than if it had 
started only a score or two of feet above baselevel. Or if the work of 
downcutting is performed in resistant rock, the stage of youth will 
linger for a longer time than if its work were in weak rock. Likewise 
the rate of development will be slower if the volume is slight than if 
it is great ; and the broadening of the valley walls by weathering will 
proceed more slowly in an arid than in a humid climate. All these 
factors, and others, introduce elements of variation in rate of develop- 
ment of valley form, but none of them introduce conditions which 
will so mask the phenomena as to render it difficult to interpret them 
and to recognize the stage of development. 

Many Stages in the Same Stream. — Owing to the different rates 
at which river valleys pass through the stages of development, and 
to causes which may arise to interfere with or modify this develop- 
ment, it is often the case that the different parts of a river system are 
in different stages of the cycle. It is necessary, therefore, to consider 
by itself each valley, or each section of similar history. One can 
rarely say that a large river system is young, or mature, though it is 
usually not difficult to classify any given part of it. 

Mature Valleys 

Contrasts with Youth. — The development of maturity of valley 
form is but a continuation of the processes outlined in the preceding 
section.. The lakes have been filled ; the waterfalls have disappeared ; 
and the river has attained a grade approximating the lowest slope 
over which its sediment load can be transported. By the gnawing 
back of the headwater tributaries the land is now all provided with 
slopes down which the water may run, and channels along which it 
may go from one tributary to another. The divides are narrowed 
and the region is traversed by a complex network of valleys. Weath- 
ering and rain wash are now the main elements in the denudation of 
the land, and the streams have for their main task the drainage of the 
land and the removal of the sediment turned over to them. Other- 
wise their work of valley formation is practically at an end, though 
here and there, by swinging against their valley walls, they may be 
engaged in actual work of excavation by lateral erosion. 

The Correction of Grade. — While in this stage the streams have 
approximated grade, and in places have reached it, there are two sec- 
tions where the grade is still in process of establishment. One of 
these is in the upper reaches, where the streams are still cutting in 
their beds, and where their valleys may even be still in the state of 



i8o 



COLLEGE PHYSIOGRAPHY 



youth, with gorges and waterfalls. The other is in the lower and 
middle course, where the stream may be engaged in correcting a too 
low grade, established earlier in the cycle. As the slopes develop 
by headwater erosion, a larger burden of sediment may come to the 




A stream valley in late youth or early maturity 



stream, and this it may not be able to move over the lower grade 
established during the stage of late youth or early maturity. In 
that case the stream aggrades its lower course, and floodplains develop, 
which are made even more extensive if the stream builds a delta at 
its mouth. The correction of the grade lower down in the river valley 
necessitates a correction above, in order to maintain an adequate 
slope. Thus floodplains develop along a large part of the mature 

river, and over them 
the river swings in a 
meandering course (Fig. 
109). 

The Characteristics 
of Maturity. — Flood- 
plains and meandering 
streams are among the 
characteristics of ma- 
turity of stream valley 
development. Other characteristics are moderately sloping valley 
walls, a well-defined drainage system with many tributaries and defi- 




Fig. 108. — Cross section of a valley in various stages of 
youth (aa' to cc'), of adolescence (dd'), of maturity (ff), 
and of old age (gg')- 



THE RIVER VALLEY CYCLE 



181 




182 COLLEGE PHYSIOGRAPHY 

nite divides, a fairly well-established grade, and the absence of water- 
falls and lakes. The degree to which these characteristics are de- 
veloped depends upon the part of the stage reached; that is, they 
are only partly developed in the stage of early maturity, but are fully 
developed in the stage of full or late maturity. It has been proposed to 
call the state of early maturity the stage of adolescence (Figs. 107, 108). 

Old Valleys 

Slow Development after Maturity. — Youth is a relatively rapid 
stage of valley development, maturity is longer, but old age is of 
almost infinite duration. Accordingly, since the surface of the earth 
is subject to changes of other kinds, it must be rare for valley develop- 
ment to proceed with such slight interruptions as to permit any con- 
siderable portion of the earth to attain the condition of old age. Few 
river valleys of to-day are in the stage of old age, and most of them 
are in youth or early maturity. Before time has sufficed for their 
development beyond this stage, some change, such as uplift or de- 
pression, so interferes with their development as to start them on a 
new cycle. Yet in past ages there have been periods during which 
the old age stage has been reached and the remnants of some of 
these are recognizable in the topography of to-day. This seems to 




The peneplain of southern New Hampshire, with Mt. Monadnock. 



argue for periods of greater earth stability during certain ages of the 
past than at present and in the recent past, a conclusion toward which 
other facts also point. 

The Characteristics of Old Age. — In the old age stage the valley 
slopes are worn to even less relief than that of full maturity. The 



THE RIVER VALLEY CYCLE 183 

water runs off from them less readily, and more is lost by evaporation, 
so that the river volume diminishes. There is also a decrease in sedi- 
ment load, and that which is supplied is of finer grain than in earlier 
stages. A larger proportion of the mineral load of the river is carried 
in solution, and the wasting away of the gentle slopes is mainly 
performed by the solvent action of underground water. Broad flood- 
plains still border the rivers, and over the flat valley bottoms the 
rivers still flow in broadly meandering courses. 

Peneplains. — Old age differs from maturity in far less notable 
ways than maturity does from youth. The prime difference is the 
decreased valley slopes; and after they have passed the stage of 
sufficient steepness for the inwash of considerable quantities of sedi- 
ment, their further lowering must be a process of exceeding slowness. 
A land form, whether plain or mountainous, thus worn down to the 
moderate slopes of the old age stage, is a peneplain (Fig. 1 10 and PI. X). 

Variations in Valley Forms 

Causes of Variations. — From the simple, ideal case of develop- 
ment in which uniformity is assured, there are wide variations in 
several directions, one of them being in the form of the valley. If 
excavated in rock of uniform texture, the valley slopes are simple, 
varying with the nature of the rock, the climatic conditions, the eleva- 
tion, the volume of water, and the stage of development; but if 
excavated in rock of varying texture, the slopes are complex during 
the stages of youth and early maturity, though with advancing matu- 
rity the influence of the rock on valley form becomes of decreasing 
importance. 

Lateral Terraces in Horizontal Rock. — This influence of varying 
rock texture may be illustrated by two cases, one in which the strata 
vary horizontally, the other in which they vary vertically. In the 
first case the strata are horizontal. Successive differences in rock 
resistance are discovered as the stream cuts its way down into the 
strata. In the stream bed these give rise to waterfalls, as we have 
seen, but on the valley walls they give rise to rock terraces. By the 
differential weathering the hard layers are etched into relief, and the 
valley sides are terraced, as is so well illustrated in the Colorado 
canyon. If the differences are but slight, and the layers of different 
texture thin, the terracing may be of only moderate degree; but if 
the beds are thick, and the differences great, extensive rock benches 
and escarpments may develop. With increasing maturity these 
recede farther and farther from the stream, often for many miles, 
until finally, as the slopes are more and more reduced, resistant beds 
no longer reveal themselves in the topography, but are covered with 
a mantle of rock waste. 

Narrow Gaps and Broad Valleys in Vertical Structures. — Where 
the strata are vertical, the resistant strata are also etched into relief, 



1 84 



COLLEGE PHYSIOGRAPHY 



while the weaker strata are more rapidly worn away. As a result of 
this process the valley alternately broadens and narrows, each narrow 
place being where the stream is crossing a resistant bed, and each 
broad place where it cuts across the weaker strata. Here also the 
influence of the variations depends upon the extent of the difference 

in resistance, and upon 
the width of the varying 
strata. With valley de- 
velopment these differ- 
ences become less and 
less noticeable. 

Superimposed Streams. 
— It sometimes happens 
that a stream flowing 
upon a land surface of 
given kind, such as a plain 
or a plateau, discovers a 
very different structure 
as it cuts its channel 
toward grade. Thus the 
Colorado River, after 
cutting several thousand 
feet in the horizontal 
strata of the plateau, 
has discovered a buried, 
worn-down mountain area 
upon which the plateau rests. Into this it is now cutting a part 
of its course. Such a river is superimposed, and the kind of valley 
that develops may be very different from that which developed before 
the discovery. Thus the Colorado flows in a narrow, steep-walled 
canyon where it crosses the buried mountain rocks, with their steeply 
inclined, variable, and resistant strata, while in the upper portion the 
valley is terraced by the etching out of the horizontal strata of 
the plateau (Figs, in, 112). 




Fig. hi. — A superimposed river, which has cut down 
through the horizontal structures of a plateau to the 
folded structures of a worn-down and buried moun- 
tain mass. 



Variations in Stream Course 

Subsequent Streams. — In the ideal case the stream courses were 
consequent upon the topography that the running water discovered ; 
and such must always be the course of a stream upon a new land. 
But during the ages required for the passage of a stream valley through 
its cycle of development, the stream may undergo very notable changes 
in position and depart very widely from the original consequent 
course. Such a course, developed by subsequent changes, is a subse- 
quent course, in distinction from the original consequent course. 

There is a wide variety of conditions which may give rise to such 
changes in a stream course, only a few of which are considered at 



THE RIVER VALLEY CYCLE 



i85 



this point, while others are taken up in later sections. The original 
consequent course may, for example, be very irregular and roundabout, 
and during its development such a course will tend to be straightened. 
Or the original course may be straight, and with subsequent develop- 
ment meandering will be set up. 

Adjustment to Rock Structure. — Among the causes for change of 
course of streams is that of adjustment to rock structure. The origi- 







. 




Fig. 112. — The Colorado Canyon with horizontal sedimentary rocks above and highly 
folded metamorphic rocks below, where the river is superimposed upon the older strata. 
(U. S. Geol. Survey.) 

nal course is determined by topography, not by rock structure, un- 
less that has determined the topography. It may happen, therefore, 
that a consequent stream is flowing on resistant rock, while not far 
away there are much weaker beds. As the surface wastes away and 
a valley develops in the weaker beds, the consequent course may be 
abandoned for a subsequent course along the weaker rock. Such a 
stream is adjusted. By the time maturity of valley form is reached, 
there is perfect adjustment to rock structure, and the adjusted course 
may depart widely from the original consequent course. Usually 
subsequent tributaries develop at right angles to a consequent master 
stream. This angular pattern of stream courses (Fig. 366) is known 
as trellis drainage. 

The Shifting of Divides. — In the wearing down and adjusting of 
stream courses, headwater erosion by adjacent streams is usually 
unequal. This is because of the advantage which (a) increased 
volume, (b) lower baselevel, or (c) weaker rock gives one stream over 



186 COLLEGE PHYSIOGRAPHY 

its neighbours. Consequently headwater erosion not only reduces 
divides in altitude, but also causes them to shift horizontally (Fig. 
359). In regions of inclined sedimentary rocks the streams obey 
a general law, which is that the divide migrates in the direction of the 
dip. When local warping of a land surface takes place, the law, 
worked out by Campbell, is that the divide will migrate toward an 
axis of uplift or away from an axis of subsidence. 

Stream Piracy and Diversion. — One important event in the ad- 
justment of stream course is the diversion of one stream by another. 
There are a variety of ways in which this may be brought about, a 
consideration of which is for the present deferred. Suffice it here to 
say that, if one stream finds conditions more favourable for develop- 
ment than another neighbouring stream, it may, by the extension of 
branches or of headwaters, eat back until it taps a part of the less 
favourably situated stream and diverts it to its own channel. Such 
robbery has been called stream piracy, and the diverting stream has 
been called a river pirate. A less sensational, and, on the whole, prob- 
ably a more definitive, term is diverting stream. 

For these and other reasons a stream course as well as a stream valley 
is subject to change. During a human lifetime both valley form and 
stream course appear to be fixed and unchangeable ; but, viewed 
from the standpoint of geological time, the stream and stream valley 
are the seat of incessant change, following laws which are usually 
not difficult to discover and interpret. 

Insequent and Obsequent Streams. — Streams in which no ad- 
justment to rock structure takes place, either (a) because of widespread 
flat-lying sediments, or (b) because the stream develops in a large 
area of a massive formation such as granite, never have subsequent 
tributaries. This is because the adjustment is complete from the 
beginning. This insequent stream pattern is often treelike, for 
which reason the drainage is said to be dendritic (Fig. 366). 

When, however, adjustment to inclined sedimentary beds results 
in subsequent tributaries of consequent master streams which happen 
to flow originally across the trend of the rock structure, still another 
type of stream is formed. The subsequent tributaries will themselves 
receive affluents. Those which flow in the opposite direction from 
the original consequent drainage are called obsequent streams. They 
develop especially upon the escarpments of belted coastal plains, 
as explained later (Chap. XIV). 

Climatic Relationships. — Rivers that flow out into deserts diminish 
in volume down-stream and eventually disappear. This is a climatic 
relationship. Rivers that flow northward, like the Mackenzie and 
Yukon in America and the Siberian rivers in Asia, have a seasonal 
climatic relationship to winter ice. This is not well understood, but 
some of these streams must have their activity in erosion, transporta- 
tion, and deposition distinctly limited by the short season. Many 
small rivers are frozen clear to the bottom when, of course, all work 



. THE RIVER VALLEY CYCLE 187 

ceases. As long as the ground is frozen they receive little sediment, 
though run-off is increased by the prohibition of percolation. Ground 
ice as well as frost in the ground later add to the summer volume of 
such streams. Even in temperate climates some streams flow through 
ice-walled gorges temporarily. While the ice is frozen to the river 
bottom they may even flow over floors of ice. Then, of course, their 
erosive power is limited, but this latter is only a local and exceedingly 
temporary climatic relationship of rivers. 

Variations due to Accidents and Interruptions 

The Accident of Glaciation. — Both the work of a stream in valley 
formation and the course of the stream may suffer interference for a 
long time by accidental causes. The spread of an ice sheet over a 
country, for instance, for the time being exterminates streams; and 
upon its withdrawal the surface of the land may be greatly modified. 
Streams may be turned out of their old valleys and forced to develop 
new ones, thus locally assuming the features of youth, though in other 
parts occupying a valley whose stage of development was attained 
before the ice invasion. Streams may even find their courses over 
an entirely new land surface, and be forced to start upon a new cycle, 
as the Niagara River has done. 

Other types of accidents occur in connection with temporary arid- 
ity, with lava flows, landslides, etc. 

Interruptions and Rejuvenated Streams. — By such interruptions 
of valley development there are all gradations from conditions in 
which the stream is only locally, or slightly, or temporarily modified 
to those under which the cycle must start anew. All these latter 
changes, involving a change of baselevel by uplift or by submergence, 
are technically known as interruptions. A stream interrupted by 
uplift is said to be rejuvenated. 

The rejuvenated stream, though inheriting some of the features of 
the earlier cycle, is given renewed opportunity for valley develop- 
ment, as a result of which it assumes some or all of the characteristics 
of youth in a whole or a part of its course. The uplift of the land, or 
the tilting of the land so as to increase the stream grade, will always, 
if extensive enough, set the stream at work again cutting its bed, and 
a gorge will be excavated in the bottom of the old valley. Such a 
stream may be said to be revived or rejuvenated. 

The Rejuvenated Rhine. — This condition is well illustrated by 
the Rhine and some of its tributaries in Germany. Below Bingen 
the river flows through a young gorge transecting a complex series 
of inclined strata, and tributaries to it also flow in gorges in which 
there are often rapids and falls (Fig. 113). The uplands between the 
gorges, and bordering the Rhine, are a succession of moderately 
undulating hills and valleys, a topography of advanced maturity or 
old age in which the revived Rhine and its tributaries have sunk 



COLLEGE PHYSIOGRAPHY 



their gorges. The upland is, in fact, a peneplain, developed by the 
wearing down of a mountain region, and now being dissected by 
rejuvenated streams. The upland slopes are so gentle as to be 



<tM ; i 



Bb'.''\m 



Fig. 113. — The gorge of the Rhine. 

occupied by farms ; but the young valleys cut in it are so steeply 
sloping that only the largest have cultivated slopes, and these only 
after great labour has been expended in terracing the steep slopes. 

Rejuvenation after Maturity. — A stream rejuvenated in the 
stage of youth would probably show little evidence of the rejuvenation 




114- — The origin of entrenched meanders. 



in the valley form; but a rejuvenated mature or old valley would 
preserve some of the inherited earlier features, as the Rhine inherits 
the old valley in which the present gorge has been sunk. Another 



THE RIVER VALLEY CYCLE 



189 



inherited characteristic may be the meandering course which a mature 
river normally has on its floodplain. When the meandering stream 
commences to cut its gorge as a result of rejuvenation, it may still 
preserve the meandering course although entrenched in the rock. Such 
a course is called entrenched or incised, and the meanders are called 
entrenched meanders or incised meanders (Figs. 114, 115). A meander- 
ing course, closely resembling that of a floodplain river, could not 
develop in a rock gorge, because the swinging which gives rise to 




Fig. 115. — Entrenched meanders in the Allegheny Plateau of Pennsylvania. (Johnstown 
Quadrangle, U. S. Geol. Survey.) 

the meandering would need first to cut the gorge walls away ; and 
incised meanders are closely bordered by the gorge walls on both 
sides, the walls themselves swinging to parallel the meandering river. 
Such a condition is possible only as a result of an inherited course 
entrenched by corrasion. 



190 



COLLEGE PHYSIOGRAPHY 



Illustrations of Entrenched Meanders. — There are many instances 
of incised meanders. The Susquehanna in southern New York and 
northern Pennsylvania is a typical illustration, and the Moselle in 
western Germany is another. Some of the meanders of the Moselle 
are perfect ox-bow forms, and one can look down from the neck of the 
ox-bow to the river on both sides, as one can from the narrow 
neck of a floodplain ox-bow ; but in the Moselle the river flows be- 
tween steeply rising valley walls, and the ox-bow is a rock hill, not 
a low alluvial plain just above the river level. During the process of 
entrenching, the lateral cutting of the stream has in some cases worn 
through the neck of the ox-bow and made a cut-off, through which 
the stream flows, abandoning the old course and leaving a rock hill 
completely encircled by a valley. Such a cut-off hill rises in the Sus- 
quehanna valley near Binghamton. It is by the undercutting of the 
rock at the neck of such an incised ox-bow that some of the huge 
natural bridges of Utah have been formed. 

Extended Rivers. — Uplift of the land often raises parts of the 
sea bottom and adds them to the continents. Over such land the 
streams are extended, and independent streams may there unite, one 
stream being ingrafted on another. Such a condition is common 
on coastal plains, and has developed in numerous instances along the 
coastal plains of southern United States. 

Drowned Valleys. — On the other extreme, a depression of the 
land may dismember streams by drowning the lower part of a valley, 

and causing the tribu- 
taries to enter the sea 
along separate courses. 
A valley which has had 
this fate is a drowned 
valley. The northeastern 
coast of the United States 
and the northwestern 
coast of Europe offer in- 
numerable instances of 
such estuaries. Chesa- 
peake Bay is a typical 
instance, and so is the 
Baltic Sea. Before sub- 
mergence, the streams 
which now enter these 
bodies of water through 
separate mouths entered 
the sea through a trunk 
stream to which they were tributary. If the land should be uplifted 
again, raising the shallow beds above sea level, the streams would 
have their courses extended and would become ingrafted upon the 
trunk stream. 




Fig. 116. — The course of the drowned lower Hudson 
River southeast of New York. 



THE RIVER VALLEY CYCLE igi 

The Effects of Depression. — While elevation of the land gives new 
life to a stream, depression tends in the opposite direction, for it lowers 
the level of the land that must be reduced by denudation by a rela- 
tive rise in the baselevel. Locally depression may revive a stream 
by bringing the baselevel so far up-stream that the slope is greater 
than is needed ; but such a change produces so slight an effect that it 
may be ignored. 

Tilting and Local Uplift. — The effects of elevation and depression 
have so far been considered as if they were uniform in amount through- 
out the stream ; but there are three possible and important ways in 
which there may be variation from such uniformity. The first of 
these is a tilting, increasing the slope toward the sea, which naturally 
intensifies the effect of the uplift by giving the stream greater power 
for excavation. The second is a tilting in the opposite direction by 
which the slope is diminished, and therefore the power of the stream 
possibly decreased. Such a tilting might conceivably proceed so far 
as to even reverse the direction of stream flow on a small land surface 
occupied by weak streams. 

The third variation is by local uplift across the stream course. 
If the rate of local uplift is too rapid, or the stream too weak to cut 
its bed as fast as the uplift proceeds, the stream may be transformed 
to a lake by the growth of a dam, and its course may be diverted or 
even reversed. Such is probably the usual result when mountains 
rise across a stream course; though there are some stream courses 
across mountains which may have existed before, or be antecedent 
to, the mountain uplift. Such a stream course is said to be ante- 
cedent. 

Illustrations of Antecedent Streams. — This explanation has been 
applied to a number of cases of rivers crossing mountain ridges or 
parts of ranges, such as the Green River across the Uinta Mountains 
of Utah and the Sutlej in the Himalayas ; but in most such cases the 
explanation is very doubtful, if not quite disproved, as in the case of 
the Green River. On the other hand, the Rhine where it crosses the 
highlands of central Germany, the Meuse in the highland of Ardennes 
in northern France, and the Kanawha River where it crosses the 
plateau of West Virginia seem to be true instances of antecedent rivers. 

Incomplete Cycles of Erosion. — Thus it is clear that, although the 
normal tendency is for river valleys to pass through a cycle of develop- 
ment from youth to maturity and old age, there are other conditions 
operating upon the surface of the lands by which this cycle of develop- 
ment is liable to interruption. And, since the cycle of development 
is a long one, rivers in one part or another of their course are certain, 
sooner or later, to experience some influence by which the. cycle is 
interfered with. The age of the earth is so great that, had the cycle 
of valley development proceeded uninterruptedly, the lands of all 
the continents would long since have been wasted away to the condi- 
tion of advanced old age. Instead of that, we have on the continents 



192 COLLEGE PHYSIOGRAPHY 

the pleasing topographic variety, and the great complexity of surface 
forms that bear the stamp of river work in all stages of valley develop- 
ment and of development interrupted, retarded, and accelerated by 
a multitude of accidents and interruptions of different origins and 
with different results. 

Underground Rivers 

Relation to Surface Drainage. — In this chapter drainage has been 
treated as a surface phenomenon ; but it has previously been shown 
that there is also an underground drainage with important results. 
One phase of the underground drainage, where solution is enabled 
to play an important role, gives rise to a system of subterranean water- 
ways, which at times are large enough to deserve the name under- 
ground rivers. 

Contrasts with Ordinary Rivers. — These underground rivers differ 
widely from surface rivers in many important respects. The under- 
ground valley is a rock-walled and rock-roofed cavern ; its form and 
direction are irregular and unsystematic, as are its tributaries ; there 
is little broadening by weathering ; there are no floodplains and no 
deltas, for the sediment load is slight ; and, since solution is the 
prime factor in the development of the underground course, the life 
history of the cavern valley is wholly unlike that of a surface valley. 

The underground river is a special phase of percolating water, en- 
larged in volume as a result of development under favourable condi- 
tions. It is, however, somewhat more than this, for although a part 
of its supply comes from percolation into the ground, and is, there- 
fore, normal underground water, another part is water that, after a 
course of greater or less length on the surface, disappears abruptly 
into the ground and thence onward follows the underground course 
until it emerges again as a spring. From this standpoint under- 
ground rivers may be classed as a special kind of river, and where 
entering other rivers, as a special phase of river tributary. 

There is, in fact, every gradation between the surface river and 
the underground river, for many surface streams disappear into the 
ground, and some have only very short underground courses, even 
no more than the passage beneath a natural bridge a few feet in width. 
The ultimate fate of the underground course is to be exposed at the 
surface when the limit of downcutting is reached, and when weather- 
ing lowers the surface to it. 

Influence of Valley Stage on Habitation 

Favourable Conditions in Extreme Youth. — In the immature stage 
of valley development the level land invites settlement and agriculture 
where soil and climate are favourable. Thus coastal plains, the plains 
of the Red River of the North, and other land surfaces, upon which 



THE RIVER VALLEY CYCLE 193 

the drainage has not been long established, may be the seat of an 
agricultural population. Where the surface is too level the early state 
of drainage may leave the land so wet that, as in the Everglades of 
southern Florida, it is uninhabitable without artificial drainage. 

Young Gorges Unfavourable to Man. — During the youthful stage 
of valley development, the flat-topped divides between the few 
streams are still good farm land, and although swamps may still 
exist, they are, in the main, better drained than in the earlier stage. 
The valley bottoms are not inviting places for human habitation, for 



Fig. 117. — Slopes so steep that they must be terraced in order to be cultivated. 
Scene in China. 

they are too narrow and too steeply enclosed. Nor are they, as a 
rule, feasible routes of travel, for their slope is variable, and often too 
steep, while their narrowness and the swinging of the stream against 
their walls, first on one side, then on the other, are opposed to passage 
up or down the gorge valleys. Young valleys are also obstacles to 
travel across country, for they interpose narrow gashes across the 
route, down whose steep slopes it is difficult, if not impossible, to 
descend, and across which it is expensive to place bridges. 

Uses of Streams in Gorges. — The water power is valuable ; but, 
being often deeply set in a valley between gorge walls, it has not al- 
ways been possible to utilize it. Now that it can be transformed to 
electric power, more and more of this hitherto inaccessible water 
power is being utilized. In arid lands the water in young stream val- 
leys is difficult to utilize for irrigation, for it may be scores or hundreds 
of feet below the level of the land through which the stream is flowing. 



194 COLLEGE PHYSIOGRAPHY 

Mature Valleys most Favourable to Habitation. — Mature valleys, 
with their floodplains and moderately sloping valley walls, are in- 
viting to settlement, for the land can be cultivated, and the grade 
and breadth of the valleys lead to their use as routes of travel, which 
become even more important if occupied by a navigable river. Towns 
and cities develop along the larger valleys, often at the junction of 
tributaries along which other routes extend, making the junction of 
the valleys a centre of converging highways. 

Among the headwaters, especially during the stage of early maturity, 
the many minor tributaries, as they gnaw their way back by headwater 
erosion, so dissect the land that it is a hilly region, perhaps too rough 
for farming and, therefore, still given over to forest, or possibly utilized 
for grazing. This is the condition of much of the plateau which 
skirts the western base of the Appalachians and extends westward 
into Ohio, Kentucky, and Tennessee. With advancing maturity this 
hilly condition disappears and the surface becomes more and more 
even and suited to agriculture. A region of full maturity is one of 
slopes sufficiently moderate for agriculture in practically all its parts. 
Full maturity and old age are the most favourable stages of valley 
development for man's uses. 

References to Literature 

R. E. Browne. Ancient River Beds of Forest Hill Divide, ioth Ann. Rept. 
California State Mineralogist, 1890, pp. 435-465. 

M. R. Campbell. Drainage Modifications and their Interpretation, Journ. 
Geol., Vol. 4, 1896, pp. 567-581, 657-678; Tertiary Changes in the Drain- 
age of Southwestern Virginia, Amer. Journ. Sci., Vol. 148, 1894, pp. 21-29; 
Erosion at Baselevel, Bull. Geol. Soc. Amer., Vol. 8, 1897, pp. 221-226. 

H. P. Cushing, H. L. Fairchild, R. Ruedemann, and C. H. Smyth, Jr. Geology 
of the Thousand Island Region, Bull. 145, N. Y. State Museum, 1910, 
194 pp. 

R. A. Daly. The Accordance of Summit Levels among Alpine Mountains, 
Journ. Geol., Vol. 13, 1905, pp. 105-125. 

W. M. Davis. The Stream Contest along the Blue Ridge, Bull. Geog. Soc. 
Phila., Vol. 3, 1905, pp. 213-244; Incised Meandering Valleys, ibid., 
Vol. 4, 1906, pp. 182-192; Geographical Essays, Boston, 1909, including 
" The Seine, the Meuse and the Moselle " ; " Baselevel, Grade, and Pene- 
plain " ; " The Peneplain " ; " Plains of Marine and Subaerial Denuda- 
tion " ; "The Geographical Cycle"; "Complications of the Geographi- 
cal Cycle"; "The Geographical Cycle in an Arid Climate"; "The 
Rivers and Valleys of Pennsylvania " ; " The Rivers of Northern New 
Jersey " ; and several other papers. 

H. M. Eakin. The Influence of the Earth's Rotation upon the Lateral Erosion 
of Streams, Journ. Geol., Vol. 18, 191 1, pp. 435-447. 

S. F. Emmons. Uinta Mountains, Bull. Geol. Soc. Amer., Vol. 18, 1907, 
pp. 287-302. 

G. K. Gilbert. Land Sculpture, in Geology of the Henry Mountains, U. S. 
Geographical and Geological Survey of the Rocky Mountain Region, 
Washington, 1877, pp. 99-150; The Sufficiency of Terrestrial Rotation 
lor the Deflection of Streams, Memoirs Nat. Acad. Sci., Vol. 3, 1885, pp. 
7-10; The Convexity of Hilltops, Journ. Geol., Vol. 17, 1909, pp. 344- 
350- 



THE RIVER VALLEY CYCLE 



195 



J. W. Goldthwait. Physical Features of the Des Planus Valley, Bull, n, 
Illinois Geol. Survey, 1909, 10,3 pp. 

J. P. Goode. The Piracy of the Yellowstone, Journ. Geol., Vol. 7, 1890, pp. 
261-271. 

D. W. Johnson. The Tertiary History of the Tennessee River, Journ. Geol., 
Vol. 13, 1905, pp. 194-231; Drainage Modifications in the Tallulah 
District, Proc. Bost. Soc. Nat. Hist., Vol. 23, 1907, pp. 211-248. 

H. B. Kiimmel. Some Rivers of Connecticut, Journ. Geol., Vol. 1, 1893, 
pp. 371-393- 

W. T. Lee. Canyons of Southeastern Colorado, Journ. Geog., Vol. 1, 1902, 
pp. 357-370; Canyons of Northeastern New Mexico, ibid., Vol. 2, 1903, 
pp. 63-82. 

W. Lindgren. The Tertiary Gravels of the Sierra Nevada, Prof. Paper 73, 
U. S. Geol. Survey, 191 1, 226 pp. 

F. Lowl. Uber Thalbildung, Prague, 1884. 

Lawrence Martin. The Physiography of the Lake Superior Region, Mono- 
graph 52, U. S. Geol. Survey, 191 1, pp. 85-117 ; The Physical Geography 
of Wisconsin, Journ. Geog., Vol. 12, 1914, pp. 226-232. 

A. Philippson. Studien iiber Wasserscheiden, Leipzig, 1886, 162 pp. 

F. von Richthofen. Fiihrer fur Forschungsreisende, Berlin, 1886. 

N. S. Shaler. Spacing of Rivers with Reference to Hypothesis of Baselevel- 
ling, Bull. Geol. Soc. Amer., Vol. 10, 1899, pp. 263-276. 

W. S. T. Smith. Some Aspects of Erosion, Bull. Dept. Geol., Univ. Cali- 
fornia, Vol. 2, 1899, pp. 155-178. 

W. H. Storms. Ancient Channel System of Calaveras County, 12th Ann. 
Rept., California State Mineralogist, 1894, pp. 482-492. 

R. S. Tarr. The Peneplain, Amer. Geol., Vol. 21, 1898, pp. 351-370. 

C. R. Van Hise. A Central Wisconsin Baselevel, Science, N. S., Vol. 4, 1896, 
PP- 57 _ 59; A Northern Michigan Baselevel, ibid., pp. 217-220; The 
Origin of the Dells of the Wisconsin, Trans. Wis. Acad., Vol. 10, 1895, 
PP- 556-560. 

C. A. White. On the Geology and Physiography of Northwestern Colorado 
and Adjacent Parts of Utah and Wyoming, 9th Ann. Rept., U. S. Geol. 
Survey, 1889, pp. 677-712. 

A. W. G. Wilson. The Laurentian Peneplain, Journ. Geol., Vol. n, 1903, pp. 
615-669. 



TOPOGRAPHIC MAPS 



Higbee, Colo. 
Boise, Idaho 
Sierraville, Cal. 
Granite Range, Nev. 



Arid Land Drainage 

Kaibab, Ariz. 
Alturas, Cal. 
Disaster, Nev. 
Tooele Valley, Utah 



Watrous, N.M. 
East Tavaputs, 
Paradise, Nev. 
Salt Lake, Utah 



Utah 



Elmira, N.Y. 
Briceville, Tenn. 
Huntington, W.Va. 
Springfield, Mass. 
Hartford, Conn. 
Monterey, Va. 
Franklin, W.Va. 
Lykens, Pa. 
Skaneateles, N.Y. 



Mature and Old Valleys 

Kaaterskill, N.Y. 
Scottsboro, Ala. 
Charleston, W.Va. 
Becket, Mass. 
Mt. Marcy, N.Y. 
Fort Payne, Ala. 
Maynardville, Tenn. 
Atlanta, Ga. 
Caldwell, Kan. 



Gaines, Pa. 
Salyersville, Ky. 
Cohoes, N.Y. 
Monadnock, N.H. 
Winslow, Ark. 
Estillville, Ky. 
Hazelton, Pa. 
New Haven, Conn. 
Ovid, N.Y. 



196 



COLLEGE PHYSIOGRAPHY 



Kaaterskill, N.Y. 



Migrating Watershed 
Gloversville, N.Y. 



Hamilton, Idaho 



Platte Canyon, Colo. 



Mountain Gorges 
Livingston, Mont. 



St. Paul, Minn. 
Niagara Gorge, N.Y. 



Post-glacial Young Streams 

Cohoes, N.Y. 
Lacon, 111. 



Skaneateles, N.Y. 
Rochester Special, N.Y. 



Rejuvenated Streams and Entrenched Meanders 



Lockport, Ky. 
Palo Pinto, Tex. 



Salyersville, Ky. 
Johnstown, Pa. 



Charleston, W.Va. 
Palmyra, Va. 



Delaware Water Gap, Pa. 
West Point, N.Y. 
Piedmont, W.Va. 
Bristol, Va. 



Stream Adjustments 

Kaaterskill, N.Y. 
Harrisburg, Pa. 
Caddo Gap, Ark. 
Lykens, Pa. 



Hollidaysburg, Pa. 
Monterey, Va. 
Fort Payne, Ala. 
Antietam, Md. 



CHAPTER VIII 

GLACIERS AND GLACIATION 

Water in its Solid Form 

Water at Freezing Temperatures. — At the freezing point, 3 2° F. 
(o° C), fresh water assumes the solid form, and its properties and the 
work which it performs are then completely changed. Water in 
its solid form can be of importance only in those climates where the 
temperature descends below 32 for a part of the year ; and it becomes 
increasingly important as the period during which such temperatures 
prevail lengthens. The parts of the earth where this condition ob- 
tains are (1) high latitudes, (2) high altitudes. 

The Freezing of Ground Water. — With a temperature below the 
freezing point it is possible to have a variety of different forms of 
solidification of water, and consequently of different kinds of work 
performed. The ground water, for example, becomes frozen and, 
as we have seen in the study of weathering, performs powerful work 
in rock disintegration. For the time being it solidifies the soil with 
an icy cement, interfering with percolation and with erosion. Ex- 
panding in the soil, it pushes fragments about, even thrusting good- 
sized fragments upward. 

Ice Work in Rivers. — Rivers are frozen at their surface, and ice 
is formed even on their beds, aiding in removal of rock fragments. 
On melting it turns loose, in liquid form, the water that has been 
temporarily locked up in the solidified state. As has been stated in 
the discussion of rivers, the formation of ice is an important factor 
in the erosive work of river water. 

Ice in Lakes. — Ice forms also on the surface of lakes, but only 
the shallowest freeze to the bottom. This is because fresh water, 
on growing colder, becomes heavier and sinks to the bottom until 
the temperature of about 39 F. is reached, after which further cooling 
makes it lighter. The lake cannot freeze over until the whole mass, 
from top to bottom, has had its temperature lowered to 39 , and then 
the surface layer lowered to 32 . When the ice forms, it expands and 
is lighter than the water and, therefore, floats. It assumes a crystal- 
line form on freezing. The crystals are of the hexagonal form with 
their axes extending vertically. Lake ice, as well as river ice, has 
geological work to perform, as will be shown later. 

Sea Ice. — Even the sea, in very cold climates, freezes at the sur- 
face ; but salt water behaves very differently from fresh. Its freezing 

197 



108 COLLEGE PHYSIOGRAPHY 

point is 2 7 or 28 , according to its salinity, and it continues to con- 
tract and grow heavier until the freezing point is reached, when it 
expands and the ice floats, as in lakes. During the freezing the salt 
is not included in the ice crystals, but is left as brine in the interstices. 
Therefore the ice tastes salt. The work of sea ice is treated later. 

Other Solid Forms of Water. — Water is also present in the atmos- 
phere in the form of water vapour, and when the temperature of the 
air descends below the freezing point, some of it will be transformed 
to the solid state if the proper point of humidity is reached. It may 
come out as frost on the ground, or, under certain conditions, as hail, 
or as snow or sleet. The hail and sleet may in some cases be frozen 
raindrops, but the snow is a crystal form which vapour assumes as it 
condenses to solid state in the air. The crystals grow by additions 
of molecules of vapour and often assume beautiful starlike form, as 
the crystals grow under the hexagonal system. In their descent 
they may be broken, or matted together, or partly melted and frozen. 

The Work of Snow. — Falling to the ground, the snow crystals 
form a blanket of snow, whose thickness varies from place to place and 
from season to season. Some of the snow disappears by evaporation, 
but in most regions the snowfall is dissipated mainly by melting dur- 
ing the return of warm weather, either during the course of the winter 
or at its close. Then, in the form of running or percolating water, 
it enters into the activities which have been considered in preceding 
chapters. While it lies upon the ground in solid form, the snow is 
usually inert and ineffective as an agent of change ; it serves as an 
agent of protection both to the land and to the plant and animal 
life which it covers with a blanket of such poor conductivity that it 
serves to maintain a far more uniform temperature than the bare 
surface could have during the changes of day and night and from day 
to day. Upon melting it springs into high activity, and becomes an 
agent of erosion. Even in the solid state, snow is an agent of erosion 
where it lies on slopes of sufficient steepness to permit it to slide away 
in avalanches. 

Snow Fields 

The Height of the Snow Line. — When the snowfall is in excess 
of melting and evaporation, a blanket of snow remains the year round. 
The line above which the snow remains permanently on the ground 
is called the snow line. The level of the snow line varies greatly with 
the latitude, for one of the chief factors in determining it is tempera- 
ture. In the Antarctic, and in parts of the Arctic, it lies at sea level, 
but in the tropical zone it is from 14,000 to 20,000 feet above sea level. 
Therefore, most lofty mountains rise above the snow line. In the 
Andes the snow line is reached at 16,000 to 18,000 feet in Bolivia, 
but it descends to 1600 feet in southern Chile. In Mexico the snow 
line lies at an elevation of about 15,000 feet, but descends to less than 
3000 feet in Alaska. No part of eastern North America rises above 



GLACIERS AND GLACIATION 



199 



TABLE SHOWING ELEVATION OF SNOW LINE IN DIFFERENT 
LATITUDES (after Paschinger) 



Latitude 


Place 


Height in Feet 


8o°-7o° N. 


Franz Josef Land 


1,000 


7o°-6o° 


Iceland 


1,800 


6o°-5o° 


Coast of Alaska 


2,500 


5o°- 4 o° 


British Columbia 


4,600 


4 o°-30° 


Asia Minor 


1 1 ,000 


3 0°-20° 


Southern Himalaya 


16,000 


20°-IO° 


Colombia 


15,000 


io°- o° 


Venezuela 

Equator 


14,000 


o°-io° S. 


New Guinea 


14,000 


IO°-20° 


Bolivia 


16,000 


20°-30° 


Northern Argentina 


15,000 


30°-40° 


Central Chile 


5,000 


40°-5o° 


South Central Chile 


2,300 


5o°-6o° 


Straits of Magellan 


1,600 


66°-7o° 


Antarctica 


At sea level 



the snow line until the mountains of northern Labrador are reached, 
while beyond, in Baffin Land, the snow line descends to 2000 feet or 
less. In Europe the snow line is reached by the Pyrenees (6500 feet), 
the Caucasus (8500 to 14,000 feet), the Alps (8500 feet), and the Nor- 
wegian highland (3000 to 5000 feet). The Himalayas and other 
lofty mountains of central Asia rise above the snow line, as do the 
lofty mountain peaks of central eastern Africa at the equator. Next 
to Australia, which does not at any point rise above the snow line, 
Africa has the least area of snow field of all the continents. But 
while Australia does not rise above the snow line, New Zealand 
does. 

Relation of Precipitation to Snow Line. — Since it is said that the 
snow line is determined by the excess of snowfall over melting and 
evaporation, it is evident that the amount of snowfall as well as the 
temperature must be an important factor in determining the snow 
line. This is well illustrated in the Alps, where the amount of snow- 
fall on the southern side is greater than on the northern side, and in 
the Himalayas, where the same is true and the snow line therefore 
descends three or four thousand feet lower than the colder northern 
side. Many other mountains illustrate the same influence, as, for 
instance, the Alaskan mountains, where the snow line also descends 
greatly on the warmer ocean side, where the heaviest snowfall occurs. 

Effect of Dry Air. — Since evaporation takes place from the sur- 
face of snow fields, there is more loss of snow on slopes exposed to dry 
winds than on slopes where the air is damper. Therefore the snow 
line is influenced to some extent by the dryness of the air, entirely 



200 COLLEGE PHYSIOGRAPHY 

apart from the fact that this condition causes less snowfall. It is 
partly for this reason that in Spitzbergen, although in latitude 78 , 
the snow line is not reached by some surfaces 2000 feet above sea 
level. 

Relation of Snow Line to Exposure and Topography. — The posi- 
tion of the snow line is also much influenced by the exposure and topog- 
raphy. The nature of the slope of the surface, the effect of winds 
in sweeping off or in adding snow, exposure to the direct rays of the 
sun, protection from the sun by the shadows of cliffs or mountains, 
and the neighbourhood of ice to cool the air are among the factors 
that cause local variation in the snow line. Due to such influences, 
there may be a difference of 1000 to 1500 feet in the elevation of the 
snow line, even in a very short distance. The effect of the exposure 
and topography in the elevation of the snow line is well illustrated 
in Alaska and in Spitzbergen. In Ice Fiord in Spitzbergen, where 
the snowfall is light, there are some places in shaded spots, in which 
the snow is driven by the wind, where the snow line is but a few hun- 
dred feet above sea level, while near by no snow lies in summer at an 
elevation of 2000 feet. 

The Nature of Snow Fields. — The snow surface that lies above the 
snow line is called the snow field (Fig. 118). Where the slopes are steep, 
as among many mountains, much of the snow is shed into the valleys, 
and an extensive part of the region above the snow line is bare of snow, 
while in the valleys it accumulates to depths of hundreds of feet. 
On the more gentle mountain slopes it may also attain great depths, 
especially in regions of heavy snowfall, like the coastal mountains of 
Alaska. The depth of snow, and the area covered, are also influenced 
by the relative proportion of snowfall to loss by melting, evaporation, 
and discharge through glaciers. The most favourable places for ex- 
tensive snow fields are those of no great ruggedness, where loss from 
melting is slight or absent, and where slopes down which glaciers 
may flow are not steep. Such conditions are met in the Antarctic 
continent and in Greenland, where the most extensive snow fields of 
the present day are found. Other large snow fields exist in other 
Arctic islands ; and there are extensive snow fields among the lofty 
mountains, notably in the Himalayas and in Alaska, where the tem- 
perature is low and the snowfall heavy. 

Largest Snow Fields in Polar Regions. — Speaking generally, the 
snow fields become smaller from polar toward equatorial regions, 
because the area that rises above snow line diminishes in extent ; and 
many snow fields in mountains of warm temperate or tropical lati- 
tudes are hardly more than large snow patches, preserved in protected 
spots. This statement is correct only in general, for where extensive 
areas of mountain rise well above snow line, and receive abundant 
snowfall, large snow fields may exist in the central temperate zone, as 
in the Alps and Caucasus, and even in the tropical zone, as in the 
Himalayas. 



GLACIERS AND GLACIATION 



20I 






202 COLLEGE PHYSIOGRAPHY 

Amount of Snow and Ice on the Earth. — It has been estimated 
by Chamberlin and Salisbury that there are, at present, on the earth 
not less than a million cubic miles of snow and ice, which, if all melted 
and returned to the sea, would cause a rise in the level of the ocean of 
about 30 feet. 

Relation of Glacier to Snow Field 

Amount of Snowfall. — The amount of snowfall in regions of exist- 
ing glaciers is suggested by the annual precipitation of 187 to 671 
inches of snow at Valdez, Alaska, and 300 to 400 inches at Field and 
Glacier, British Columbia. These are from places far below the snow 
line. High up in the snow fields of these mountain glaciers the pre- 
cipitation is doubtless much greater. In Greenland and Antarctica 
a very much smaller snowfall nourishes the largest glaciers in the world, 
because the climate is so cold that much less, or perhaps none, of the 
snow is lost by melting and evaporation. 

The Change from Snow to Ice. — As the snow fields accumulate, 
the lower portions slowly change to ice. The change of snow to ice 
is a familiar phenomenon during winter, when the melting of the sur- 
face snow furnishes water which percolates into the snow and freezes. 
The snow banks that are last to disappear in spring illustrate this 
clearly ; and there is every gradation from the snowflake crystals to 
granular, icy snow, and to solid ice. 

Doubtless this process operates also in the snow fields, for in parts 
of some of these there are periods of melting. But it is not the sole 
process by which snow is changed to ice, for this change occurs in 
regions such as central Greenland, where the melting point is never 
approached, even in summer. Experimentally snow can be trans- 
formed to ice under mere pressure, and the crystalline structure of the 
resulting ice is granular, as in glaciers. The exact process by which 
this change takes place is not yet demonstrated, but it seems to be 
a molecular rearrangement, as a result of which the molecules of the 
individual snowflake crystals join together to form larger crystals 
of compact ice. The air included between the snow crystals gathers 
in bubbles, scattered through the newly formed crystals. 

The Beginning of Flowage. — When the snow field becomes thick 
enough, its lower portion is transformed to ice ; and, upon attaining 
the requisite thickness, this ice commences to flow. The exact 
thickness required for the beginning of flowage is unknown, and it 
doubtless varies with the temperature of the ice and the slope on 
which it rests. A depth of several hundred feet is a minimum for 
ice flowage, but it is probable that in a cold region, like Spitzbergen, 
the requisite thickness is several times the minimum. 

Relation to Pressure. — The flowage of ice is a direct result of the 
pressure, and the ice moves away from the pressure as a mass of 
wax will flow away from the pressure when a weight is placed upon it. 



GLACIERS AND GLACIATION 203 

The flow of wax is the result of its viscosity, but it is not definitely 
known that ice is a viscous substance, though in large masses under 
pressure it flows much as viscous substances do. The exact physical 
processes by which the flowage is accomplished are not demonstrated. 
It may be actual viscous flowage, or it may be alternate melting and 
freezing at points and along planes under pressure, or it may be a molec- 
ular rearrangement under stress, or a movement along gliding planes, 
or a combination of two or more of these. The solution of this is a 
physical problem; to the physical geographer, the fact of prime 
importance is that the ice flows under pressure as a viscous substance 
does. 

Zones of Fracture and Flowage. — During the transformation of 
snow to ice, and during the later motion, the ice develops a nodular 
crystalline structure, with crystals an inch or two or three inches in 
diameter. Ice is, therefore, a crystalline rock, > one of the purest 
of rocks on the earth's surface. Like other solid rocks, it is brittle 
under ordinary atmospheric pressures, and may, therefore, be easily 
broken ; but, under pressure of two or three hundred feet, strains and 
stresses no longer cause rupture, but give rise to flowage. Accordingly, 
a large mass of ice consists of ,an upper zone of fracture, and a lower 
zone of flowage, as the earth's crust does. From the behaviour 
of ice at the surface, therefore, one cannot draw accurate con- 
clusions as to its behaviour under the pressure of several hundred 
feet of ice. 

Flowing Ice Forms Glaciers. — Since ice under pressure flows, 
large snow fields, whose base is ice, contribute flowing ice. Such 
flowing ice is called a glacier. The size of the glacier varies with the 
extent of the contributing snow field. It may be small and short, 
or it may be many miles long, and many miles broad, according to 
the snow supply. Ordinarily the flow is down grade ; but, where the 
pressure is sufficient, the ice may flow over level ground, or even up 
grade, if the pressure head be sufficient. Where the up-grade move- 
ment is extensive, the surface grade of the ice must be toward the 
direction of flowage, though the ground over which the ice flows may 
slope away from it ; but locally ice may be forced up grade even with- 
out such an opposite ice surface slope. 

Types op Glaciers 

The Four Classes. — There is every gradation from snow fields to 
glaciers fed by snow fields, and from small, motionless ice masses to 
great glaciers. According to their size, origin, and position there 
are many differences among glaciers, and many names have been 
proposed for the different forms assumed. Among these, however, 
there are four types which have received quite general recognition : 
(1) valley glaciers, (2) piedmont glaciers, (3) ice caps, (4) continental 
glaciers. 



204 COLLEGE PHYSIOGRAPHY 

Glaciers in Valleys. — Of these the simplest as well as the most 
common and best known is the valley glacier, which, as its name 
indicates, is a glacier in a valley, down which it flows. Since valley 
glaciers were first studied in the Alps, they are sometimes called 
Alpine glaciers. There are many differences in form and size of valley 
glaciers, according to the topography and the supply. They grade 
into mere snow fields where smallest, or stretch a score or two of 
miles along mountain valleys where largest, becoming great rivers 
of ice which deeply flood the valleys. The valley glaciers radiate 
outward from the mountain snow fields, from which they move the 
snow outward and downward to a warmer climate, where the ice 
disappears by melting. 

Glaciers at the Bases of Mountains. — Where valley glaciers 
descend to the foot of a mountain and out upon an open slope, as into 
a broad valley or upon a plain, the glacier end spreads into an ice fan, 
or bulb glacier, or piedmont bulb. If two or more such bulbs coalesce, 
a broad-spreading glacier end is formed, to which the name piedmont 
glacier is applied. A piedmont glacier, therefore, has valley glacier 
feeders, but is itself a low-lying ice plateau, spreading with moderate 
flow over the low grade at the mountain base. The extent of its 
spreading will depend upon the amount of ice supplied, and the topog- 
raphy. 

Small Ice Caps. — If the snow fields are extensive enough, and 
the loss by melting, evaporation, and ice drainage are not sufficient 
to prevent, they may completely submerge an area beneath a snow 
cap, which, since only the upper portion is snow while the lower por- 
tion is ice, is commonly called an ice cap. An ice cap most easily 
gathers upon a surface that is not very rugged, and, with especial 
ease; in a cold climate where melting is slight.. Accordingly, ice 
caps are common in the Arctic regions, some of them being only a 
square mile or two in area, and with little or no motion, others cover- 
ing very large areas, as in Vatna Jokull, Iceland. 

Ice Sheets. — The ice cap merges imperceptibly into the continental 
glacier, which, in a sense, is only a large ice cap. Greenland and 
Antarctica contain the two largest existing continental glaciers, but 
during the Glacial Period continental glaciers also existed in north- 
western Europe and northeastern North America. In the conti- 
nental glacier there is a great ice cap, burying all the land, with the 
ice moving outward in all directions from the centre of accumulation, 
and, near its margin, being deflected by its valleys so as often to 
terminate in valley glacier tongues, or distributaries. The continen- 
tal glacier is perhaps too large to be called a glacier, and it is sometimes 
referred to as an ice sheet. In its origin, nature of movement, and 
work performed, the ice sheet is, however, so like a glacier that, in spite 
of the size and other differences, it is properly to be classed as a great 
glacier. 



GLACIERS AND GLACIATION 205 

Valley Glaciers 

Snow Supply. — Glaciers really constitute a form of snow drainage ; 
and, were it not for this leading away of the snowfall, the snow fields 
might accumulate indefinitely. In valley glaciers the process starts 
with the snow itself, which not only falls directly from the sky into 
valleys, but also slides into them from the steeper slopes. During 
every snow storm, and immediately after it, there is downsliding of 
the freshly fallen snow, as there is from the steep roof of a house. 
And at irregular intervals there are other, and often far greater, 
avalanches of snow that has accumulated on slopes until it has become 
so unstable that it can no longer remain. Very often thousands of 
tons come crashing down the mountain slopes, and one of the dangers 
of mountain climbing among the snow fields is from the snow slides 
or avalanches, which, in lofty snow-covered mountains, often occur 
with alarming frequency. 

Added to this supply of snow is a not inconsiderable amount which 
is blown into the valley by the winds which sweep over the snow fields. 
The effect of these winds may often be seen where the snow has been 
swept away from exposed places, or from around rock masses, as it 
is from around boulders in a field. A considerable part of the blown 
snow collects in the more protected valleys, where it settles in the 
quieter air. In Spitzbergen there are many little glaciers or glacierets 
under the lee of cliffs. To these the wind sweeps the snow from the 
neighbouring hill or plateau tops, which in summer are free from snow, 
although higher than the glacierets. 

Thus the valley glacier is supplied from (a) direct snowfall, (b) 
avalanching from the valley slopes, (c) by the indrift of wind-blown 
snow. The area in which these supplies are added is called the glacier 
reservoir. 

The Wasting of Snow. — Naturally the reservoir extends down to 
the snow line ; that is, to the line where snow supply and snow dissi- 
pation are balanced. The glacier itself, flowing down grade, with a 
large supply behind, can extend below the snow line into the zone where 
wastage exceeds snowfall. This is the zone of the dissipator. In the 
dissipator the wastage is primarily by melting, though there is also 
loss by evaporation. The term ablation is applied to the combined 
processes by which the glacier wastes. 

It was said that a glacier was a form of snow drainage, and that 
the first process in the drainage was movement of the snow itself. 
This is continued by the motion of the ice in flowing down the valley, 
and is completed by the melting of the ice in the dissipator and its 
run-off in streams of water. Thus, snow that falls, even in the zone 
of perpetual frost, at last finds escape to the sea, whence it originally 
came as vapour. 

Parts of the Glacier. — The upper part of the glacier is the snow 
field. This grades into a zone of granular snow, called the neve or 



206 



COLLEGE PHYSIOGRAPHY 



fim> and this, in turn, into the ice stream. These three zones are not 
definitely bounded, nor are they capable of exact definition. Ice 
may exist beneath parts of the snow field and the neve ; and snow may 
rest upon the upper part of the ice stream. In general the snow field 
has little or no motion from flowage, the neve is slowly moving, and 
the ice stream moves only by flowage. The neve is the zone in which 
the snow becomes transformed to the crystalline ice. It may be 
entirely absent or invisible in large glaciers where the transformation 
occurs beneath great accumulations of snow. 

Forms of Valley Glaciers. — Most commonly a valley glacier con- 
sists of a broad, branching supply ground, or reservoir, from which 
a tongue of ice protrudes down the valley to a greater or less distance. 
The surface slope is roughly accordant to that of the valley bottom, 
being somewhat steeper, and presenting the average slope, not the 
details. Large irregularities of the bottom may, however, be repre- 
sented, as where domes of ice are raised in passing over buried rock 
hills, and where abrupt descents are represented by a roughly parallel 
descent of the glacier surface. At its front the glacier has normally 
a relatively rapid slope due to melting, and not at all related to the 
slope of the valley bottom. 

Avalanches from steeply sloping Glaciers. — The slope of the valley 
glacier naturally varies greatly. Some have an average angle of slope 
of but a few degrees, and in walking up their surface one seems to be 
scarcely rising. Others are so steeply inclined that it seems a wonder 
that they are able to maintain themselves. Indeed, such glaciers do 
occasionally slide out of their valleys. In the spring of 1901, for 
example, such a fall occurred in the Alps, and the avalanche to which it 

gave rise swept across the road 
over the Simplon Pass, burying 
a village and killing most of the 
inhabitants. A similar glacier 
fall occurred in Yakutat Bay, 
Alaska, in 1905, sliding out of 
a steeply perched valley and the 
ice falling over 1000 feet into 
the fiord. Since there were no 
inhabitants here, there was no 
destruction of life or property ; but a huge water wave was generated 
in the fiord, which swept the neighbouring coast to a height of no 
feet ; and fifteen miles away, where the author observed its effects, 
to a height of 15 or 20 feet. 

Cross-section of Ice Tongues. — In cross-section the surface may 
be fairly flat in large glaciers well above the snow line, and there may 
even be a rise at the sides, where snow has slid from the mountain 
slopes. Here the glacier crowds up to the mountain side, and its 
surface plane is in contact with it. 
Farther down, and especially in the dissipator, the form more 




Fig. 119. — Cross-section of a valley glacier. 
(Shaler and Davis.) 



GLACIERS AND GLACIATXON 



207 



commonly considered typical of glaciers is found. This is a gentle 
rise toward the centre from each side, and a fairly sharp descent as 
each mountain wall is approached, forming a marginal valley with 
the glacier for one wall and the mountain side for the other. This 
valley is due to melting, for the rate of melting is increased along the 
glacier margin by the warming of the mountain surface in the sun. 
A valley may not develop in places where the rate of ice movement 
is sufficient to crowd the ice against the valley side ; and the rate 
needed to bring this about need not be so great in the shady as on the 
sunny side. Toward the end of the glaciers, where melting is most 
rapid, ice thickness least, and motion slowest, the marginal valley 




Fig. 120. 



Cascading Glacier, Alaska, descending out of a high mountain valley, 
receded from 1905 to 1910, but advanced between 1910 and 1913. 



becomes best developed. This broadening of the lateral valleys, 
together with more rapid motion in the centre, causes the lobate 
terminus characteristic of glacier ends on the land. 

Other Kinds of Glaciers. — Besides valley glaciers with the char- 
acteristics mentioned, there are many variations from this, which 
may be thought of as typical or normal. There are, for example, 
glaciers on ledges of various shapes on the face of the cliff, called 
cliff or cornice glaciers. Some are circular, or semicircular, or linear, 
or irregular. Other glaciers terminate on the face of a cliff, in a broken 
end, like a frozen cascade (Fig. 120), and known as cascading glaciers. 



208 



COLLEGE PHYSIOGRAPHY 



Ice blocks discharged from the terminus of a glacier ending on a cliff 
may accumulate below and, becoming recemented, form a new ice 
mass called a recemented or reconstructed glacier. Other glaciers, 
instead of rising to a well-developed, broad snow field, may 




Fig. 



A through glacier (on right) between Nunatak Fiord and the Alsek River, 
Alaska ; medial moraines (on left) . 



head on a low, flat divide from which another glacier descends in 
the opposite direction. Such a glacier, which is double-ended, and 
continues from end to end, is a through glacier (Fig. 121). In Alaska 
there are high level, glacier-like masses whose termini and even whole 
extent are buried beneath angular debris. For these the name rock 
glaciers has been proposed, but the author's interpretation is that 




irt of terminus of Childs Glacier, Alaska, compared in height with the 
Capitol in Washington, which is 287! feet high. 

these represent a phase of moraine-covered glacier, preserved and 
modified by the presence of perpetual frost in the moraine. These 
are some of the subtypes of valley glaciers, which will serve to indi- 
cate that there are variations from the class described as normal. 



GLACIERS AND GLACIATION 



209 



Size of Valley Glaciers. — In the Alps, where there are a great 
many glaciers, there is every gradation from glaciers a few hundred 
feet long to the great Aletsch 
which is 10 miles long, or, with 
its snow field, fifteen miles. 
The average length of the 
better-known glaciers of the 
Alps is from 3 to 5 miles ; but 
the majority of the Alpine 
glaciers are less than a mile in 
length. The Aletsch is about 
a mile wide, but most of the 
glaciers of the Alps are much 
narrower. 

Far larger valley glaciers are 
formed in the Caucasus, Hima- 
layas, southern Andes, and the 
mountains of the Alaskan coast. 
Here glaciers 20 to 40 or even 
more than 50 miles in length 
are found; and widths of from 3 to 5 .miles are not uncommon. 
The Muir glacier of Alaska, for instance, is about 35 miles long and 
from 6 to 10 miles wide, the total area of the ice surface being about 
350 square miles. Other large valley glaciers in Alaska are the 
Hubbard, Seward, Miles, and Columbia (Figs. 122, 123, 125, 126). 

Tidal Glaciers. — The smaller glaciers terminate only slightly below 
the snow line, but the larger ones may descend far below it. The 




Fig. 123. — Three of the largest Swiss glaciers 
(in black) compared in size with the Hubbard 
Glacier in Alaska. 




Fig. 124. — An iceberg stranded at low tide. The portion below the dotted line is sub- 
merged when the iceberg floats. 
P 



2IO 



COLLEGE PHYSIOGRAPHY 



Aletsch, for instance, terminates at an elevation of 4440 feet, which 
is about 4000 feet below the snow line. The great majority of valley 
glaciers terminate on the land, but some of the larger ones, in regions 
where the snow line is low, push on to sea level, and there discharge 




Fig. 125. — Map of tidal and land-ending termini of Nunatak Glacier, Alaska, showing 
advance from 1909 to 19 10, submarine topography of Nunatak Fiord, and two cascad- 
ing glaciers in hanging valleys. 



their ice into the sea as icebergs. This is true of the Muir, Taku, 
Hubbard, Columbia, and other Alaskan glaciers. Glaciers terminat- 
ing in the sea are called tidal glaciers (Figs. 1-25, 126, 130, 132). 



GLACIERS AND GLACIATION 



211 



Thickness of Ice Streams. — Little is known with regard to the 
thickness of glaciers. It is estimated that the Alpine glaciers attain 
a depth of from 800 to 1200 feet, and the fronts of tidal glaciers, below 
as well as above sea level, are sometimes 900 to 1000 feet high. It is 
probable that the great ice streams like the Muir, which is 900 feet 
thick at the end, are much thicker than the Alpine glaciers. The 
Grand Pacific Glacier, near the Muir, is known to have been over 
2500 feet thick in 1894 at a point about twelve miles from the terminus. 




M UIR^LAC:[E : {?\ ^^/^g .. 7 



126. — Muir Glacier in 1892 and in 1913. In 1892 the ice stream was nearly 2400 
feet thick £>\ miles from the terminus. (Left-hand map after Reid.) 



It is probable that some Alaskan glaciers reach depths in excess of 
3000 feet ; yet, since ice will move more rapidly with increased 
pressure, the thicker the glacier, the greater tendency there is for the 
ice to flow down the valley, and hence to put a check upon the depth. 
The glacier depth is normally greatest in the middle, partly because 
the ice surface is often highest there, and partly because the valley 
depth is greatest there. At the margins the ice may thin to a depth 
of but a few feet. The forms of the cross-section will vary greatly, 
according to the form of the valley into which the glacier is moulded. 
Its lower surface has the curve of the valley bottom, but its upper 
surface is variable. It may be a straight line, though usually it is 



212 



COLLEGE PHYSIOGRAPHY 



gently curved upward toward the middle ; and in the zone of the dis- 
sipator it curves sharply downward near the margins (Fig. 119). 

Rate of Motion. — There is much difference in the rate of motion 
of valley glaciers. Some of the smallest are almost, if not quite, 
motionless, while large glaciers move at the rate of several feet a day. 
The rate of motion in a glacier increases from the margin toward 
the center (Fig. 127). Thus in the Mer de Glace, in Switzerland, the 
daily rate of motion in summer and autumn was from 13 to 10^ 
inches near the sides, and much less at the margins, while in the 
center it was from 20 to 27 inches. This is one of the most rapidly 
moving, as it is one of the largest, Swiss glaciers. Reid found that 




Fig. 127. — Glacier in the French Alps, showing more rapid motion near center than at 
sides. Elevations in meters. (Mougin, Ministere de l'Agriculture de France.) 



the Muir Glacier, near its end, was almost if not quite motionless 
at the sides, but rapidly increased in rate toward the center, where 
its motion was 7 feet a day. While this is rapid motion for a 
glacier, it is probable that some of the larger glaciers move even 
faster; and some of the tongues extending to the sea from the 
Greenland ice sheet flow at a rate of 60 to 75 feet a day. Childs 
Glacier in Alaska moved at the rate of 8 to 40 feet a day in 1910 
(Fig. 128), but in the previous year it was moving at the rate of only 
4 to 6 feet a day. 

It is impossible to obtain accurate measurements of rate of flow of 
a glacier from top to bottom, but there is reason to believe that the 
basal layers are retarded by friction. Measurements made by Tyn- 



GLACIERS AND GLACIATION 213 

dall near the side of a glacier showed a decrease in movement down- 
ward. It cannot be said, however, that the exact nature of the change 
in rate in the glacier as a whole is definitely determined by this obser- 
vation. 

The rate of motion varies with the supply, being greatest in those 
glaciers which have large supply. It also varies with the slope, though 
it is not true that the steepest glaciers flow fastest, because steep 
valleys are apt to be small and with small ice supply, while many 
large valleys, with moderate slope, have so large a supply that there 
is rapid ice flow. 

There is also a variation with the temperature, for the ice flows 
fastest when near the freezing point. Thus the glacier is thought by 
some to move faster in summer than in winter. The variation from 
side to centre, mentioned above, is due to the influence of friction, and 




Fig. 128. — Railway bridge over Copper River, Alaska, costing $1,400,000, which was 
threatened by the advance of Childs Glacier in 1910. 

to the thinness at the edge. Since friction retards motion, the nature 
of the valley floor has influence on the rate of motion, as irregularities 
in the bed of a stream have on the flow of the water. Still another 
influence on rate of flow is the presence of debris in the ice. When 
heavily charged with rock fragments flowage is retarded. 

Advance and Recession. — The position of the end of a glacier 
is determined by the balance between supply and wastage. It can 
rarely happen that so delicate a balance will be able to maintain an 
ice front at a given point for a long period of time, for, with climatic 
change from year to year, the supply may vary, or the rate of ablation 
may vary. This gives rise to fluctuations in the ice front, some of 
them minute and seasonal, some of notable extent. 

Cycles of Advance and Recession. — At the present time there 
seems to be a general condition of wastage, and ice fronts are, in the 
main, in recession. This has been true in the Alps, Pyrenees, and 



214 



COLLEGE PHYSIOGRAPHY 



Caucasus for several decades, though prior to 1855 there were ad- 
vances. The Alaskan glaciers, and those of other regions, are, in 
general, thought to be in recession. There are reasons for believing 
it probable that there are cycles of advance and recession, due, per- 
haps, to climatic variations ; and careful records are now being kept 
in the hope of discovering the cause for variations in the position of 
glacier fronts. 

Advance interrupting Recession. — Even during periods of general 
recession individual glaciers may advance, as the Vernagtferner in the 




Fig. 129. — Views of Hidden Glacier, Alaska, from the same point before and after a two- 
mile advance. At B the ice was 1100 feet thick after the advance. 



Austrian Tyrol has done, and as the Yakutat Bay glaciers have 
since 1901. In the latter case the advance has been due to the ava- 
lanching of great masses of snow into the glacier reservoirs during the 
vigorous earthquakes of 1899. This spasmodic and great addition 
to the supply has started a wave of advance which, sweeping rapidly 
down the glacier, has pushed forward the fronts of glaciers that were 
hitherto receding. In one case a glacier front, the Hidden Glacier, 
was pushed forward two miles in a brief interval of time. Another 
advanced a mile in nine or ten months, which is at a rate of not less 
than 20 feet a day (Fig. 129). 




Fig. 130. — The receding Nunatak Glacier in 1899, 1905, 1906, and 1910. (Upper view 

by Gilbert.) 

21S 



216 COLLEGE PHYSIOGRAPHY 

Rates of Recession. — When glaciers whose ends are on the land 
are receding, the rate is less rapid, for the recession can go no faster 
than ablation removes the ice. Before its advance the Hidden Gla- 
cier was receding at the rate of between one-half and one foot a day 
as an average for 6 years. This rate is much more rapid than that 
of the recession of the glaciers of the Alps. A tidal glacier may re- 
cede even much faster when the supply diminishes, for the discharge 
of ice in the form of icebergs is more rapid than that through melting 
alone. Thus Nunatak Glacier, in Alaska, had an average daily reces- 
sion during 6 years, between 1899 and 1905, of about 2J feet; Muir 
Glacier, between 1892 and 19 13, receded at an average rate of over 
5 J feet a day, and Grand Pacific Glacier for two months in the summer 
of 191 2 receded at the rate of 80 to 120 feet a day (Figs. 126, 130). 

Ice Structure. — As has been stated, glacier ice is coarsely crystal- 
line. The ice is not uniform throughout, but is veined and stratified. 
Some of the differences are due to the size of crystals, some to the 
presence or absence of included air bubbles, and some to layers of 
included debris. Very often layers or veins of clear, transparent ice 
reflect a blue color and are called blue veins, forming a striking 
contrast to the opaque, whitish ice. 

The cause for the veining of glacier ice is not perfectly understood, 
and it is probable that the cause is different from glacier to glacier 
and from place to place in the same glacier. Among the causes for 
veining are (a) freezing in cracks which have been filled with water ; 
(b) stratification in the snow fields ; (c) differential motion and shear- 
ing, due to the fact that ice moves at different rates in its different 
parts. The bands and veins of different kinds of ice are often greatly 
contorted, as layers of metamorphic rocks are, showing that, after 
formation, they have been subjected to plastic motion. They lie 
at all angles in glaciers, from horizontal to vertical. 

Surface Features. — • The surface of a valley glacier is a snow- 
covered waste above the snow line ; but in the zone of the dissipator 
it assumes greater variety. Here the veined structure is revealed, 
there is a greater or less burden of rock material, and the surface is 
broken more or less by fissures or cracks. In summer an ice surface 
itself is granular and crumbles, for ablation has weathered out the 
crystal grains so that they adhere loosely, if at all. Immediately 
beneath, however, is massive ice, though water is sinking into this 
ice in the pores and between the crystal faces. 

Moulins and Surface Melting. — As the ice melts there are innu- 
merable tiny streamlets, which, uniting, sometimes give rise to small, 
short streams ; but these soon find escape through a hole in the ice, 
a moulin, which the running water may enlarge so as to form a large 
pit in the bottom of a roughly circular area — resembling a sink hole 
of a limestone country. The water that falls into the moulin may 
cascade to the glacier bottom, or may find escape along a channel in 
the ice. Beneath the ice it may excavate pot holes in the rock, 



GLACIERS AND GLACIATION 217 

some of which are called giant kettles or cauldrons. Such pot holes 
may be seen at Lucerne. Near the front and margins innumerable 
streamlets run down the steep ice slopes to the bordering land. 
During the summer days the ice surface may waste from 1 to 4 
inches a day. 

Glacier Wells and Rock Tables. — The presence of rock fragments 
on the glacier surface often clearly shows that ablation is rapid. If a 
fragment is small enough to be warmed through on exposure to the 
sun, it will melt its way into the ice ; and it is not uncommonly the 
case that the ice surface is pitted by little circular wells, at the bottoms 
of which lie small stones or a thin layer of sand or mud. On the other 
hand, if the fragment is too large to be warmed through, it protects 




131. — Crevasses in Seward Glacier near Mt. St. Elias, Alaska. (Ogilvie, International 
Boundary Survey.) 

the ice beneath from melting. Then as the ice surface melts down, 
this part is left standing with a rock cap, forming a glacier table. As 
the ice pedestal melts the stone slides off, leaving an ice pyramid, 
which then slowly melts away. A similar change takes place when 
sand or mud accumulates in a depression to a sufficient depth to pro- 
tect the ice. Then, with further melting, a sand or mud-covered 
pyramid or ridge is left, from which the loose material is later washed 
away. 

Crevasses. — One of the most striking features of a glacier surface 
is the crevasse, a yawning fissure extending down into the ice. The 
crevasse is due to a straining of the ice to the rupture point. It 
starts as a mere crack, then is widened by further pulling apart and 
by melting. If we think of the ice as a plastic mass with a brittle or 
a rigid crust, it is easy to see that, as the under ice flows, the rigid ice 
that is borne along by it will be ruptured when subjected to straining. 



218 COLLEGE PHYSIOGRAPHY 

It is also easy to see that there will be a limiting depth for the cre- 
vassing, for it cannot extend into the plastic ice below. Crevasses 
are but 200 or 300 feet deep, and commonly less (Figs. 131, 132). 

The abundance of crevasses varies with the amount and rate of the 
straining. In rapidly flowing glaciers, even on a regular bed, there 
is great crevassing, and the glacier may be so broken as to be impas- 
sable from one side to the other. By melting, the surface becomes 
transformed to a maze of ridges, pinnacles, and seracs, with inter- 
vening yawning crevasses. A similar condition appears where a 
glacier flows over a sharp incline in the valley bed, giving rise to what 
is called an ice fall or an ice cascade. Such ice falls interpose serious 
obstacles to travel over a glacier surface. Below the ice cascade the 
crevasses may be closed again, or, if not, their bottoms are soon 
reached by the general lowering of the surface through ablation. 

Crevasses also develop through the strain introduced by differential 
motion, and individual crevasses may appear in any part of the 
glacier. There is often a zone of crevassing extending from the sides 
out toward the center of the glacier as a result of the more rapid rate 
of motion in the center. This exerts a strain at an angle with the 
direction of motion of the glacier, and the ice is pulled apart by the 
tension, giving rise to fissures at right angles to the strain. Thus 
these crevasses point up the glacier at an angle of about 45 ° to the 
direction of motion. These fissures are sometimes developed in such 
numbers as seriously to interfere with passage over the glacier surface. 

Any tension sufficient to rupture brittle surface ice may develop 
crevasses. Such strains may be set up either by differential motion of 
the flowing lower ice, or by topographic influence of the slope and 
motion of the rigid upper ice. Under the multitude of conditions 
of movement and topography affecting glaciers there are innumerable 
detailed causes for crevassing. A glacier that is stagnant and uncre- 
vassed may, upon being forced to move, become greatly crevassed, as 
the advancing Yakutat Bay glaciers were. The thrust may even 
cause horizontal or thrust faulting in the ice, and the rigid margin of 
an advancing glacier may be broken into fragments, which, with the 
continuation of the push, fall from the sides as detached ice blocks. 
This is one of the causes for the discharge of icebergs from the face of 
a tidal glacier. 

Crevasses are not confined to the lower portion of the glacier. 
They occur also in the neve and on the glacier above the snow line. 
In such positions the snow often partly or completely covers them and 
hides them from view. This is one of the dangers which mountaineers 
must guard against when travelling above the snow line. 

Transportation of Rock Material. — Rock fragments are supplied 
to glaciers for transportation in several ways. Some is blown in by 
the wind, and still more falls from the valley sides, where the rock 
fragments are loosened by weathering. Far greater quantities come 
with the snow in avalanches. Thus rock fragments, of varying size, 



GLACIERS AND GLACIATION 



2IQ 




Fig. 132. — Crevassed tidal terminus of Nunatak Glacier. Alaska. 



220 



COLLEGE PHYSIOGRAPHY 



are scattered through the ice of a valley glacier, the quantity varying 
according to conditions. To this supply of rock fragments the glacier 
adds still more, which it picks up itself from its bed. Some of these 




Fig. 133. — Debris-laden basal ice of Hubbard Glacier, Alaska. Such ice is especially 
effective in glacial erosion. 



fragments are plucked loose by the powerful thrust of the ice, and 
some are ground off by the scouring of the valley bottom as the 
moving ice drags rock fragments over it. When on or near the sur- 
face of a glacier, they are prevailingly angular. 

Unlike the other agents of transportation ice carries rock fragments 
irrespective of size. A boulder tons in weight may be transported 
side by side with a grain of sand or clay. 
There is no such assortment according to 
size and specific gravity as is a necessary 
result of transportation by wind and running 
water. 

The Supply of Debris. — Owing to the 
nature of the source of the rock fragments, 
the debris in a valley glacier is carried mainly 
(a) near the bottom, (b) at or near the top, 
though there is (c) some rock material be- 
tween the top and bottom incorporated in 
the ice when it was formed out of the snow. 
The debris at the surface is especially abun- 
dant because it settles there from the air and 
falls from the cliffs. It cannot sink to the 
base of the ice, and that which falls into 
crevasses goes but a short distance into the 

F \ G he I app^rInce e of ta d6bri 1 s n - ice > and is soon exposed at the surface again 
covered ice. (Gilbert.) when ablation lowers the surface to the ere- 





GLACIERS AND GLACIATION 



221 



vasse bottoms. Ablation also concentrates the incorporated [debris 
of the upper glacier at the surface by removing the ice from it. 

The Debris-laden Basal Ice. — The bottom is a zone of abundant 
debris, because the ice is there busily at work eroding its bed and mov- 
ing the loosened fragments away. Most of this debris remains at 




Fig. 135.— Surface of Valdez Glacier, Alaska, completely covered with ablation moraine. 

















/€&■ 


i^^lsi - 





Fig. 136. — Shrubs growing in ablation moraine on Grinnell Glacier, Alaska. 



or near the bottom, because there are no such uprising currents as in 
rivers. Yet there is some uprising, and bottom debris is, therefore, 
brought up into the ice, especially near the front and the margins 

(Fig- 133)- 

The Debris-covered Terminus. — Above the snow line little debris 
is seen on the glacier surface, for it is buried by the snowfall. In the 



222 



COLLEGE PHYSIOGRAPHY 



dissipator, however, it is brought into view and often into relief. 
Rock fragments are scattered all over the surface, but are especially 
abundant near the margins where they have fallen from the enclosing 
cliffs. The protection given by these zones of rock fragments causes 
them to form ridges which seem to be ridges of debris, but are really 
ice ridges, thinly veneered with a protecting coat of debris (Fig. 134). 

Moraines on the Ice. — The debris carried by a glacier is called 
moraine. The bands near the margins of the glacier are lateral 
moraines. They are supplied chiefly by mechanically weathered 
material which falls from the valley walls, but some of the material 
in lateral moraines is supplied by the uprising of ice layers near the 
margins of the glacier. 

Bands of medial moraines (Fig. 121) often extend down the central 
part of a valley glacier, some of them representing the lateral moraines 




Fig. 137. 



Glacial boulders, the one on the left from till deposit in central New York, the 
one on the right from the Greenland ice sheet. 



of tributary or uniting glaciers. Sometimes there are several ribbons 
of medial moraines, each marking the incoming of a branch higher 
up. Other medial moraines come from buried rock knobs, being 
exposed at the surface farther down the glacier by ablation. 

In some cases, as in Alaska, so much debris is incorporated in the 
glacier that ablation concentrates it in a uniform sheet, completely 
hiding the surface of the ice with a coat of ablation moraine (Figs. 

i35. 136)- 

Moraine Deposited by Melting Ice. — The rock fragments in the 
base of the glacier constitute the ground moraine. As the glacier 
moves forward to its front, where it terminates by melting, it brings 
up to the front a large part of the moraine load, though some is 
removed by the running water of the melting ice before it reaches the 



GLACIERS AND GLACIATION 



223 



front. The ice at the front melts and flows away as running water, 
but it can carry with it only a part of the rock load that the ice bears. 
This falls to the base of the glaciers, and there builds up a deposit, 
called the terminal moraine. If the ice front remains in approxi- 
mately one position for a long enough period, a very considerable 
deposit may be made. Terminal moraines of some extinct glaciers 
of large size in the Alpine valleys are two or three hundred feet high. 
Deposits by Valley Glaciers. — Being an agent of erosion and trans- 
portation, glaciers are necessarily also agents of deposition. Since 
the ice is ultimately transformed to water, both ice and water are 




Fig. 138. — -Terminal moraine of Columbia Glacier, Alaska, where advance in 1910 resulted 
in the formation of a push moraine of till, peat, and vegetation. 



involved in the deposition of glacier-borne debris. Those accumu- 
lations made by the ice are glacial deposits; those made by the glacier- 
supplied waters are called glacio-fluviatile deposits. The term glacial 
drift is often applied to these ice and ice-born-river deposits. 

Boulder Clay or Till. — The deposits made directly by the ice are 
characterized by their heterogeneous nature. Both large and small 
fragments occur side by side in the same deposit, with little or no 
assortment. There is, therefore, an absence of stratification, though 
there may be a rough lamination due to the effects of ice motion. 
Owing to the fact that the glacier glides along its bed, there is much 
clay, and the deposit is often called boulder clay, indicating that the 



224 



COLLEGE PHYSIOGRAPHY 



matrix is clay, set with boulders, some of which are of large size. 
Another name, more commonly used, is till. 

Shapes of Glacial Boulders. — The shapes of the stones in the 
boulder clay are distinctive. Though prevailingly angular when 
supplied to the ice, they are abraded into faceted, or soled, subangular 
form (Fig. 137). They usually bear glacial scratches or stria. In- 
deed they are so different from stream-worn or sand-blasted stones 
that they may be distinguished at a glance, even in the consolidated 
and indurated glacial till of past ages. Such glacier-derived shales 
made from boulder clay or till are called tillite, and are usually iden- 
tified by the presence of subangular, striated, glacial boulders. 

Ground Moraine. — When a glacier has abandoned a valley, the 
till occur in a sheet spread over the region occupied by the glacier, 




Fig. 139. — Glacial streams depositing gravel and sand in the interior flat of Variegated 
Glacier, Alaska. If the moraine-covered ice should now melt away there would be an 
oval area of stratified gravels completely surrounded by unstratified till. 



having been left there when the ice melted. The till sheets represent 
that part of the moraine in and under the ice that was not carried 
away by streams when the ice melted. It is sometimes called the 
ground moraine, and in regions occupied by valley glaciers is not 
commonly very thick. It may, in fact, fail to even veneer the bed 
rock, and may be represented primarily by boulders and little pockets 
in depressions, the rest having been carried away when the ice melted 
or by later erosion. 

Terminal Moraine. — The medial and lateral moraines, also low- 
ered to the valley bottom, occasionally form prominent lines of 
moraine, composed of angular fragments among which are many 
boulders. There are also terminal moraines, marking the sites of halts 
in the receding ice. These moraines are formed partly by the falling 



GLACIERS AND GLACIATION 



225 



of fragments from the ice front, partly by the dragging forward of 
debris beneath the thin ice terminus, though, judging from conditions 
in living glaciers, the latter is a less effective cause for terminal moraines 
than the former (Fig. 138). 

Lateral Moraine. — The process of dragging beneath the ice by 
lateral shove may also help to accentuate lateral moraine deposits, 
which are often very pronounced in valleys formerly occupied by 
glaciers. Running water is another factor at work in the deposit of 
both terminal and lateral moraines, so that they are often complex 
both in form and in composition. 

Glacio-fluviatile Deposits. — There is some lateral drainage in the 
valley between the glacier and the mountains, and here complex mar- 
ginal deposits are made, which may later be interpreted as lateral 
moraines. There is also drainage beneath the ice, for the multitude 




m 

^ . .. . 



Fig. 140. — An Alaskan glacier which readvanced and covered its earlier gravel deposits 
When it melts, a superposition of till upon stratified drift will be found. 



of rills on the glacier surface, descending through moulins, forms a 
subglacial drainage of importance. This drainage emerges from near 
the center of small valley glaciers, and from near the margins of larger 
glaciers, often flowing as violent torrents, heavily laden with sedi- 
ment. Their volume, which is greatest in summer, and increases 
with the daytime melting, is easily seen to be the result of melting 
of the glacier, though some may be contributed from land streams 
descending the mountain slopes (Figs. 80, 139, 140). 

Rock Flour. — The sediment load is given to the water as the 
ice melts and loosens rock fragments which it is carrying. A large 
part of the sediment is derived from the basal layers, where there is 
much finely ground rock, called rock flour. The streams bear so 
much of this rock flour that they are clouded with it, and retain a por- 
tion of the fine sediment even when standing for a time. The streams 
issuing from most valley glaciers are so milky in colour from the quan- 
tity of finely ground rock flour in suspension, that their water has 
been called glacier milk. In some regions, where the rocks have a 
Q 



226 



COLLEGE PHYSIOGRAPHY 



strong colour, or the load is excessive, the glacial streams are discoloured 
brown, or other colour. 

The Reason for Deposition. — The glacial streams are at times 
such torrents that they bring from their ice caves, or tunnels, stones 
and even small boulders, as well as rock flour and sand. As the 
torrents rush these materials along, rapidly rounding them by attri- 
tion, one can hear them bumping together as they roll along the 
stream bed. When confined within an ice tunnel, and under a 
head of water from up the glacier, the glacial streams can bring out 
of the ice a volume of sediment which cannot be transported down 
the slopes of the valley. Consequently, deposit quickly commences, 
and an alluvial fan is started with the apex at the ice tunnel. Over 




Fig. 141. — Relations of outwash gravels (stippled), terminal moraine (circles), lake deposits 
(horizontal lines), and overridden gravels (large dots), to three stages in glaciation of 
Russell Fiord, Alaska. 



this fan the stream spreads in a multitude of distributaries which 
with their smaller volume are the seats of still more deposit (Figs. 
80, 141). 

Outwash Gravel Plains. — Such a deposit, called an outwash gravel 
plain, or valley train, may spread over the valley bottom from side 
to side and accumulate to a depth of scores of feet. It is stratified, 
and the gravel is coarsest near the glacier, where the stones may be a 
foot or two in diameter, and it may grade down to a sand plain. Ordi- 
narily, however, it is a gravel plain with well-rounded stones, and 
crossed by numerous channelways. Very often the alluvial fan grows 
upward over the terminus of the glacier, burying the ice, especially 
where it has reached a state of stagnation. Later melting of the 
buried ice gives rise to pits, hollows, and kettle-shaped depressions 



GLACIERS AND GLACIATION 



227 




Fig. 142. — Kettles in pitted valley train of Hidden Glacier, Alaska, in 1905. 



Such a feature is called a 



am 




HENEY GLACIER 

jK,Copper River Region, 

Alaska. 



byF. E. Williams. 
y-Photographic Stations! 
- VT - -tonal Geographic 
Society' s Alaskaa 
Expedition of 1910 
_, ^fc L. Martin 
*£AM^el5 i n .Charge. 



in the outwash gravel plain. 
(Fig. 142). 

Marginal Lakes and their Deposits. — At the margins of valley 
and piedmont glaciers small lakes are sometimes held in between the 
ice and the land (Fig. 143), like the Berg Lakes of Bering Glacier in 
Alaska (PL IV), those at the borders of Malaspina Glacier near the 
Chaix Hills, and the Margelen 
See of Aletsch Glacier in Switzer- 
land. They often drain out over 
adjacent cols or else beneath the 
ice, in the latter case causing 
destructive floods. In these mar- 
ginal lakes the streams from the 
glacier deposit sediment, and 
when the glacier melts and the 
lake disappears, these form de- 
posits of lake clay. 

These are some of the princi- 
pal glacio-fluviatile deposits con- 
nected with valley glaciers ; but 
there are other forms, whose de- 
scription is taken up among the 
deposits of continental glaciers. 



Fig. 143. — Marginal lakes between an Alaskan 
glacier and its terminal moraine. 



228 



COLLEGE PHYSIOGRAPHY 



Erosion by Valley Glaciers. — Valleys that have been occupied 
by glaciers show many signs of powerful glacial erosion. Many of 
the pebbles and boulders that were left by the glacier are polished and 
striated by the attrition to which they have been subjected as they 
have been ground against one another or against the valley bed. The 
rocks of the valley floor and sides are likewise polished, striated, and 
grooved (Fig. 144). It is as if a great sandpaper, studded with large 
rock fragments, and pressed heavily down on the valley bottom, had 




Fig. 144. — Glacial striae and grooves on a valley side in Alaska. 



been dragged over it. Here and there, too, are places where the 
blocks of rock have been torn off bodily or plucked (Fig. 145). Irregu- 
larities of the rock floor are smoothed and rounded into swinging 
curves, and dome-shaped bosses, known as roches moutonnees or sheep 
backs. Because of the steepness and plucked character of the lee 
slope, they look rough from that side, but the name was given on 
the basis of their smooth appearance as seen from the abraded side 
(Fig. 146). 

These features are so characteristic that one can tell of the presence 
of former glaciers from such signs alone. Coupled with this is the 
fact that streams from the glaciers are burdened with rock flour and 
partly ground-up rock, while the lower layers of the glacier are 
charged with rock fragments. These phenomena make it clear that 
glaciers are eroding their beds, though at what rate cannot yet be told. 
Yet, though the rate be ever so slight, if it proceeds through long 



GLACIERS AND GLACIATION 



229 



, 




. 




k .-- 


<** < »*^JHii^i3fe«f ^^^^^^#Mlii^Jy^ilteiiiii j 1 ii 




^ • '" *w*vl ^Sft^V s '*** ■^"t*^ j*' ; ■- ^^^H^B Hf 




; "'»-. * . ~'y*t'+ y '' i' ' u -" i 




,*■ •• JsW'^i / ',"v ■«■' ■ ' ^ "S 








^j^'^mjB*-*^ «•'<**"■<■ • \jMka£ ^rjiti« '"^trwl 




iJE*ff* " * -'**»* i \%^ TflaJte? *••*■■»■ w^- * 'i • ••' ! ■ ^*8L!vfcii£ 








* ^^^SSmEyHlf*' ^^'"""^ijinS^aBlffig • • •^-"•"'•C' 4* - i^i«P 








^^^^K!K' .* *■ --^Sgl^ Pj^^Mig^jjigf'^PR; 







Fig. 145. 



Rock surface in California made irregular by glacial plucking. 
U. S. Geol. Survey.) 



(Gilbert, 




Fig. 146. — Roche moutonnees in the Sierra Nevada. (Gilbert, U. S. Geol. Survey.) 



230 



COLLEGE PHYSIOGRAPHY 



enough time, it will be competent to notably deepen and broaden the 
valley, as the slow work of rivers and weathering can. 

Characteristics of Ice-sculptured Valleys. — There are clear and 
conclusive evidences that time has not been lacking, for there are 
features characteristic of glaciated regions that admit of no other 
explanation than that of profound deepening and broadening of 
valleys by glacial erosion. Of these features one is the U-s.hape of the 
glaciated valleys (Fig. 147) where the sides are steep, as in young 
valleys, but the valley bottom is broad and more like that of 
maturity. No known process of river work will produce such a 




Fig. 147. — U-shaped valley in Alaska. 



valley. The projecting and overlapping spurs that characterize 
steep-sided valleys in the stage of youth are truncated, and in some 
cases even erased, by the powerful erosion of the glacier (Fig. 149). 
Some of these glaciated valleys are so straight that in Alaska they 
have been called canals (Fig. 154). 

Hanging Valleys. — In valleys with these peculiarities there is 
a peculiar relation between tributary and main valleys. A normal 
stream valley system has its tributaries entering the main valleys at 
or near the level of the main stream (p. 177) ; but in glaciated valleys 
many of the tributaries enter the main valley hundreds, or even a 
thousand feet or more, above the main valley bottom. Such a valley 
is called a hanging valley. In it a stream flows with a certain slope, 
then, reaching the lip of the hanging valley, finds an abrupt change 



GLACIERS AND GLACIATION 



231 




Head of an ice-sculptured valley in the Alps with much bare rock and 
small rock basin lakes. Grimsel Pass, Switzerland. 



i 0* 




Ifek 'h§'^ 


1 w* '. 


&*. • ■■"' 





m 



£*yi J 






149. — Oversteepened side of a glaciated Alpine valley without overlapping 
spurs. Lauterbrunnen Fall. 



2 3 2 



COLLEGE PHYSIOGRAPHY 




Fig. 150. —Relationships of tributary glaciers 
to hanging valleys. (After Davis ) 



in slope down which it leaps in a single fall or in a series of cascades 
to the main valley bottom. It is a wholly inexplicable phenomenon 
on any theory of river erosion, but finds ready explanation under the 
theory of glacial erosion (Figs. 150, 151). 

This explanation is that before the advent of the glaciers there were 
valleys, the tributaries entering the main valley with accordant grade 

(p. 177), as they should. Then 
both main and tributary valleys 
were occupied by glaciers which 
broadened and deepened them, 
but eroded the main valley the 
most. When the ice disappeared, 
the tributaries were discordant, 
and the depth of the main valley 
below the mouth of the tributary 
is the measure of the extent to 
which it was lowered by glacial 
erosion in excess of the lowering 
of the tributary. In some cases 
this discordance indicates glacial 
erosion of 1000 to 2000 feet, ac- 
companying which was notable 
broadening. 

In the recent geological past 
valley glaciers have been far more extensive than now in many 
regions : in Alaska, Patagonia, New Zealand, the Alps, Norway, 
and many other places. During this period powerful glaciers, com- 
pared with which the largest glaciers of the Alps are mere pygmies, 
ploughed along through the mountain valleys for many thousands 
of years. It would seem strange if such glaciers failed to produce 
notable changes. It is, therefore, significant that the assemblage 
of forms here described are common to all such regions of former 
great glaciers, while they are not characteristic features of regions 
that were not glaciated. The great majority of students of glacial 
phenomena accept the evidence as conclusive that glaciers have 
been among the powerful agents of erosion. It is true that there 
are still some who are unwilling to accept the evidence ; so at an 
earlier time there were some who long held out against the evidence 
that streams eroded their own valleys. In some cases, at least, the 
failure to accept the proofs of glacial erosion is due to the fact that the 
best-known glaciers are among the weakest. To use these glaciers 
of the Alps as a basis for interpretation of the power of great glaciers 
is quite like using a sluggish, modern brook as a basis for understand- 
ing the formation of the Colorado Canyon. 

The full acceptance of the evidence of powerful glacial erosion carries 
with it the necessity of assigning to it a very profound influence in 
shaping the topography of many mountain regions of former great 



GLACIERS AND GLACIATION 



233 



glaciers. Deep valleys have been eroded and the mountain topog- 
raphy sharpened. According to Penck and Bruckner the present- 
day topography of the Alps is profoundly modified, and in places 



.■...-■■ 


• ' ■'. 


; .,::\::i; v ''.^-- i .- : '>--l;-- ; .-''- b '". : .-V-:-'---. 

' ■. ; 
^. ,. YUKON RIVE 


; P f ; 



- - \' 


. : .,,,,, , -, . 








;,;;..;.; ;;|,;. : ; ..,,:$-■ :vv: " . 




, ? «#8llF 


- ■ ] 




^■'■m 


m&*^ 


*?■■ ■■*■> ' 


| 


,,,xO^ 






%: ^ 


' ^^" 


>v^ & ^ 








<&am^ 


COLL 


EGE FIORD 








:?■:;■ -■ ; v:" : ^ :: — r x^i^Mr 


S^St^&il^ 





Fig. 151. — Accordant and hanging valleys. Upper view (Duclos) shows Klondike River 
joining the Yukon in the never-glaciated district near Dawson. Lower view shows a 
hanging valley in College Fiord, Alaska, made by glacial erosion. 



determined, by glacial erosion. Many thousands of waterfalls occur 
where the streams from hanging tributaries descend the steepened 
slope of the main valleys, perhaps the most noted being the falls of 
the Yosemite (PL IX). 



234 



COLLEGE PHYSIOGRAPHY 



The Cirque or Kar. — Another noteworthy feature of glaciated 
mountain regions is the cirque or kar, an amphitheatre-like valley with 
steeply rising walls (Figs. 152, 153). Other names used for this 
form are corrie in Scotland, botn in Norway, cwm in Wales, oule in 
the Pyrenees, caldare and zanoga in the Carpathians. Cirques are 
apparently due to the erosive action of glaciers, eating backward at 
their heads. The exact nature of the process is not clearly under- 
stood, but it seems to be partly the erosive action of the 
downsliding snow against the cirque walls, and partly the scouring 







Fig. 152. — Cirques in the Rocky Mountains. (Darton.) 



at the base of the slopes by outward motion of the ice that 
forms beneath the snow. At the base of the slopes the snow 
accumulates and here there is probably a downward, plunging 
motion, sharply eroding and deepening the valley bottom near 
the cliff base. This outward movement is indicated by the common 
presence of a great crack, called the bergschrund, extending deep 
in the neve near the cirque head. It has been suggested that alter- 
nate melting and freezing at the base of the bergschrund, introducing 
a sort of frost quarrying process, is the main factor in cirque formation ; 
but it does not seem an adequate explanation, though it may be 
contributary. 



GLACIERS AND GLACIATION 235 




Fig. 153. — Head of cirque in Canyon Creek, Glacier National Park. Cliff 3100 feet high. 



236 



COLLEGE PHYSIOGRAPHY 



Whatever the exact nature of the process, the result seems estab- 
lished. By it cirques are formed, enlarged, and caused to extend head- 
ward. By this cirque recession mountain ranges are sharpened, and 
divides are pushed backward. The cirques on one side of a mountain 
range often push their heads back into the area of others on the oppo- 




■rrso o i i Jst/uEj 

Fig. 154. — Cross-section of a fiord in Alaska. Vertical and horizontal scales the same. 



site side of the divide. There are all stages in the process, and it is 
apparently a potent factor in shaping the upper portions of mountains. 
Although the efficiency of ice as a powerful agent of erosion was 
proposed many years ago and has ever since been maintained by a 
body of workers, it is in only comparatively recent times that the full 
significance of hanging valleys and other features indicative of glacial 

erosion has been generally recog- 
nized. To those who recognize 
the significance of the evidence 
it seems so clear that the wonder 
is it was so long overlooked, and, 
when first proposed, was not 
accepted by some who now fully 
admit its convincing nature. 
While there are a few who still 
refuse to admit the force of the 
evidence, to the great majority of 
students of glacial phenomena it 
seems as obvious as that rivers 
have formed their valleys. 

Glaciation in Fiords. — Where 
glacial erosion along a coastal 
region extended below sea level, 
or where the sea has been ad- 
mitted by subsidence to the glacially eroded land, fiords are produced 
(PI. V). To this origin is to be assigned a large part of the fiorded 
coast of Alaska, Patagonia, and Norway, and other coastal regions. 
The problem of fiord production without submergence has been 
studied by Muir, Gilbert, and others. One of the significant fea- 
tures is the submerged hanging valley (Fig. 155). In connection with 
deposition in Alaskan fiords during or after glaciation the glacio- 
fluviatile deposits have filled some deep fiords completely, and in 




Fig. 



55- 



Two submerged hanging valleys 
in Alaska. 



GLACIERS AND GLACIATION 



2 37 



others, terminal moraines in the sea, or moraine bars (Fig. 156), have 
been built by the receding tidal glaciers. 

Distribution of Valley Glaciers. — The distribution of valley gla- 
ciers is practically the same as that of snow fields (p. 199), for in snow 







Set/rocft 

q^Mora/ne for 

\Su6merfed 
1 MoWnefar 



/1a*?o0wa lownctai ft,ponG/ /lore'ne 6er £*SefMMA 



B/ocStsrene 0a/ 



>///////// //// /W 



Fig. 156. — Map and profile of Alaskan fiord with moraine bar near Ripon Glacier. 



fields glaciers are born. Even where snow fields give rise to ice sheets, 
valley glacier distributaries extend out from the ice sheet margins. 
Glaciers occur in all zones, but in the tropical zone are found only in 
the loftiest mountains, and even there are usually small. They increase 
in size and number toward the polar regions, and their termini descend. 
In Europe valley glaciers exist in the Pyrenees, Alps, Carpathians, 
Caucasus, and Norway. No part of the British Isles rises high 



238 



COLLEGE PHYSIOGRAPHY 




Fig. 



157. — Radiating valley glaciers on Mt. Rainier in Washington and Mt. Sanford in 
Alaska. (U. S. Geol. Survey.) 



GLACIERS AND GLACIATION 239 

enough for permanent snow fields. There are no glaciers in eastern 
North America, except possibly in the mountains of northern Labrador. 
In the Arctic islands to the north of Labrador there are many glaciers. 
There are a few small glaciers in Mexico, within the tropics, then 
none south of the San Bernardino, Sierra Nevada, and Cascades in 
California, Oregon, and Washington. There are many in northern 
United States, as on Mounts Rainier and Hood, and in the Glacier 
National Park and still more in British Columbia. North of this the 
number and size of the glaciers increase rapidly, culminating in Alaska 
(Figs. 157, 159). 

No one can tell how many valley glaciers there are on the earth. 
There are certainly scores of thousands, for where the mountains are 
snow-capped, every valley extending from the snow fields has its 
glacier. There are about 2000 in the Alps, but in Alaska there are 
certainly tens of thousands. Only the very largest and most conspic- 
uous are named, and even these are not yet thoroughly explored. 
Many glaciers larger than the Aletsch do not yet bear a name. Other 
regions of great and numerous glaciers are also almost unknown, as 
Patagonia, the Himalayas, and the islands of the Arctic. 

Piedmont Glaciers 

Malaspina Glacier, an Ice Plateau. — The Malaspina Glacier, at 
the base of Mt. St. Elias in Alaska, is the type of this class of glaciers. 



Fig. 158. — Malaspina Glacier and Mt. St. Elias. (Russell.) 

Several large valley glaciers and a number of small ones feed it, form- 
ing a low ice plateau fringing the seaward base of the mountains. 
This ice plateau is about 70 miles long in an east-west direction, and 
from 20 to 25 miles broad, and the total area is about 1500 square 
miles, making it larger than Rhode Island (Figs. 158, 159). -1 

Relation to Valley Glaciers. — While forming a continuous plateau, 
each part, or lobe, of the glacier is supplied by one of the great valley 
glacier tributaries ; that is, the glacier is composed of a series of coa- 
lesced piedmont bulbs. , Accordingly, the different portions of the ice 



240 



COLLEGE. PHYSIOGRAPHY 



plateau are somewhat independent of the other portions, but are 
influenced by the particular valley glacier which supplies the ice. 

Where the valley glaciers emerge from their mountain valleys, 
the ice is in vigorous movement and the surface is often broken by 
crevasses. Spreading beyond the mountain base, the ice flows, less 
rapidly, but is still broken by many crevasses. With decreasing 
motion and increasing ablation, these crevasses gradually die out 
toward the margins. In general, the ice plateau has a fairly level 




Fig. 159. 



Malaspina Glacier and Yakutat Bay, Alaska, (Model by Lawrence Martin. 
Copyright, 1909, by the University of Wisconsin.) 



surface, with undulations and minor irregularities. Its general ele- 
vation is from 1500 to 2000 feet, but it descends rapidly near the 
margin, where the base stands at about the sea level. 

The Forest-covered Margin of Malaspina Glacier. — Most of the 
surface is of clear ice, though there are swirls of moraine upon it, and 
there are morainic bands between the lobes and around the margins. 
In the latter position the ice is completely covered with ablation 
moraine, which in places covers a width of five miles. In some por- 
tions of this moraine-covered area the ice has reached such a state of 
stagnation, and has become so deeply coated with moraine, that a 
forest grows upon it. There, spruce trees., Cottonwood, and alders 
and other plants form a dense thicket, although a few feet beneath 
the surface in which they grow are hundreds of feet of glacial ice. It 



GLACIERS AND GLACIATION 



241 




Fig. 160. — The forest-covered border of Malaspina Glacier. Dead trees in foreground on 
outwash gravels. Living conifers in background on moraine-covered ice. (Russell.) 

is estimated that there are from 20 to 25 square miles of forest on this 
piedmont glacier (Fig. 160). 

Advance of One Lobe of the Malaspina. — In 1906 the eastern lobe 
of the Malaspina Glacier was subjected to a powerful thrust as a 




Fig. 161. — The crevassed eastern border of Malaspina Glacier destroying trees. The 
broken blocks which appear like rock are glacial ice. 
R 



242 



COLLEGE PHYSIOGRAPHY 



result of the abrupt advance of the Marvine Glacier. By this the ice 
was broken into a sea of crevasses, making this part of the glacier 
surface impassable. The advance and breaking of the glacier extended 
to the eastern margin, and overturned and destroyed most of the forest 
that had previously grown there (Fig. 161). The central and western 
portions of the glacier were undisturbed. 

Outwash Gravels at Border of Malaspina. — The westernmost 
lobe of the Malaspina Glacier is rapidly retreating, but it still reaches 
the sea in Icy Bay, where icebergs are discharged from it. It also 
reaches the sea further east at Sitkagi Bluffs, though not there discharg- 
ing icebergs. The remainder of the margin of the glacier rests on land, 




Fig. 162. — Site of a former piedmont glacier northwest of the Alps. 

Bruckner.) 



(After Penck and 



which, so far as can be seen, is chiefly made of outwash gravels de- 
posited by the streams that issue from the ice. There are several 
large streams and innumerable small ones, issuing from ice tunnels 
along the margin. The larger streams are great torrents, heavily 
charged with debris, where they issue from the ice caves. They soon 
branch and subbranch into a multitude of distributaries, and quickly 
deposit the coarsest part of their load, thus building up a series of 
alluvial fans fringing the ice margin. The great amount of water is 
due to the fact that there is here an extensive surface of ice below snow 
line and close by the sea from which warm, damp winds prevail. 

Other Piedmont Glaciers. — Near the Malaspina Glacier are other 
smaller piedmont glaciers and piedmont bulbs of individual glaciers 
which spread out at the mountain base without coalescing with others. 



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BERING GLACIER 



Part of the Piedmont Bering Glacier, Alaska. The foothills south of Martin River Glacier contain 
valuable bituminous and anthracite coal deposits. The streams from the ice have isolated Bering 
Lake from Controller Bay. There are many more streams than are shown on the part of the out- 
wash gravel plain east of Okalee Spit. Contour interval 200 feet. (From Chitina Quadrangle, 
United States Geological Survey.) 



GLACIERS AND GLACIATION 



2 43 



Farther west is another large piedmont glacier, the Bering (PL IV). 
Its area is not known, though apparently smaller than the Malaspina, 
which it otherwise resembles. Doubtless this type of glacier is also 
represented in other parts of the world, though we know of none that 
rival either the Malaspina or Bering. 

Importance of Piedmont Glaciers. — During the Glacial Period, how- 
ever, when valley glaciers were far more expanded than now in many 
mountains, the piedmont type of glacier was common. In the Alps, 
for example, ice bulbs expanded at the mountain base both on the 
north and south sides, and a piedmont glacier overspread a part of 
Switzerland (Fig. 162). They were likewise formed in the North 
American Cordillera. Therefore, although piedmont glaciers are not 
now very common or widespread, they are of importance as existing 
illustrations of a former condition whose effects are now plainly to be 
seen. 

Continental Glaciers 

Ice Caps and Ice Sheets. — As already stated, there are many ice 
caps on the islands of the Arctic, some of them of very small extent, 




Fig. 163. — Vatna Jokull and other ice caps in Iceland. 



while on the larger islands there are more extensive ice sheets. There 
are, however, only two ice sheets that are of sufficient proportions to 
warrant the name continental glacier, one covering much of Greenland, 
the other in the Antarctic. There are all gradations, from the ice 
sheet that covers all the land, as in Greenland, to vast snow fields sub- 




244 



GLACIERS AND GLACIATION 



245 



merging most of the land, as in Spitzbergen, and thence to snow fields 
feeding typical valley glaciers, or to small ice caps a few square miles 
in area (Fig. 163). The smaller ice caps resemble the large ice 
sheets in a small way ; and the great snow fields and glaciers of the 




Barrier 



* 



Fig. 165.- — Beardmore Glacier and other distributaries which connect the continental 
glacier of the interior of Antarctica with the piedmont glacier called Ross Barrier. 
(Hobbs, after Scott and Shackleton.) 



Spitzbergen type are intermediate in character between ice sheets 
and valley glaciers. These glaciers may, therefore, be omitted 
from further description, and attention be paid primarily to the two 
great areas of existing continental glaciers. 



246 COLLEGE PHYSIOGRAPHY 

The Antarctic Ice Sheet. — Within a few years much has been 
learned about the conditions in the Antarctic, though this vast region 
is still in large measure unexplored and unknown. It is an ice-bound 
region, the land covered with snow and ice, and the sea covered with 
floating ice. The snow line extends to sea level, and snow banks and 
glaciers project from the land around the entire coast line. Whether 
the land is one great continent, mostly buried beneath snow and 
ice, or whether it is a series of ice-submerged islands is not yet known. 
It is, however, certain that the South Pole, which lies in the midst of 
this region, is located on a continental glacier, — the largest at present 
existing on the earth. What the size of the continental glacier is, 
cannot be stated, but it cannot be less than 5 million square miles in 
area (Fig. 164). 

The Continental Glacier of Antarctica. — Around the coast line is 
mountainous land with snow and glaciers upon it. But back from the 
coast the snow has accumulated as an ice cap, which Shackleton found 
to be a vast, snow-covered ice plateau, rising to about 10,000 feet 
no miles from the South Pole, where he turned back. Amundsen 
and Scott subsequently found that it extended to the pole itself. 

Beardmore Glacier and Other Distributaries. — From the ice 
plateau of the Antarctic continental glacier there are outlets along 
valleys in the mountains, down which valley glacier tongues extend. 
These vary in size, but one, the Beardmore Glacier (Fig. 165), is over 125 
miles long, and from. 10 to 20 miles wide, with a total area of over 5000 
square miles ; yet it is merely one of many distributaries of the vast 
continental glacier. The depth of the inland ice is unknown, but it 
may well be several thousand feet. It is able to accumulate until 
its depth necessitates outward flowage, for there is no other source of 
loss than evaporation, and the drifting of the loose snow by the wind. 
There is no melting, and probably no other form of precipitation than 
that of snow. 

The Great Ice Barrier. — The shores of Ross Sea, near Victoria Land, 
are bordered by an ice cliff 500 miles long, called the Great Ice Barrier. 
This cliff (Fig. 166), which rises from 50 to 280 feet above the water, 
is the edge of a vast plain of ice — a sort of piedmont glacier — which 
stretches for a distance of over 300 miles southward and is evidently 
afloat. Into it pour numerous great glaciers, and it is moving sea- 
ward at the rate of about 1600 feet a year. Although supplied with 
ice from the glaciers that enter it, and given its motion by them, it is 
said to be composed of compacted snow ice, not of glacier ice. The 
explanation is that the annual snowfall adds layer upon layer on the 
ice barrier while the sea water melts the glacier ice at the bottom. 
It is a peculiar form of glacier. 

Antarctic Icebergs. — From the Great Ice Barrier, and from other 
parts of the coast of Antarctica, icebergs are discharged into the sea. 
Those from the ice barrier are especially noteworthy for their great 
size and tabular form. Sometimes huge sections of the ice cliff break 



GLACIERS AND GLACIATION 



247 



off and float away, looking like great islands of ice. Scott reports 
a tabular iceberg 5 or 6 miles long and about as wide, though few 
exceed a square mile in area. Usually they rise no more than 150 feet 
above the water, though one measured 240 feet. 

The Greenland Ice Sheet. — Though smaller, more is known about 
the continental glacier of Greenland than about that of Antarctica. 
Greenland has an area of about 827,275 square miles, and all but a 
fringe of coast line is covered with an ice sheet whose total area is 




Fig. 166. — The Great Ice Barrier of Antarctica. (Scott.) 



estimated to be about 715,400 square miles, or over eight times the 
area of Great Britain. This ice rests upon a low mountainous land, 
judging from the topography along the coast, where the mountains 
commonly rise 2000 to 3000 feet or more above sea level (Fig. 167). 
The Interior of Greenland a Snowy Desert. — Greenland has been 
crossed in the south by Nansen, De Quervain, and Koch, and in the 
north by Peary and Rasmussen, but the greater part is an unknown 
desert waste of snow. The surface rises toward the interior, which 
is a great, snow-covered ice dome, attaining an elevation of at least 
8000 to 10,000 feet, and with the ice several thousand feet in depth. 



248 



COLLEGE PHYSIOGRAPHY 



So far as known, no land rises above this interior, which, next to that 
of Antarctica, is the most absolute desert in the world. There is no 
sign of life upon it; the temperature never rises to the melting point ; 
but, through all seasons of the year, there is snow and bitter cold. 
The snowfall that descends upon the interior finds its way outward 




SO fo 

Fig. 167. — The continental glacier in Greenland. Coastal fringe of land stippled. 



in two ways : (1) by the winds, which prevailing blow down the sur- 
face of the ice sheet ; (2) by the slow outward flow of the ice, which 
spreads seaward under the load of the accumulating snow. We 
have no knowledge as to the rate of flow of the ice sheet, but the fact 



GLACIERS AND GLACIATION 



249 



that there is little or no crevassing indicates that it is a very slow 
movement. 

Smooth Ice of Border Region. — Spreading northward, southward, 
eastward, and westward from the central area of accumulation the ice 
reaches lower altitudes, and, in the south, lower latitudes. It there- 
fore reaches into the zone of ablation. There is, therefore, a fringe, 
broadening southward, where the surface is exposed to summer melt- 
ing; and there the snow is removed and the glacier ice revealed. 




Fig. i( 



The Cornell Glacier, a distributary of the Greenland ice sheet. Mt. Schurman, 
a nunatak. (Tarr and Bonsteel.) 



In its general characteristics this ice is like that of valley glaciers. 
The movement is still slow, and there is little crevassing, though some 
domes of crevassed ice appear, where the glacier is evidently passing 
over the crests of buried hills. There is no moraine at the surface, 
though dust, blown from the land, is present in the bottom of minute 
wells which they have melted in the ice. Everywhere is a vast expanse 
of clear ice, little broken, but with undulating surface, rising toward 
the snow-covered interior (Fig. 169). 



250 COLLEGE PHYSIOGRAPHY 

Nunataks. — In the intermediate coastal fringe the conditions are 
entirely changed. Here peaks, called nunataks, project above the 
ice, forming rock islands in a sea of ice, and from them bands of 
moraine extend seaward. 

Glacier Distributaries. — The outward-flowing ice moves more 
freely down the valleys between the peaks, giving rise to valley tongues, 
or distributaries. The largest of these reach the sea in the fiords, 
which are the continuation of the valleys through which the ice is 
flowing (Fig. 168). These rapidly moving valley tongues are cre- 
vassed, and some are a sea of crevasses. Like valley glaciers, they may 
bear lateral moraines in their lower portions, derived from the border- 
ing cliff which here rises above the ice surface. Between the valley 
tongues the margin of the ice sheet rests against the land. Thus the 
border of the Greenland ice sheet is very irregular, with a land margin 
for a large part of the distance, but with tongues projecting beyond 
this into the sea. While there is loss of ice by ablation, a large part 
of the Greenland Glacier discharges into the sea through the rapidly 
moving distributaries. Between the tidal glacier tongues the hilly 
land rises, so that a large part of the Greenland coast is a land fringe, 
with the ice sheet extending up to it, and discharging down its valleys. 
This fringe of land broadens toward the south. Where it is high 
enough, it has its own individual valley glaciers, and, where a broad 
enough area is present, as on Disco Island, its own ice cap. 

Some of these ice distributaries are very large. The largest, so far 
as is known, is the Humboldt Glacier, which is 60 miles wide where 
it enters the sea. There are many that are 5 miles in width, and their 
ice cliffs rise 200 to 30a feet above the water. They advance at a 
rate that, for glaciers, is very rapid. Thus, one which is five miles 
wide moved during the period of observation at the rate of 5 feet a 
day; another, larger glacier, at a rate of 48 to 65 feet a day; and 
one, the Upernavik Glacier, has a reported movement of 75 feet 
a day. 

Greenland Icebergs. — With such rapid motion, there is, naturally, 
abundant discharge of icebergs, otherwise the glacier fronts would be 
pushed far out to sea. Almost constantly pieces are crashing from the 
ice front, and as they fall into the water, or rock back and forth in it, 
they generate ring waves, which sweep far out in the fiord and cause 
breakers on the coast. Some of the ice fragments fall from the cliff, 
a few hundred pounds or a few tons at a time, or, now and then, great 
masses hundreds of tons in weight. In other cases it appears prob- 
able that large blocks are broken off by the buoyancy of the water 
into which the glacier advances till its end is afloat. 

The fiord is dotted with ice, and in places quite filled with it ; but 
it does not remain there, for the winds that blow off the glacier, and 
the fresh water that flows from the glacier, cause an outward current, 
and the bergs travel out to the open sea. There they drift about, 
and many of them travel southwards past Newfoundland before they 



GLACIERS AND GLACIATION 



251 



finally melt. It was an iceberg from Greenland which caused the 
wreck of the steamer Titanic in 191 2. 

While not comparable in size with the huge tabular icebergs of the 
Antarctic, many of the Greenland icebergs are nevertheless of great 
size, the largest being a mile across. They rise 100 to 200 feet out 
of the water and sink 6 or 7 times as far below the surface, so that an 
iceberg may be 1000 or 1500 feet high, and much larger in other 
directions. As it floats slowly in the current its irresistible force is 
sometimes shown when it runs into a shoal and begins to break to 
pieces, though no reason for the breaking can be seen. They also 
change in shape through melting and turning over. By this wide 
dispersal of the ice, the snow that falls upon the interior of Greenland 
is finally returned to the sea, and, as it melts, rock fragments that 
* the glacier bore along are dropped over the sea floor. 

Marginal Phenomena. — Where the Greenland ice sheet rests on 
the land, it usually has a slope of greater or less steepness, due to 




Fig. 169. — The Greenland ice sheet with a nunatak and the Cornell Glacier distributary, 
terminating in Ryder Fiord, which was excavated by glacial erosion. 



melting in the neighbourhood of the warmed land, and, as in valley 
glaciers, there is often a depression between the ice and the land, in 
which a muddy stream flows. Here and there, also, there are marginal 
lakes, in which the glacial streams are building deltas and other 
deposits. There is, however, no such volume of water in this climate 
as there is in the warmer climate in which the Alaskan and other 
valley glaciers of the temperate latitudes terminate. 

The lower layers of the ice sheet are charged with rock fragments, 
which have been removed from the surface over which the ice has 
moved. These are revealed at the base of the ice along the land 
margin, but a short distance above the base the ice is clear and free 
from morainic materials. These rock fragments show signs of the 
powerful grinding to which they have been subjected, and the stones 



252 COLLEGE PHYSIOGRAPHY 

are often well striated, while the rocks on which the glacier rests are 
also striated, polished, and worn into the roches moutonnees form. 
At the base of the ice cliff a terminal moraine is forming by the accu- 
mulation of the rock fragments brought forward to the edge of the ice. 
It fringes the ice margin closely and, therefore, leaves a definite record 
of the stand of the ice whenever it remains long enough at a given 
position. 

Glacial Erosion in Greenland. — The Greenland ice sheet has 
formerly been more extensive than now, and has left records of its 
presence, similar to those left by valley glaciers. There are roches 
moutonnees forms ; striated pebbles and boulders, often of a different 
kind from rocks on which they rest; grooved and polished rock 
surfaces ; and in favourable situations, thin deposits of till. But the 
Greenland Glacier does not carry a great enough load of morainic' 
material to form extensive deposits when the ice melts away. The 
fiords through which the ice formerly extended are smoothed and 
straightened by glacial erosion, and that they are also deepened is 
proved by the presence of many hanging valleys. That the move- 
ment of a glacier 2000 feet deep, or more, should wear away the bed 
upon which it presses with a weight of 800 pounds per square inch, 
dragging its rock-shod mass heavily over the valley bottom at the 
rate of from 25 to 50 feet a day, is not difficult to believe. That it 
should wear the valley bottom deeply requires only beliof in the con- 
tinuation of the process for a long period of time. Here again, where 
we know glaciers formerly to have been, we find the evidence that 
they have eroded profoundly. A very considerable part of the 
irregularity of the coast line of Greenland is the work of glacial ero- 
sion, dissecting a previously less rugged land (Fig. 169). 

References to Literature 

Louis Agassiz. Etudes sur les Glaciers, Neuchatel, 1840, 347 pp. ; Nouvelles 

Etudes et Experiences sur les Glaciers Actuels, Paris, 1847, 598 pp. 
E. C. Andrews. The Ice Flood Hypothesis of the New Zealand Sound Basins, 

Journ. Geol., Vol. 14, 1906, pp. 22-54. 
W. W. Atwood. Glaciation of the Uinta and Wasatch Mountains, Prof. 

Paper 61, U. S. Geol. Survey, 1909, 96 pp. 
W. J. Bently. Studies of Frost and Ice Crystals, Monthly Weather Review, 

1907. 
T. G. Bonney. Ice Work, Past and Present, New York, 1896, 284 pp. 
E. Bruckner. Klimaschwankungen und Volkerwanderungen im XIX Jahrhun- 

dert, I tern. Wochensch. f. Wissenschaft, Kunst und Technik, March, 

1910. 
J. V. Buchanan. On Ice and its Natural History, Nature, Vol. 78, 1908, pp. 

379-382. 
E. C. Case. Experiments in Ice Motion, Journ. Geol., Vol. 3, 1895, pp. 918- 

934- 
T. C. Chamberlin. Glacial Studies in Greenland, Journ. Geol., Vol. 2, 1894, 
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669-681, 833-843; Vol. 4, 1896, pp. 582-592; Vol. 5, 1897, pp. 229-240; 
Bull. Geol. Soc. Araer., Vol. 6, 1895, pp. 199-220. 



GLACIERS AND GLACIATION 253 

T. C. Chamberlin and R. D. Salisbury. Geologic Processes, New York, 1905, 
pp. 244-323. 

W. O. Crosby. The Hanging Valleys of Georgetown, Colorado, Amer. Geol., 
Vol. 32, 1903, pp. 42-48. 

W. M. Davis. Glacial Erosion in France, Switzerland, and Norway, Geo- 
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E. von Drygalski. Zum Continent des Eisigen Siidens, Berlin, 1904, 668 pp. ; 
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S. Finsterwalder. Der .. Vernagtferner, Wissenschaftliche Erganz. zur 
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A. Forel and M. Lugeon. Les Variations Periodiques des Glaciers des Alps, 
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E. J. Garwood. On the Origin of Some Hanging Valleys in the Alps and 
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G. de Geer. Guide de l'Excursion au Spitzbergen, nth International Geo- 
logical Congress, Stockholm, 1910. 

G. K. Gilbert. Glaciers, Harriman x\laska Expedition, Vol. 3, 1904, 231 pp.; 
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A. Heim. Handbuch der Gletscherkunde, Stuttgart, 1885, 560 pp. 

J. Henderson. Extinct and Existing Glaciers of Colorado, University of 
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H. Hess. Die Gletscher, Brunswick, 1904, 426 pp. ; Alte Talboden im Rhone- 
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W. H. Hobbs. Characteristics of Existing Glaciers, New York, 191 1, 289 pp. 

D. W. Johnson. Hanging Valleys of the Yosemite, Bull. Amer. Geog. Soc, 
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W. D. Johnson. The Profile of Maturity in Alpine Glacial Erosion, Journ. 
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A. C. Lawson. The Geomorphogeny of the Upper Kern Basin, Bull. 3, Dept. 
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D. F. Lincoln. Glaciation in the Finger-Lake Region of New York, Amer. 

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W J McGee. Glacial Canyons, Journ. Geol., Vol. 2, 1894, pp. 350-364. 

Lawrence Martin. The Glaciers of Alaska, pp. 1-22, the Glaciers and Glacia- 
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E. de Martonne. Sur la Formation des Cirques, Annales de Geographie, 

Vol. 10, 1901, pp. 10-16. 

F. E. Matthes. Glacial Sculpture of the Bighorn Mountains, 21st Ann. Rept., 

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254 COLLEGE PHYSIOGRAPHY 

F. Nansen. First Crossing of Greenland, New York, 1892 ; Farthest North, 

2 vols., New York, 1897. 

0. Nordenskjold. Die Polarwelt und ihre Nachbarlander, Leipzig, 1909, 

220 pp. ; Einige Beobachtungen iiber Eisformen und Vergletscherung der 
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V. Paschinger. Die Schneegrenze in Verschiedenen Klimaten, Petermanns 
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R. E. Peary. Northward over the Great Ice, New York, 1897; The Inland 
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A. Penck. Glacial Features in the Surface of the Alps, Journ. Geol., Vol. 13, 
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A. Penck and E. Bruckner. Die Alpen in Eiszeitalter, 3 vols., Leipzig, 
1909. 

A. Penck, E. Bruckner, and L. du Pasquier. Le Systeme Glaciaire des Alps, 
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H. Philipp. Ergebnisse der Filchner Vorexpedition nach Spitzbergen 1910, Peter- 
manns Mitteilungen, Erganzungsheft 179, Gotha, 1914, 79 pp. 

G. Quincke. The Formation of Ice and the Grained Structure of Glaciers, 

Nature, Vol. 72, 1905, pp. 543-545. 

C. Rabot. Revue de Glaciologie, Ann. du Club Alpin Francais, Vol. 28, 
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la Societe Fribourgoise des Sciences Naturelles, Vol. 5, 1909; Glacial 
Reservoirs and their Outbursts, Geog. Journ., Vol. 25, 1905, pp. 534-548. 

H. Reck. Glazialgeologische Studien iiber die Rezenten und Diluvialen 
Gletschergebiete Islands, Zeitschrift fur Gletscherkunde, Vol. 5, 191 1, 
pp. 241-297. 

H. F. Reid. The Mechanics of Glaciers, Journ. Geol., Vol. 4, 1896, pp. 912- 
928; Glacier Bay and Its Glaciers, 16th Ann. Rept., U. S. Geol. Survey, 
Part 1, 1896, pp. 421-459; Glaciers of Mt. Hood and Mt. Adams, Zeit- 
schrift fiir Gletscherkunde, Vol. 1, 1907, pp. 112-132; Variations of 
Glaciers, reports of the International Committee on Glaciers, Journ. Geol., 
Vol. 3, 1895, pp. 278-288; and annually in the same journal; see also 
Archives des Sciences Physiques et Naturelles, Vol. 2, Geneva, 1896, 
pp. 129-147, and annually up to 1905 ; and Zeitschrift fiir Gletscherkunde, 
Vol. 1, 1907, pp. 161-181, and annually in the same journal. 

1. C. Russell. Glaciers of North America, Boston, 1897, 210 pp.; Existing 

Glaciers of United States, 5th Ann. Rept., U. S. Geol. Survey, 1885, 
pp. 303-355; Glaciers of Mt. Rainier, 18th Ann. Rept., U. S. Geol. Sur- 
vey, Part 2, 1897, pp. 349-415 ; Second Expedition to Mt. St. Elias, 13th 
Ann. Rept., U. S. Geol. Survey, Part 2, 1892, pp. 7-91 ; The Quaternary 
History of Mono Valley, California, 8th Ann. Rept., U. S. Geol. Survey, 
Part 1, 1889, pp. 261-394; A Note on the Plasticity of Glacial Ice, Amer. 
Journ. Sci., Vol. 153, 1897, pp. 344-346; The Influence of Debris on the 
Flow of Glaciers, Journ. Geol., Vol. 3, 1895, pp. 823-832. 

R. D. Salisbury. The Greenland Expedition of 1905, Journ. Geol., Vol. 3, 
1895, pp. 875-902; ibid., Vol. 4, 1896, pp. 796-810. 

R. F. Scott. Results of the National Antarctic Expedition, Geog. Journ., 
Vol. 25, 1905, pp. 353-373; H. T. Ferrar, ibid., pp. 373-386. 

N. S. Shaler and W. M. Davis. Illustrations of the Earth's Surface, 
Glaciers, Boston, 1881. 

W. H. Sherzer. Glaciers of the Canadian Rockies and Selkirks, Smithsonian 
Contributions to Knowledge, Vol. 34, 1907, 135 pp. 

R. S. Tarr. The Margin of the Cornell Glacier, Amer. Geol., Vol. 20, 1897, 
pp. 139-156; Former Extension of Cornell Glacier near the Southern 
End of Melville Bay, Bull. Geol. Soc. Amer., Vol. 8, 1897, pp. 215-268; 
Valley Glaciers of the Upper Nugsuak Peninsula, Amer. Geol., Vol. 19, 
1897, pp. 262-267; Lake Cayuga a Rock Basin, Bull. Geol. Soc. Amer., 
Vol. 5, 1894, pp. 339-356; Some Instances of Moderate Glacial Erosion, 



GLACIERS AND GLACIATION 255 

Journ. Geol., Vol. 13, 1905, pp. 160-173; The Properties of Ice (with 
J. L. Rich), Zeitschrift fur Gletscherkunde, Vol. 6, 1912, pp. 225-249; 
Physiography and Glacial Geology of the Yakutat Bay Region, Alaska, 
Prof. Paper 64, U. S. Geol. Survey, 1909, pp. 11-144; Glacial Erosion in 
Alaska, Pop. Sci. Monthly, Vol. 71, 1907, pp. 99-119; Some Phenomena 
of the Glacier Margins in the Yakutat Bay Region, Alaska, Zeitschrift 
fur Gletscherkunde, Vol. 3, 1909, pp. 81-110; The Theory of Advance of 
Glaciers in Response to Earthquake Shaking, ibid., Vol. 5, 1910, pp. 1-35 ; 
The Glaciers and Glaciation of Alaska, Science, N. S., Vol. 35, 191 2, pp. 
241-258. 

R. S. Tarr and Lawrence Martin. The Glaciers and Glaciation of Yakutat 
Bay, Alaskan Glacier Studies, Washington, 1914, pp. 23-231 ; An Experi- 
ment in Controlling a Glacial Stream, Annals Assoc. Amer. Geographers, 
Vol. 2, 191 2, pp. 29-40. 

T. Thoroddsen. Die Gletscher Islands, Petermanns Mitteilungen, Vol. 32, 
1906, pp. 163-208. 

J. Tyndall. The Glaciers of the Alps, New York, 1896; Forms of Water, 
New York, 1872. 

W. Upham. Physical Conditions of the Flow of Glaciers, Amer. Geol., Vol. 
17, 1896, pp. 16-29. 

G. F. Wright. Ice Age in North America, New York, 1889, 191 1, 741 pp. 

Zeitschrift fur Gletscherkunde. Fur Eiszeitforschung und Geschichte des 
Klimas. Edited by Eduard Bruckner, Vienna, Austria. Annual volumes 
beginning in 1907. 

TOPOGRAPHIC MAPS 

Glaciers 

Controller Bay, Alaska, 601 A. Mt. Hood, Oreg. 

Coast Survey Chart of Glacier Bay, No. 8306. Mt. Adams, Wash. 

Chitina, Alaska, 601. Glacier Peak, Wash. 

Valdez Bay and Vicinity, Alaska, 602 B. Shasta Special, Cal. 

Nizina District, Alaska, 601 B. Fremont Peak, Wyo. 

Cirques or Kare, and Glaciated Mountains 

Chief Mt., Mont. Cloud Peak, Wyo. Mt. Lyell, Cal. 

Copper Mt., Alaska Yosemite Valley, Cal. Juneau Special, Alaska 

Grand Teton, Wyo. Gilbert Peak, Utah Silverton, Colo. 

Lake Placid, N.Y. Kintla Lakes, Mont. Georgetown, Colo. 

For tidal glaciers, bulb glaciers, glaciated troughs, hanging valleys, valley 
trains, glaciated mountains, and fiords see the eight coloured contour maps in 
pocket of Tarr and Martin's Alaskan Glacier Studies; recent Professional 
Papers and Bulletins of the Alaskan Division of the U. S. Geol. Survey; and 
Coast Survey Charts, Nos. 8002 and 8550. 



CHAPTER IX 

THE GLACIAL PERIOD 

Importance or Glacier Study to Man 

While glaciers are among the noteworthy phenomena of physical 
geography, and possess features of general interest, there is an added 
reason for studying them in the fact that one of the most recent epi- 
sodes in geological history was the great extension of glaciers in moun- 
tain regions where they still exist, as already stated, and also their 
development in regions where glaciers are no longer present. Among 
these places are northeastern North America and northwestern 
Europe — regions now densely settled. The effects of this former gla- 
ciation are plainly stamped upon the surface of the country. They 
have exerted a profound influence on the development of the regions 
from which the ice has disappeared, particularly in the United States 
and Europe. 

Evidence of Former Glaciation 

The former presence of ice sheets in Europe and North America 
where no glaciers now exist is plainly indicated by a number of phe- 
nomena : (i) over wide areas there are both stratified and unstratified 
deposits like those now being made in association with glaciers. There 
are pitted plains, there are moraines, there is till, and all the kinds of 
deposits to be expected where glaciers have been, and many of them of 
a kind that no other agency than ice is now known to make. 
(2) Scattered through and over these deposits are rock fragments, large 
and small, of a totally different kind from those of the region, but 
known to exist in other sections, which other evidence indicates to 
have been the region from which the ice sheets moved. Some of these 
fragments are boulders of huge size, often hundreds, and even thou- 
sands, of tons in weight. No other agency than ice is competent to 
transport such huge masses so far from their source, which is often 
scores and even hundreds of miles distant. (3) The boulders and 
pebbles are striated, as are those carried by living glaciers ; and ice 
is the only agency known to be capable of this result. (4) The bed 
rock is also grooved, striated, polished, and rounded into the roches 
moutonnees, just as is the case at the front of the Greenland ice sheet 
and the valley glaciers of the Alps, Alaska, and other mountain regions. 
These grooves point toward the region from which the rock fragments 

256 



THE GLACIAL PERIOD 257 

have been moved ; and they extend with a regularity and definite- 
ness that no other agent of erosion than glacial flow could give. 
(5) The roches moutonnees forms also point toward the source from 
which the ice came, for one side is worn more than the other. The 
side from which the ice came, called the stoss side, is smoother and 
worn more than the opposite or lee side ; and from the roches moutonnees 
as well as from the striae one can tell the direction of the ice motion. 
Thus three evidences clearly point the same way : (a) the rock frag- 
ments, (b) the striae, (c) the roches moutonnees. (6) There are hang- 
ing valleys, U-shaped valleys, truncated spurs, and other evidences of 
powerful glacial erosion in places where the ice moved freely along 
valleys. (7) Associated with all these phenomena there has been a 
rejuvenation of streams, as a result of partial or complete filling of 
valleys that existed prior to the Glacial Period. By this rejuvenation 
lakes have been formed in great abundance, rivers have been forced 
to cut gorges, and waterfalls have been developed in great numbers. 
(8) Along a sinuous belt, extending from sea level and passing across 
plains and over hills and even low mountains, such as the Appalachians, 
there is an accumulation resembling the terminal moraines of valley 
glaciers, and on the outer side of this, there are deposits of outwash 
gravels. (9) That this terminal moraine belt traces the former front 
of the glacier is clearly indicated by the fact that on one side of it all 
the phenomena mentioned above are well developed, while on the 
other side the phenomena are more or less completely absent. The 
fact that there were earlier advances, in which ice sheets reached 
farther than the terminal moraine of the last advance, has made this 
moraine a less definite line of demarcation than it would otherwise be. 

Early Explanations of Phenomena 

The Problem of the Erratics. — Some of the phenomena mentioned 
above were recognized very early. Thus the recognition of foreign 
rocks gave rise to the appellation erratics, still used for glacial boul- 
ders. The erratics are often large and unnatural, they are often 
grouped, and they are sometimes strangely perched, even so that they 
may be rocked by the hand. Among primitive people these phe- 
nomena led to mythical explanations, such as the work of fairies or 
giants. 

Supposed Relation to the Flood. — When more thoughtful attention 
was given to the explanation of the erratic boulders and the asso- 
ciated deposits, the Biblical Deluge was adopted as the transporting 
power ; and when it became evident that even this could not propel 
huge boulders so far, nor make the striae on the pebbles, boulders, and 
bed rock, much less deposit the clay and stones side by side, it was 
supposed that the Deluge swept along with it great icebergs. Much 
controversy arose over this theory, embittered by the theological 
element involved. Even before the glacial theory was proposed, diffi- 



258 COLLEGE PHYSIOGRAPHY 

culties began to appear, so that when Agassiz proposed the glacial 
theory there were some who were ready, to consider its possibility, 
though it was not without a bitter fight that its acceptance became 
general. How to obtain such a quantity of ice to float, how to make it 
pick up rock fragments from a valley bottom and carry them to the 
top of a neighbouring hill, how to account for the formation of long 
parallel grooves in the rock by swirling water currents, were difficulties 
that helped in the abandonment of the iceberg theory. And with 
the discovery of the belts of lateral moraines, of the phenomena of 
ice erosion, and a multitude of other features, the glacial theory has 
become well founded, for it explains all the features, which no other 
explanation can. 

Agassiz 's Explanation. — The glacial theory, now universally ac- 
cepted as demonstrated, was a theory which naturally had its birth 
in a region of living glaciers. The goat herders of the Alps recognized 
the fact that the glaciers had formerly been more extensive, for they 
observed the same phenomena both at the glacial fronts and on the 
valley slopes and bottoms at a distance. Scientific men before Agassiz 
recognized the evidence of former extensive glaciers, but it remained 
for Agassiz to extend the theory to regions outside the Alps, and to 
postulate ice sheets for extensive areas in Europe and America where 
there are now no glaciers. This was brilliant generalization. Agassiz 
made the mistake of over-enthusiasm, for he eventually applied the 
theory even to Brazil. 

Extent of the Glaciation 

The Scandinavian Ice Sheet. — At first it was thought that the 
glaciation encircled the poles and spread out from polar centres, as 
the Antarctic Glacier does to-day. But this has now been found to be 
incorrect. In Europe the great centre of glaciation was Scandinavia, 
which was covered with an ice sheet that spread into Russia, Germany, 
Holland, and the North Sea. It was 1500 feet thick at the Hartz 
Mountains and perhaps 6000 to 7000 feet in Scandinavia. It cbvered 
an area of about 770,000 square miles and coalesced with an ice sheet, 
probably 4000 to 5000 feet thick, which developed on the British Isles. 
Northward and westward the ice reached into the Arctic and Atlantic 
oceans. The centre of this Scandinavian, or Baltic, Ice Sheet was in 
Sweden, a little east of the highland of Norway, rather than on the 
highest part of the upland (Fig. 170). 

Mountain Glaciers of Europe. — At the same time the mountains of 
Europe were centres of glaciation, the Alps especially (Fig. 171). 
Glaciers filled all the Alpine valleys, spreading southward beyond 
the mountain base in Italy, and northward upon the plateau of 
Switzerland, southern Germany, and Austria, where it formed pied- 
mont bulbs and piedmont glaciers. At the same time the Car- 
pathians, Caucasus, Pyrenees, Apennines, and Urals had their 



THE GLACIAL PERIOD 



2 59 



expanded glaciers, and there were local ice tongues in the small 
mountains of Europe such as the Vosges, Black Forest, the Auvergne 
district of France, and the island of Corsica. 

The Labrador and Keewatin Ice Sheets. — Apparently contempo- 
raneous with the European ice sheet a continental glacier covered 
northeastern North America, occupying an area estimated as 4 mil- 
lion square miles. It had two main centres, one in Labrador, the 
other, the Keewatin, west of Hudson Bay. From these two areas, 
and perhaps one or more small centres such as Newfoundland, the 
ice spread outward in all directions, reaching southward into the United 




Map showing the 

maximum extension of the .- 

Ice Sheets in the /•' 

Glacial Period / s . 



I Areas not affected by extreme glaciation 
The Scandinavian Centrr 
C = The Ccrdi Heron Centre 
K = The Keewatin Centre 
L = The Labrador or Laurentide Centre 
Arrows indicate, the direction of Ice-fioio 



Fig. 170. — The world at the maximum of glaciation. Areas in Asia and South America 
much generalized. Glaciation in Alaska shown inaccurately, see Fig. 172. (After 
Encyclopaedia Britannica.) 



States to the islands south of New England, thence westward to the 
Ohio valley, then, crossing the Mississippi, the front swung north- 
westward, crossing into Canada near the Rocky Mountain front in 
western Montana (Fig. 172). 

These great ice sheets were truly continental glaciers ; and from 
Greenland we doubtless have a picture of the conditions that existed 
here during their existence. Apparently the snow accumulated until 
great ice caps developed, covering all the land, and spreading slowly 
outward from the centres of dispersion. How deep the ice was at 
the centres cannot be told ; but since it was sufficient to propel the 
ice across the St. Lawrence valley and up the southern slopes of that 
valley, it must have had a great depth. The ice rose over the tops of 
Mt. Marcy in New York (5344 feet), Mt. Katahdin in Maine 



26o 



COLLEGE PHYSIOGRAPHY 




THE GLACIAL PERIOD 



261 




WILLIAMS ENGRAVING C0.,N..Y, 

Fig. 172. — Territory covered by the maximum extension of the glaciers in North America. 

(5150 feet), and Mt. Washington in New Hampshire (6279 feet). 
To have reached such elevations the ice surface in the distant centre 
of dispersion must have been much higher. It may well have risen 
to an elevation of 10,000 feet. 

Doubtless these ice sheets were vast deserts, as interior Greenland 



262 



COLLEGE PHYSIOGRAPHY 



is to-day. Where they entered the sea they discharged icebergs. 
Where they terminated on the land they wasted away by ablation, 
and from their fronts huge torrents of sediment-laden water issued, 
for they extended southward into a temperate climate. How long 
the ice sheets lasted cannot be told, but from the work they performed 
the period of time must have been great. 

Mountain Glaciers in Other Parts of the World. — Unless possibly 
in the Antarctic, there were no ice sheets comparable to these of North 

America and Europe in 
other parts of the world. 
Yet glaciers were more 
extensive than at pres- 
ent in many places in 
both hemispheres and 
even in places where 
there are now no gla- 
ciers. There were far 
more extensive valley 
and piedmont glaciers in 
Alaska, British Colum- 
bia, the Rocky Moun- 
tains, the Cascades and 
Sierra Nevada, the 
Andes as far as the 
southern tip of South 
America, New Zealand, 
the Himalayas, and 
other parts of the world. 
These were apparently 
true mountain glaciers, 
the greatly expanded 
ice mass of the North 
American Cordillera, for 
example, not being of such a nature as to be properly alluded to 
as an ice sheet, nor its source as a centre of glaciation. Within it, 
however, as in the present mountains of Alaska, there were innumer- 
able centres of ice dispersion. The glaciers of the Antarctic, of 
Greenland, Iceland, Spitzbergen, and other islands of the Arctic ex- 
tended farther than now. It cannot be asserted that this former 
extension of mountain glaciers was contemporaneous with the conti- 
nental ice sheets of Europe and America, though there is no evidence 
that it was not. 

Some Arctic Lands not Glaciated. — It is a curious fact that the 
great ice sheets were developed, not in the coldest regions of the 
present day, nor on land that is at present lofty. They occurred on 
two sides of the Atlantic Ocean; and elsewhere, excepting in the 
frigid zones, the extension of glaciers was confined to mountain regions. 




Fig. 173. — Stand Rock in the Driftless Area of Wiscon- 
sin, a feature which could not possibly have persisted in 
glaciated territory. (After Salisbury and Atwood.) 



THE GLACIAL PERIOD 



263 



In the mountains, however, there was no expansion of glaciers at all 
comparable to the ice sheets of Europe and North America. No ice 
sheets developed in Alaska or in Siberia, not even in the parts north of 
the Arctic Circle, though the mountains of Alaska and northeastern 
Asia both contained valley and piedmont glaciers of far greater size 
than at present (Figs. 170, 172). 

The Driftless Area of the Mississippi Valley. — In the state of 
Wisconsin in the upper Mississippi valley and, to a smaller extent, in 
the adjacent states of Minnesota, Iowa, and Illinois is an area of over 




Fig. 174.- 



Part of the border of the Driftless Area in Wisconsin, with moraines and drum- 
lins to the east. (After Alden and Thwaites.) 



10,000 square miles which was not covered by the Labrador and 
Keewatin ice sheets of the continental glacier, between which it lay. 
It contains no glacial drift and is, therefore, known as the Driftless 
Area. The lobes of these ice sheets even coalesced south of the Drift- 
less Area, and advanced over 300 miles farther south at the maximum 
of glaciation (Fig. 210). 

It seems probable that the driftless character of this area is due to 
the temporary protection afforded by the highland of northern Wis- 
consin, in conjunction with the presence of the deep basin of Lake 
Superior north of it and the basin to the east now occupied by Lake 
Michigan. It is not driftless because of altitude, for it rises no higher 
than the adjacent land. If the period of expansion of the continental 
glacier had lasted longer, this Driftless Area would surely have been 
overridden (Figs. 173, 174). 



264 



COLLEGE PHYSIOGRAPHY 



Glacial Deposits 

Deposits of Melting Ice and of Glacial Streams. — As in the case 

of existing glaciers, deposits were made by the vanished ice sheets, 




Fig. 175. — The ground moraine or till sheet in Minnesota. 




Fig. 176. — Boulder clay or till resting on solid rock, Vermilion iron range of Minnesota. 



both directly by the ice and through the intervention of water. Thus 
we find both glacial deposits and glacio-fluviatile deposits. The latter 
are assorted and stratified, the former mainly unassorted mixtures 
of fragments of various sizes and kinds, without stratification. Very 



THE GLACIAL PERIOD 265 

often the deposits consist of a mixture of the two, for ice and water 
were often working side by side. Some of these deposits were accu- 
mulated beneath the ice, still more were laid down at the front of 
the ice or beyond its front. 

The Till Sheet. — Of the glacial deposits the drift or till sheet is by 
far the most extensive (Fig. 175). It consists of rock fragments 
dragged beneath the ice and carried in it, and left at the glacier 
front, where it had been brought when the ice melted. It may be 
classed as the ground moraine. Owing to its origin, the till consists 
of ground-up and partly ground-up rock fragments, the former being 
clay or rock flour, the other larger rock fragments up to the size of 
large boulders. These are mixed together, since they lay side by 
side in the ice (Fig. 176). The stones include a great variety of 




Fig. 177. — View of stony fields and stone walls in Maine, showing the type of soil 
left in some places by the continental glacier. (J. Ritchie, Jr.) 

kinds, gathered up from different rock outcrops over which the ice 
moved, and all mixed together. They are often polished and 
striated by the abrasion against one another in the differentially 
moving ice layers, and against the bed rock over which they were 
dragged. The bouldery nature of the till led to its being also 
called boulder clay ; its compactness in places has given rise to the 
name hard pan; and its blue colour in unweathered outcrops has led 
to the name blue clay. 

Composition of Till. — The till varies greatly in composition ac- 
cording to the kind of rock over which the ice bed passed, and the 
extent to which the rock has been ground up. Some till, derived 
from soft rocks, and subjected to thorough grinding, is mainly clay, 
though hard rock derived from more distant outcrops and not ground 
up may be scattered through it (Fig. 178). In central New York 
boulders of hard, crystalline rock, brought from Canada by the ice, 



266 



COLLEGE PHYSIOGRAPHY 



and easily distinguished from the shale and sandstone rock of the 
region, are locally called " hardheads." In other places the ice 
mainly encountered resistant rocks, and these are less thoroughly 
ground up. This is true in many parts of New England, south of the 
Adirondack Mountains, in Wisconsin, and in various parts of Europe, 
as in Scotland. In such places till contains a smaller proportion of 
clay and far more boulders. Indeed, in places the surface may be 
so strewn with boulders that no agriculture is possible on the 
boulder-covered fields (Fig. 177). 




Fig. 178. — A boulder train, or deposit of glacial erratics of a distinctive and uncommon 
kind of rock, crossing three mountain ridges and valleys in western Massachusetts. 
(After Taylor.) 



Thickness of Till. — The till sheet is variable in depth also. In 
some places the ice was heavily charged with rock fragments, notably 
where it passed over weak rocks, as in the states of the Mississippi 
valley. There the till is deep. Elsewhere the glacier had a small 
load, as in the regions of resistant rock like New England, the Adiron- 
dacks, northern Wisconsin, and east-central Canada. There the 
till sheet is thin, and there are areas of bare rock, but there are some 
areas of bare rock or thin drift which are due to the washing off of 
the ground moraine by glacial streams or rain-born rills of the closing 
stages of the Glacial Period when vegetation was still absent and ero- 
sion correspondingly rapid. 

The irregular distribution of the till is dependent also on the move- 



THE GLACIAL PERIOD 



267 



ment of the ice. Where it was moving vigorously it carried the rock 
fragments away, and where its motion decreased they were accumu- 
lated beneath the ice. This is often seen on a small scale where a 
tail of boulder clay has accumulated on the lee side of a crag, giving 
rise to the phenomenon of crag and tail. It is also seen where till 
has been dragged into valleys transverse to the direction of ice motion. 




Fig. 179. — Topographic map of drumlins in Wisconsin. 
(Sun Prairie Quadrangle, U. S. Geol. Survey.) 

On a much larger scale it is illustrated by the deep sheet of till in the 
Mississippi valley, where the ice spread out with slow motion. The 
lower ice layers can be overburdened just as certainly as a river 
can be. . 

Speaking generally, the till mantles the rock surface with a sheet 
varying in thickness from place to place, roughly parallel to the rock 
topography. It is not exactly parallel, however, for there are un- 
dulations due to irregularity of deposit, or to sculpture subsequent 
to deposition. 



268 COLLEGE PHYSIOGRAPHY 

Drumlins. — Of these irregularities the most notable are the pecul- 
iar forms known as drumlins. These are oval hills sometimes oc- 
curring singly, but usually in clusters, and in the latter case the sur- 
face rises and falls in a series of billowy curves like the waves of the 
ocean. While the oval is the characteristic form, there are ridge- 
shaped drumlins, and there are double curves, and other variations 
in form. In all cases the long axis is in the direction of ice motion, 
and the end pointing toward the source of the ice, the stoss end, is 
steeper than the opposite or lee end. The drumlins vary in size, some 
being but a few hundred feet long and a few yards high, while others 
are a mile in length and ioo to 200 feet high. Perhaps a normal 
size may be given as a half mile long, an eighth of a mile wide, and 




Fig. 180. — Longitudinal profile of a drumlin in Massachusetts. (J. L. Gardner.) 

1 50 feet high at the highest point ; but there are wide variations from 
this (Figs. 179, 180). 

Distribution of Drumlins. — Drumlin clusters form a landscape 
of such a characteristic kind that they can be easily recognized on a 
map. There are such clusters in many places, some of the best 
known being in eastern Wisconsin ; in central New York on the 
Ontario plain between Rochester and Syracuse, and north of this on 
the Canadian side of Lake Ontario ; in the Connecticut valley ; and 
in eastern Massachusetts and southern New Hampshire. Boston 
is built partly on drumlins, and drumlins make the islands of Boston 
Harbour, as well as the State House hill and Bunker Hill. Drumlins 
occur in great numbers in the lowlands of central Ireland, whence 
the name comes; they also occur in the lowlands of Scotland, on the 
North German plain, and elsewhere in Europe. In some of these 
clusters there are hundreds of individual drumlins. 



THE GLACIAL PERIOD 



269 



Origin of Drumlins. — There are two theories for the origin of 
drumlins: (1) that they are irregularities built up beneath the ice by 
irregular deposit, as sand bars are built in an overburdened river, (2) 
that they have been carved out of a sheet of till, at first deposited fairly 
uniformly, and later sculptured — being a phase of roches moutonnees 
carved in a till sheet. It has not been demonstrated which of these 
explanations is the correct one, and it is quite possible that one theory 
is correct for some cases, the other for others. 

Moraines. — When the ice halted for a long enough time, morainic 
deposits accumulated at the margin of the ice sheet, forming a terminal 



n\\ 



Fig. 




Moraines in Finland. Eskers at right angles to them and abundant lakes 
inside the morainic border. (Hobbs, after Sederholm.) 



moraine. The conditions involved in the accumulation of the moraine 
were complex and variable, including (a) the sliding down of debris 
from the front of the ice, (b) the dragging up and accumulation of 
debris beneath the thin edge of the ice, (c) the deposit by glacial water 
along the ice margin, (d) oscillation of the ice margin, (e) burial of 
ice blocks and thin glacier margins, and (/) the influence of pregla- 
cial topography. With the ever varying combinations of these factors 
the terminal moraine has been given great complexity and variety 
of form, composition, and depth. 

Form of Moraines. — A terminal moraine may be, but a few feet 
high and a few yards broad, or it may be several hundred feet thick 



270 



COLLEGE PHYSIOGRAPHY 




Terminal Moraine 



Ground Moraine/Lake Clay t 
Delta and Floodplain deposits. 

I* j o I 2 J Milts 



Fig. 182. — Lateral and terminal moraines in central New York. 



THE GLACIAL PERIOD 



271 



and several miles broad; it may be a single, continuous belt or a 
series of separate or overlapping ridges. In form the moraine 
may be a ridge, or a complex of 
low undulating hills (Fig. 191). 
Most characteristically it has 
the knob-and-basin topography, 
consisting of hillocks or hum- 
mocks with intervening kettle- 
shaped depressions in the bot- 
toms of which small ponds and 
swamps often lie. The hum- 
mocks may rise one or two 
hundred feet above the kettle 
bottoms, or the difference in 
elevation may be only a few feet. 
There is no rule, and seemingly 
no system, for the conditions 
during formation were most com- 
plex and variable (Figs. 181,182). 

Composition of Moraines. — In composition a given part of the 
moraine may be all till, or all sand, or gravel, or clay, or, what is 
more common, it may consist of a mixture of two or all of these. The 
composition is as variable as the form. Very commonly there are 




Fig. 183. 



The glacial lobe near New York. 
(Salisbury.) 




Fig. 184. — Moraines in southeastern Wisconsin (oblique lines); interlobate moraine near 
Richmond, Palmyra, and Delafield (cross-hatched) ; drumlins (solid black) ; striae 
shown by arrows ; older drift (horizontal lines) near Evansville and Albany ; ground 
moraine and outwash in white. (After Alden.) 



many boulders in the moraine, far more than in the ground moraine. 
This is because boulders within the ice were brought up to the front 



272 



COLLEGE PHYSIOGRAPHY 




and there left in the accumulating deposits, and, therefore, far more 
would thus be concentrated along the frontal belt than existed in 
any single part of the ice. The terminal moraine is often a belt of 
boulder-dotted hills in a region where boulders are elsewhere rare. 
Sometimes so many boulders are accumulated that the surface is 
literally covered with boulder piles. The bear den moraine is the 
extreme of this condition, for here the boulders are piled one on the 
other, and over considerable areas no soil appears. 

Moraines Bordering Glacial Lobes. — The terminal moraine marks 
essentially the position where the ice front stood for some time. From 
it, therefore, the form of the ice margin can be determined. The 
moraine clearly proves that the ice pushed its front forward in a series 

of lobes where valleys gave freer 
opportunity for movement, or 
where, for other reasons, the 
strength 1 of flow was increased. 
As a result of this fact the ter- 
minal moraine sweeps in a series 
of lobes across the country, giv- 
ing rise to the lobate moraine. 
Where the lobes coalesce there 
is often a band of interlobate 
moraine, where the terminal 
moraines of the two lobes unite 
to form a single band at the junc- 
tion of the ice lobes (Fig. 184). 
Recessional Moraines. — When the ice advanced over the country 
it doubtless halted here and there and built moraines ; but later, being 
overridden by the ice, these were erased. At its outermost stand a 
terminal moraine was built (Figs. 174, 183). Then, as the ice sheet 
began to melt away, and the front of the glacier receded to more and 
more northerly positions, there were periods of halting during which, 
of course, morainic deposits began to accumulate at the ice margin. 
Some of these halts gave rise to well-defined belts of recessional 
moraine, which are in no way different from the outermost terminal 
moraine excepting in position (Figs. 185, 186). 

Glacio-Fluviatile Deposits. — If from small living glaciers great 
torrents of sediment-burdened water issue, it is -to be expected that 
similar, or greater, torrents should have issued from the margin of the 
continental glaciers. That this was the case is abundantly proved 
by the glacio-fluviatile deposits, not only on the outer side of the 
terminal moraine, but all over the land across which the front of the 
melting ice sheets receded. These water-laid deposits vary greatly 
in character, and some are of such indefinite nature that it is difficult 
to state their origin ; but there are some deposits of such definite 
form that their origin is not difficult to understand. All such deposits 
were formerly alluded to as " modified drift." 



Fig. 185. — Recessional moraines in Ontario. 
(Taylor.) 



THE GLACIAL PERIOD 



273 



Eskers. — Among these one of the most striking forms is the 
esker, sl long, winding ridge of gravel or sand, in some cases two or 
three hundred feet high, and very often 5, 10, or 15 miles long, and 
sometimes even much longer ; but they are rarely more than a few 




SCALE Of MILES 



50 50 100 

Illinoian drift is shaded 
Wisconsin drift alone shows 
direction of ice currents. 



JAMS ENGRAVH 



Fig. 



Recessional moraines between Wisconsin and New York. (After Taylor and 
Leverett.) 



hundred yards broad at the base. Their tops may be even, like a 
railway embankment, or they may be undulating ; and the course is 
commonly notably serpentine. In fact they were formerly called 
serpent kames in America, but now the Irish name esker is generally 
adopted, though the Swedish name osar is sometimes used. They are 
very common in regions of former glaciation, especially in hilly dis- 
tricts (Figs. 181, 187). 



274 



COLLEGE PHYSIOGRAPHY 



Cause of Eskers. — The esker is a deposit in a glacial tunnel, usu- 
ally, if not always, at the base of the ice. Here the water flows with 
great velocity, often under head, and confined in a tunnel. It is 
often able to carry even good-sized cobblestones, and these are rapidly 
rounded in the swift current. If more load is given than the stream 
can carry, or if it is forced to build up its bed by the upward growth 

of an alluvial fan at the outlet 
of the tunnel, some of the coarser 
material must be laid down on 
the subglacial stream bed, the 
stream at the same time enlarg- 
ing its tunnel by melting the 
roof. Thus a long, narrow, 
winding deposit of gravel is 
made, with ice roof, and ice 
walls. When the stream ceases 
to flow and the ice walls melt 
away, the gravel slides into a 
state of repose and the embank- 
ment-like esker is formed (Fig. 
1 88). Under the conditions of 
its formation it is naturally given 
a rough stratification. 

Eskers cannot develop, at least 
not in a very perfect form, in ice 
that has much movement, for 
the tunnels would soon be closed. 
Nor can they develop at the base 
of a thick ice sheet, for the pres- 
sure would close the cavity. 
The most favourable position for 
esker development is under the 
thin, stagnant front of a glacier, 
or beneath a detached ice block, 
conditions that were common 
along the front of the receding 
ice sheets. Doubtless also esker 
development is favoured along 
the margins of valley glaciers, or glacier lobes, where the ice is thin, 
the motion slight, and the volume of water great. It is from such 
places that glacial torrents issue from living glaciers, and doubtless 
eskers are forming in some of them, as, for example, in Alaska, where 
small eskers are found on ground from which the glaciers have receded 
within a century. 

Karnes. — Hummocky deposits of sand and gravel, often with 
perfect stratification, are called kames. There seem to be various 
conditions under which kames can develop, all depending upon the 




Fig. 187. — Maps of eskers (black) in Wis- 
consin and in New York. (Upper map 
after Alden.) 



THE GLACIAL PERIOD 



275 



transportation of coarse sediment by glacial streams. Some kames 
may form beneath the glacier where surface streams fall through 
moulins, though the more common result of such a fall is the excava- 
tion of a pot hole. If, however, the stream bears much sediment, it 
may accumulate in a hummocky kame deposit. 

An esker often merges into a kame area; and, after a certain dis- 
tance, the esker form again develops. In this case it is evident that 
the kame is formed beneath the ice, and the conditions are appar- 
ently the enlargement of the subglacial cavity and the irregular dep- 




FlG. I 



Esker near Yakutat Bay, Alaska. 



osition of the sand or gravel in which are incorporated blocks of 
ice from the roof of the tunnel, whose later melting forms depressions 
in the gravel. 

Other kames were formed along the margin of the glacier where 
gravel came to rest on the edge of the ice, or on detached stagnant 
blocks. Later melting of the ice gave rise to kettles and hummocks. 
This process is seen in course of development along the fronts of some 
of the Alaskan glaciers. Patches of kame form parts of many mo- 
raines left by the continental glaciers, and in some cases there are 
extensive areas of this type of moraine which has been called kame 
moraine, or kettle moraine. 

Esker Deltas. — When a glacial stream, on emerging from its ice 
tunnel, enters a lake, it builds a delta with the load of sediment that 



276 



COLLEGE PHYSIOGRAPHY 




Ground Moraine, 
lake clay, deltas/lood- 
pldins, 



Fig. 189. — Terminal moraines and valley trains of outwash gravel in central New York. 

it carries. Lakes were often developed in such positions along the 
front of the continental glacier, where the ice formed a dam across a 
valley sloping toward it. With the disappearance of the ice dam the 
lake is drained, but the delta remains, and if an esker was built in 
the ice tunnel, it will be seen extending up to the delta. Such an 



THE GLACIAL PERIOD 



277 



esker-fed delta may be called an esker delta, though it has been given 
the less descriptive name sand plain in New England, where there 
are numerous perfect instances of this type of land form. 

Outwash Gravel Plains. — Where the glacial streams issued upon 
a slope leading away from the ice, or where they made extensive 
enough deposits to grade up such a slope, outwash gravels were ac- 
cumulated, as they are in front of living glaciers. On open land these 
outwash gravels are in the form of great, coalescing alluvial fans or 
outwash aprons. Southern Long Island was built up by such out- 
wash gravel deposits during the Glacial Period, just as such accumula- 
tions are being formed around the margin of the Malaspina Glacier 
to-day; but where the streams were confined in valleys, they ag- 
graded the valley, making a flat valley floor of gravel. It is to such 




Fig. 190. — Diagram to show relationships of outwash, T, to moraines, M. (After Penck.) 



narrow, outwash gravel plains that the name valley train is most 
appropriately applied (Figs. 189, 192). The ice-born streams carried 
their deposits great distances beyond the ice front, from southern 
Illinois far down the Mississippi, for example. 

On the surfaces of outwash gravel deposits the old courses of the 
braided streams may still be traced, and the well-rounded pebbles 
testify to the rapid motion of the streams that brought them. Such 
plains are often pitted with little kettles ; and some good-sized ones, 
where buried ice and stagnant ice blocks have melted out and allowed 
the gravels to settle. Swamps, ponds, and small lakes occupy some 
of these kettles. 

Near the point of emergence of the glacial stream, the outwash 
gravel plain consists of coarse fragments, and it is often cut into ter- 
races by the rapid erosion of the stream. The very smooth river-laid 
part of the outwash gravel plain grades into a hummocky kame topogra- 
phy, where the gravels were deposited on the ice, whose later melting 
has given rise to the kame topography. In such places the outwash 
gravel and moraine merge imperceptibly into one another, a condi- 
tion which finds expression in the term moraine-headed terraces. 

Loess of the Glacial Period. — Among the fine-grained deposits of 
the Glacial Period are thick deposits of loess. In the United States they 
are especially well developed in the Mississippi valley. The origin 
of this loess, whose character is discussed in the chapter on the work 
of the wind (pp. 72-73), is not agreed, but it seems probable that some 



278 COLLEGE PHYSIOGRAPHY 




Fig. iqi. — Hummocky recessional moraine in central New York. 




FlG. hi-'. Valley train of out wash gravels near upper view. 



THE GLACIAL PERIOD 



279 



of it is wind-laid and some water-laid. Just after the Glacial Period, 
when grass and trees had not yet readvanced over the glaciated lands, 
the wind must have been supplied with great quantities of fine-grained 
dust, especially on the outwash plains and valley trains. 

Marginal Lakes 

Ice-dammed Lakes. — A continental glacier, covering all the 
land, has along its front some valleys sloping away from it, some toward 




Fig. 193. — Two stages in the recession of the continental glacier in a hilly region. Upper 
diagram with ice-dammed lakes, lower with valley drainage developed between the 
glacier and the terminal moraine of the previous stand. 



it. In the latter case, water naturally accumulates (Fig. 193) in a lake 
with an ice dam and an outflow across the lowest point in the rim. 
Lakes of this origin abounded along the front of the ice sheets of both 
Europe and America, during all stages in their recession. Some of 
them were very small in size and quickly filled with deposits ; others 
were so large, or existed for so short a time, that the deposits in them 
were not noteworthy. Sometimes their outlets fluctuated greatly 
as the ice front advanced or receded. Through the outlets of the 
great temporary lakes, large volumes of water flowed and cut broad 



280 



COLLEGE PHYSIOGRAPHY 



channels in drift or rock, where now no water flows (Fig. 194). This is 
illustrated along the divide between Sweden and Norway, where there 
are broad outflow channels, some of them sunk into the mountain 




Fig. 194. — Glacial Lakes Tonawanda and Iroquois with the five spillways, of which the 
Niagara Spillway at Lewiston was lowest and, therefore, persisted. (Taylor.) 



rock by the water that flowed westward from lakes held up between 
the ice sheet in Sweden and the Norwegian mountain divide. 

Features made by Marginal Lakes. — The former presence of such 
lakes is clearly proved by a number of phenomena, notably (a) the 
outflow channels, (b) beaches along the abandoned lake snores, (c) 
deltas where tributary streams entered the temporary lakes, but now 
perched upon the hill slope and often dissected by the stream that 
built them, (d) a sheet of lake clay (Fig. 195) on the bottom of the 




mmwWMmm, 




Diagram to explain superposition of lake clay upon unstratified till. 



extinct lake, (e) occasional iceberg deposits, where floating ice in the 
lake became stranded and, on melting, dropped some of the load of 
debris that it carried, especially boulders. 



THE GLACIAL PERIOD 



281 







Glacial Lake Agassiz. — Among the many temporary lakes mar- 
ginal to the ice sheet of North America one of the most noteworthy 
was that that developed in the northward-sloping valley of the Red River 
of the North, in North Dakota, Minnesota, and Canada. It started 
as a small lake with an overflow southward into the Mississippi, but 
as the ice dam melted back it grew in size until finally it had a length 
of about 700 miles, a maximum width of 250 miles, and an area of 
110,000 square miles, or more than the combined area of all the Great 
Lakes. Lake Winnipeg and other smaller lakes lie in depressions in 
the bed of this great extinct Lake Agassiz, as it has been called. 
(Fig. 196.) 

Lake Agassiz, though covering a great area, was shallow, and it 
received a vast quantity of sediment from the ice front. This, settling 
upon the flat bottom of the Red 
River valley, has given rise to a 
very level plain, of fine-grained silt, 
which is now the seat of extensive 
wheat cultivation. Beaches and 
deltas mark the border of this ex- 
tinct lake, and its outflow channel 
is easily recognized. 

The Glacial Great Lakes.— The 
receding ice sheet also interfered 
with the Great Lakes-St. Lawrence 
drainage, for at first it occupied the 
entire basin, then, receding north- 
eastward it lay as a dam across it. 
At first small lakes developed at 
the ends of the Great Lakes, and 
in other valleys sloping northwards. 
Then, as the ice receded, these grew larger, and one by one coalesced, 
until finally the ice disappeared from the St. Lawrence valley. The 
beaches and deltas of these lake stages have been carefully studied, 
and the various outflow channels have been identified. 

The history of the lake stages marginal to the receding glacier in 
the Great Lakes region has been complicated not merely by the re- 
cession of the ice, but also by the fact that the land has been rising. 
For this reason the three upper Great Lakes flowed into the St. 
Lawrence through the Ottawa River at one stage, and then the ocean 
waters reached up the St. Lawrence into Lake Ontario. Later uplift 
so tilted the land that the upper Great Lakes flowed out through the 
channel past Detroit. The maps (Figs. 197, 198, 200, 204), with 
descriptions beneath, state in sequence the main episodes in this 
complicated history of marginal lakes associated with ice recession 
from the Great Lakes region. 

Some of the beaches of these lakes are so well developed that they 
were recognized as beaches before their cause was known. Such is 







Fig. 196. — Map to show area and posi- 
tion of maximum territory covered by 
various stages of Lake Agassiz. (After 
Upham.) 



282 



COLLEGE PHYSIOGRAPHY 



LEGEND 

Area covered by Lakes 
Lake outlets 

Glacial drainage -= -= ftf 

Successive Ice borders Mffl^- ~ - ^M 

Ice borders in New York largely conjectural. 
Correlative Moraines from Wisconsin 
westward not determined. -V 

SCAL E OF MIL E 8_ J !f' 

IQ^jjlontreaV f 




LEGEND 

Area covered by Lakes 

Lake outlets ._ 

Glacial drainage 

Correlative Ice border. ft]; 

SCALE OF MIL! 




LEGEND 

Lakes 



Area covered i 

Lake outlets c== 

Glacial drainage - = v^- ,. 

Correlative Ice border M^s.^-M' 

(From Michigan westward undetermined.) 



'■W Montreal 




Fig. 197. — The Glacial Great Lakes. Upper map shows an initial stage with independent 
bodies of water like Lakes Maumee and Chicago, which had separate outlets. Middle 
map has Lake Whittlesey draining into Lake Chicago. Lower map shows Glacial 
Finger Lakes draining westward into Lakes Warren and Chicago. Glacial Lake 
Duluth not shown in early stages because of incomplete information. (Taylor and 
Leverett.) 



THE GLACIAL PERIOD 



283 




LLIAMS ENGRAVINt. C0..f 



Fig. 198. — The Glacial Great Lakes. Upper map shows Lakes Duluth, Chicago, and 
Lundy nearing the end of the Mississippi outlets. Midila map shows Lakes Algonquin 
and Iroquois and the Mohawk outlet, as well as the KirkfWd outlet which deprived 
Niagara of a large part of its water. Lower map shows the Nipissing Great Lakes and 
outlet into Ottawa River, which was subsequently abandoned for the present St. Lawrence 
outlet through uplift and tilting north of the Hinge Lines. (Taylor and Leverett.) 



284 



COLLEGE PHYSIOGRAPHY 



the Iroquois beach in New York south of Lake Ontario along which an 
Indian trail ran, and later a road, called the "'ridge road." The silt 
deposited on the temporary lake bottoms has helped to level the sur- 
face and has made a fertile soil, the seat of fruit and other thriving 
agricultural industries. 




Marginal Channels 

Moraine Terraces. — Water flowing along the ice margin, and at 
times deflected from it, has in some places caused deposits which are 
morainic in their character. Some of these deposits form even-topped 
terraces, called moraine terraces. They are really aggraded, mar- 
ginal, stream valleys with the ice for one wall, which, on melting 
away allowed the deposit to slide into an angle of repose, giving rise 
, A to the terrace face. In other 

portions of the marginal valleys 
lakes were developed and filled, 
giving rise to even broader ter- 
races. 

Abandoned Marginal Gorges. 
— Some of the marginal streams, 
because of steeper slope, or less 
sediment load, cut into their beds 
instead of aggrading them ; and 
some have aggraded in places 
and cut in others. Where the 
streams were able to erode their 
valley bottoms, marginal chan- 
nels resulted, often as well-developed gorges. Some of these are still 
occupied by water, while some have none, or have streams far too 
small to have made the valleys in the bottoms of which they meander, 
cutting neither the bed nor the sides. Occasionally the marginal 
channels are to be found on the hill slopes in apparently unnatural 
positions, contouring the hillside instead of extending down the slope 
as gorge-forming streams normally do. Such a channel offers clear 
evidence of the presence of some retaining wall, such as an ice margin, 
along which the water flowed (Figs. 199, 201). 

Marginal channels with the same characteristics exist along the 
borders of present-day glaciers, and they are to be expected wherever 
ice sheets have stood, especially in hilly regions. They abound in 
the glaciated region of Europe and America, and they, together with 
overflow channels, often serve as the site for roads and even for rail- 
ways across divides. Associated with moraines, they are an aid to 
the determination of the position of the front of the receding conti- 
nental glacier (Fig. 337). 






Fig. 199. — Map and profile of marginal chan 
nel at Slaterville Springs, N. Y. (Rich.) 



THE GLACIAL PERIOD 



285 




JVIU.IAMS ENGRAVING SO., N.Y. 

Fig. 200. — Three stages in the recession of the ice sheet from New York, showing glacial 
lakes and changing outlets. (Fairchild.) 



286 



COLLEGE PHYSIOGRAPHY 



Glacial Erosion 

Moderate Erosion on Plains. — The presence of striae on the bed 
rock and of roches moutonnees forms in the ledges, as well as the 
scratched stones and boulders that the glacier carried, testify to the 
fact that the continental glacier, like other glaciers, was an erosive 
agent (Fig. 202). Similar testimony is offered by the deposits of drift 
that the ice has left, for these materials were dragged from the sur- 
face over which the ice flowed. Yet the evidence is convincing that, 
in places, the glacier did little more than scrape off the products of 
preglacial weathering, and in places did not even accomplish this 
much. Probably the greater part of the land surface over which the 




CATARACTS 



QYbniivi- 



Fig. 201. — Marginal channels in central New York. (Fairchild.) 

ice sheets passed was lowered but little, though there is no means of 
estimating the exact amount. 

Profound Erosion in Valleys. — While of slight effectiveness in 
general, the ice sheet becomes a powerful agent of erosion locally. 
Wherever the topography tended to concentrate the ice flow, as along 
a valley, evidence of glacial scouring appears, and in some favourable 
localities there is convincing evidence of profound glacial erosion. 
This is well illustrated in the Finger Lake region of central New York. 
Here the upland was but little worn, and rock, decayed before the 
advance of the ice, was not all removed ; but the north-south valleys, 
along which the ice flowed freely, were broadened and deepened, their 
slopes were steepened, the valley spurs were worn away, and the 
tributary valleys were left hanging. In other words, the same topo- 
graphic features were developed as are found in Greenland, Alaska, 
Norway, Scotland, New Zealand, and other glaciated mountain lands. 
Throughout the region of former glaciation the same evidence is pres- 
ent and forms are found that no known agent of erosion excepting 
ice could produce (Fig. 203, Pis. V, IX). 



Plate V 



H u 



G A C H 



U N . T A i 




>t. 3364 




X 



V 



5005 

5280 
5150 
4852 



&( 



t5 



x ; 



H^ 



F I 



^ 



Ji 



Pt.Doran 



H 



1> 




766.5 



w 



ILLIAMS ENSBAV'ing CI., 



HARRIMAN FIORD, ALASKA 

A fiord due to glacial erosion in youthful mountains. Barry Glacier receded notably from igio to 
1913, as observed by B. L. Johnson. In 1899 it terminated near Pt. Doran, where it built a 
moraine bar represented by the cross-hatched shoals, and extended as high as the heavy dotted 
line. The stream from the small ice tongue southwest of Cascade Glacier is in a hanging valley. 
(See Fig. 154.) Contour interval above and below sea level 100 feet. (From map by National 
Geographic Society's Alaskan Expedition of 1910, Lawrence Martin in charge.) 



THE GLACIAL PERIOD 



287 




•■• -* 




Glacial groove and striae on Keweenaw Point, Michigan. 



Rock Basins. — Ice erosion has deepened some valleys locally, 
forming rock basins in which lakes exist. Such are Lakes Cayuga 
and Seneca, two of the Finger Lakes. It is highly probable that a 
large portion of the depth of Lakes Ontario, Erie, Huron, Michigan, 
and Superior is due to. glacial erosion, though upon this all are not 
agreed. Certainly many lakes lie in basins partly or wholly excavated 
by ice erosion. The formation of rock basins is due to (a) weak rock, 
and (b) differential erosion as a result of rapid flowage or thick ice. 




Fig. 203. — Hanging valley near Michipicoten, Ontario. 



288 COLLEGE PHYSIOGRAPHY 

The Formation of Lakes 

The Abundance of Glacial Lakes. — The glaciated lands are dotted 
with lakes, varying in size from Lake Superior to small pools. There 
are said to be 10,000 lakes in Minnesota alone, and in Europe and 
America there are hundreds of thousands of lakes and ponds that 
have come into existence as a result of the glacial invasion. These 
lakes are of variable origin and often due to a combination of two or 
more causes. 

Causes of Glacial Lakes. — Mention has just been made of the 
rock basin lakes, and the instances given — the Great Lakes, and the 
two Finger Lakes, are also partly due to drift deposit. Many lakes 
are due solely to this cause ; for if a moraine, or an esker, or other 
glacial deposit has been laid down across a valley it serves as a dam 
until it is cut through by the outlet stream. Still another cause for 
lakes is the irregularity of the drift deposit itself, already spoken of 
in the description of moraines, kames, drumlins, and other deposits. 
In the depressions on such drift forms, ponds and lakes abound. Tens 
of thousands of them are present in the terminal and recessional mo- 
raines. 

These are the three main classes of glacial lajtes, but there are many 
variations according to the nature of the deposit, or the dam, or the 
topography. Some lakes are long and finger-like, when occupying 
a glacially sculptured valley. Such lakes abound in mountainous 
regions, as in Scotland, Norway, and the Alps, where Lakes Como 
and Maggiore are typical instances. Other lakes are circular, as is 
often the case with lakes in kettles. Still others are broadly branch- 
ing, where the dam causes the water to spread over and partly sub- 
merge a low, hilly land, as is often the case in Maine, New Hamp- 
shire, the Lake Superior region, and Canada. There is infinite variety 
of form, size, and depth among the glacially formed lakes. 

Lakes an Evidence of Youth. — The lakes caused by the ice sheet 
are necessarily youthful phenomena in the drainage systems that the 
glacial invasion has rejuvenated. They, together with other phenom- 
ena, testify to the recency of the retreat of the ice. Slowly they are 
being filled, and some have already been destroyed by filling or by the 
cutting down of the barrier, or by both processes combined. In the 
meantime they act as temporary baselevels to the streams above, 
checking the development of the rejuvenated streams. 

Diversion of Streams 

The Deep Burial of Preglacial Stream Courses. — The rejuvena- 
tion of the streams of the glaciated country has not only led to the 
development of lakes in their valleys, but also to numerous changes 
in stream courses. Sometimes the drift has been deposited so deeply 
as to quite effectually obscure the preglacial topography in regions 



THE GLACIAL PERIOD 



289 




v -sc^f 



Fig. 204. — Lakes along the border of the continental glacier in central New York, showing 
coalescing of water bodies and occupation of lower outlets with northward retreat of 
the ice. (Watson.) 



290 



COLLEGE PHYSIOGRAPHY 



APUUA \ 






XrnjjK 


1 cl/yle 




m\km 




V , EA^HQMER 




/ HCWR/ \ 




gro\tonW cor t^Ta^n^ ' 




\ ,^/mH^AN BLOlkETT MILLS 




)d/ \ 




OtfREEVILLE 
^/^""\ MESSENGERVILLE 




ITHACA U 




V 




LISLE* 





A LAKE "% X 
> WASCO 5K%JEATELES / 
m^ fLAKE, P 


Ja^uiJa \ 
muff 1 \f 




Vioravia v y 


Vcl) 

TRUJXTO/ 


YLE 


} ^LAKE\ Y 
LCACKE NCOMO \ / 


EA§j4il>1ER 




/\ ) HOFDER/ 






grcYon^/corj^la\j 

\ ^(^EAN BLOC 

.vttS-EVILLE 
CAYUGA >X \ 
?LA>S£r^ \ 
ITHACA HARFORD 


k<ett mills 
senWrville 




RICHFORD" 


lTslc 





Fig. 205. — Preglacial and present drainage near Ithaca, N. Y. (Carney.) 

of low relief. In the Middle West depths of 200 to 300 feet are re- 
corded in the well borings, and it is estimated that on the lower penin- 
sula of Michigan there is an average depth of 300 feet of glacial drift. 
On such surfaces an essentially new drainage system has developed, 
often along quite independent lines from those of the preglacial 
streams. The streams developing on such drift topography have the 




Fig. 206. — Map showing preglacial drainage (dotted lines) and present streams and lakes 
near Madison, Wisconsin. (Thwaites.) 



THE GLACIAL PERIOD 291 

features of youth — lakes, steep-sided valleys, imperfectly developed 
tributaries, and flat-topped divides. It was largely because of this 
youthful topography that extensive areas in the Central States were 
too swampy for tree growth, and were, accordingly, open prairies 
when first seen by white men. 

Partial Burial of Stream Courses. — In other cases, and especially 
in hilly regions, drift deposits have led to local diversion of stream 
courses. There are many conditions under which such diversion has 
been brought about. For instance, deposits in a stream valley have 
buried projecting rock spurs, and the streams, flowing in these deposits 
after the ice disappeared, and cutting into them, have been super- 
imposed on the buried rock ledges to one side of the axis of the pre- 
glacial valley. Again, drift deposits have turned streams out of their 
valleys and forced them along new courses for a part or whole of their 
length. In some of the valleys of New York the drift is 200 or 300 
feet thick (Figs. 205, 206). 

Reversal of Drainage. — Still another case is where valleys which, 
before the ice came, sloped northward, are now occupied by south- 
flowing streams, thus completely reversing the direction of stream 
flow. Such reversal has been brought about by a combination of 
processes such as (a) lowering of divides by glacial erosion, (b) lower- 
ing of divides by the erosion of glacial waters, (c) deposit of sediment 
in lake and stream bed, thus grading up a slope away from the ice, and 
(d) tilting of the land. In this way profound changes in drainage 
have been brought about. The St. Lawrence drainage has lost 
heavily by this process, many tributaries that formerly flowed north- 
ward having been turned into the Susquehanna and Mississippi 
systems. The entire headwaters of the Ohio, for example, above 
Cincinnati are apparently normally north-flowing, having had their 
course inverted by the effect of glaciation (Fig. 207). 

Establishment of New Stream Systems. — Through the effects 
of glaciation the drainage has been completely rearranged in places, 
elsewhere slightly modified. Niagara River did not exist, at least 
not along its present course, before the Glacial Period ; nor did 
the St. Lawrence system, which is apparently made by the union of 
several streams, some tributary to Hudson Bay, some to the Missis- 
sippi, some to the Gulf of St. Lawrence. It is a composite river system 
with a combination of features of youth and inherited maturity. In 
the more hilly and mountainous regions the rivers have been less 
completely changed, though often locally diverted ; in the less hilly 
regions, like the Mississippi valley, many completely new courses 
have been established. 

Postglacial Gorges and Waterfalls. — With all this rejuvenation 
the streams have naturally set to work upon their task of establishing 
themselves in their new condition. Thus they are at work toward 
the establishment of grade. In this process they have excavated 
gorges, as at Niagara and hundreds of other instances, and in excavat- 



292 



COLLEGE PHYSIOGRAPHY 




Fig. 207. 



Preglacial north-flowing drainage (above) and present west-flowing Ohio 
drainage in the eastern part of the state of Ohio. (Tight.) 



THE GLACIAL PERIOD 



293 



=s^^ 


5 


E 3° 

Q 2 




■^^1==^ 





ing them they have developed a still greater number of waterfalls. 
Thus postglacial gorges and waterfalls characterize the region of 
recent glaciation because the streams that have been rejuvenated 
as a result of the glaciation have 
not yet had time in most cases 
to cut their valleys down to 
grade (Fig. 208). 

Gorges in Hanging Valleys. 
— Among the causes for gorges 
and waterfalls is the change 
in slope which many tributary 
streams find in passing out of 
their hanging valleys to the 
main valley bottom. This is 
well illustrated in the Finger 
Lake valleys of central New 
York, notably Seneca and Cay- 
uga, where scores of tributary 
streams, flowing through mature 
valleys in the uplands, have an 
abrupt change of slope as they near the main valleys, and descend in 
precipitous courses through narrow, deep gorges, in the bottoms of 
which the streams leap from ledge to ledge in a series of rapids and 
falls (Figs. 60, 76). This condition is also illustrated in the English 
Lake District, in the Scottish Highlands, in Norway, and in all other 
regions where glacial erosion has lowered the valleys through which 
the ice flowed freely. 




Fig. 208. — Postglacial and interglacial gorges 
near Ithaca, N. Y. 



Influence of Glaciation on Topography 

Changes not Revolutionary. — Throughout this chapter instances 
have been given of the influence of glacial action on the topography, 
and enough has been said to show that it has been profound. Yet 
the impression must not be gained that this influence has, in general, 
been revolutionary, for it is true that, in the main, the general topog- 
raphy is much as it was before the ice came. Large sections have 
been modified only in detail. 

Erosion Generally Superficial. — The modification of topography 
has been brought about (a) by erosion, (b) by deposition. Erosion, 
as already stated, has worked locally, greatly changing the topography 
of mountain regions by the deepening of valleys and by cirque reces- 
sion, and elsewhere broadening and deepening valleys through which 
the ice flowed freely. But over the larger part of the area in United 
States covered by the continental glacier the effect of glacial erosion 
has been somewhat superficial. In Canada, however, and especially 
in the region near Lake Superior, Hudson Bay, and Labrador the 
continental glacier swept away all the soil from broad areas of 



294 



COLLEGE PHYSIOGRAPHY 



resistant rock, leaving much naked upland, drift only in the valleys, 
and there very stony drift, and many rock basins now j occupied by 
lakes. 

Deposition on Uplands. — Deposition has also produced unequal 
effects. Over much the larger part of the area covered, there is only 
a thin veneer of till on the hillsides and hilltops, scarcely obscuring 
the natural rock topography, and, in places, not even covering the 
rock. 

Deposition Greatest in Valleys. — In the valleys, however, deposit 
has been greater and more varied, and there one finds moraines, 
kames, eskers, and other glacial and fluvio-glacial deposits. But 
even here deposit has produced only local and minor features, often 
quite out of scale with the major features of the glacial topography. 
In America deposition has produced its greatest effect on gently-un- 




FiG. 209.- — Drift deposits completely mantling the preglacial topography. 

dulating surfaces near the ice border, as on the plains of the states in 
the Middle West. There, in places, the entire topography is drift- 
made (Fig. 209). The drainage features, as already noted, were 
profoundly modified and altered by the glacial invasion, and this is 
one of the most notable effects of the presence of the ice sheet. 



Influence on Man 

Glacial Soil. — In a multitude of ways the changes wrought by 
the ice sheet have had an influence on the occupants of the glacial 
region. The soil conditions have been completely changed, for the 
soil that previously existed as a result of rock decay has been swept 
off and replaced by glacial drift. In some sections bare rock slopes 
have been left, but generally a glacial deposit, of greater or less thick- 
ness, mantles the rock. This drift varies greatly and often within a 
very narrow area, being now clay, now sand or gravel, here stony, 
there free from stones. It is probable that the average quality of 
the soil has been improved ; though locally, as in the low, hilly, eastern 
and southern part of New England, where the soil is often strewn with 



THE GLACIAL PERIOD 295 

boulders brought from the higher land of the interior, the reverse is 
doubtless the case. 

On this glacial soil plants encroached rapidly after the Glacial Period, 
so that its flora is now as abundant and varied as in the never-glaciated 
southern states. It is likewise fully as productive of plants used by 
man as before glaciation. 

The animals that lived in the glaciated parts of North America 
and Europe before the ice age have likewise found the vegetation 
supported by the glacial soil to be suited to their demands and re- 
turned to all parts of the glaciated area. The mammoth and masto- 
don, however, have become extinct, though the presence of their 
skeletons in glacial deposits show they were here before the ice age. 
It is not certain that they were exterminated as a result of glaciation. 

Sand, Gravel, and Clay as Economic Resources. — The glacial 
drift includes some material of local economic value, such as sand for 
building purposes, gravel useful in roads and cement, and clay ex- 
tensively used in the manufacture of brick, tiles, flower pots and other 
coarse earthenware. 

Water Supply. — Glacial drift is also of great value as a storage 
place for underground water. It undoubtedly has greater average 
depth than the preglacial deposits, and much of it is more porous, 
so that it has greater storage capacity. This is of importance for 
local water supply, and it is a factor of significance in maintaining the 
flow of streams in the glaciated country. The storage of under- 
ground water contributes toward the same end as the storage of water 
in lakes, and this regulation of river flow is a matter of much impor- 
tance in streams that are navigable, or whose water is utilized for 
power. 

Lakes, Falls, and Water Power. — The many lakes to which the 
glacier has given rise are useful in a number of other ways, as is shown 
in the next chapter. The influence of waterfalls has already been 
considered. Suffice it to repeat here that the abundant water power 
of glaciated regions has been a factor of profound significance in 
the industrial development of many regions, such as Wisconsin, New 
England, and many other regions. The Merrimac River of New 
Hampshire and Massachusetts, for example, has falls produced by 
glaciation at the manufacturing cities of Manchester, Lowell, and 
Lawrence. Had it not been for the glacial invasion, a large proportion 
of the waterfalls of the world would not exist. 

Other Influences. — In a multitude of other ways the course of 
human affairs has been influenced by the ice invasion. Highways 
of travel have been modified by glacial erosion and deposition, de- 
termining the course of roads, canals, and railways on the land; 
and interior water routes have been made or modified. The sites of 
towns and cities have been determined, the levelness or ruggedness 
of farm land has been shaped, and local scenery, often of considerable 
economic value, has been evolved directly or indirectly by glacial 



296 



COLLEGE PHYSTOCxRAPHY 




THE GLACIAL PERIOD 297 

action. Yosemite and Niagara Falls illustrate the latter influence, as 
do also many lakes to which man resorts. 

Best Portions of United States, Great Britain, and Germany 
Glaciated. — It is a noteworthy fact that several of the greatest 
agricultural nations of the world, and the three leading manufacturing 
nations, which are also the most advanced industrially, are located 
partly in the belt of former glaciation. It would be absurd to claim 
that this is entirely the result of glaciation, for other factors are plainly 
to be seen; but it is nevertheless true that the effects of glaciation 
are to be reckoned as of fundamental importance. 

Influence on History and Development of United States. — That 
this is true in the case of the United States is easy to see when one 
remembers that manufacturing in New England developed first as a 
response to the water power, which is still utilized ; that water power, 
similarly caused by glacial action, is extensively used at various points 
as far west as Minneapolis ; that the Great Lakes waterway is a prod- 
uct of glacial action; that the route of the Erie Canal was made 
possible by glacial lake deposit, and by glacial lake outflow into the 
Mohawk ; that the level surface of the Central States, and the tree- 
lessness of the prairies which induced early and rapid settlement and 
development of agriculture, are due to glacial deposition ; and that 
there are a multitude of minor influences of the glacial invasion. 
Surely, had there been no recent glaciation, the industrial history of 
the United States would have been notably different, and it is very 
doubtful if its development would have been anywhere near so rapid 
as it has been, or if its history would have been even approximately 
what it has been. Who can tell what the history of the French settle- 
ment of America would have been but for the influence of the Great 
Lakes? Or of the Central States, but for the ease of entering and 
settling them from the east? Or of the settlement of the Western 
States, but for the fact that people had so easily pushed their way 
westward across the Mississippi valley plains before the time of the 
discovery of gold in California? Yet each of these events in our 
history was influenced profoundly by conditions which the glacial 
invasion introduced. 

Complexity of the Glacial Period 

Advances and Recessions. — For the sake of clearness in the pre- 
ceding discussion the subject has been treated as if there had been 
but a single period of glaciation. As a matter of fact, the ice invasion 
has been far more complex. There have been advances and recessions, 
as well as minor oscillations of the ice front, and there have been periods 
when the continental glaciers have either entirely disappeared or 
have receded far back toward the centres of dispersion. As yet there 
is not unanimity of opinion as regards the detailed history of this com- 
plexity, though there is general agreement that there was a complex 



298 COLLEGE PHYSIOGRAPHY 

series of advances and recessions, during each of which the ice margin 
phenomena were repeated, though partially or completely erased by 
each advance (Fig. 210). 

Evidences of Complexity. — The evidences of advance and reces- 
sion of the ice front are of several kinds, among which are the follow- 
ing : (1) There is more than one till sheet in many places, one over- 
lying the other. (2) The till sheets vary in character, and the lower, 
or older, is often much weathered, as if long exposed to weathering 
before the overlying drift was laid down upon it. (3) The older 
till sheets extended farther south in the Mississippi valley than the 
drift of the last glacial advance, and they are not only more weath- 
ered, but have drainage of a much more mature pattern than that of 
the last drift. (4) There are buried gorges beneath the upper till, 
which were excavated in the interval between the ice advances, and 
since they are larger than the postglacial gorges, the interval between 
advances is believed to have been longer than postglacial time. 
(5) The till sheets of the different advances are often separated by soil 
beds, and peat deposits containing plant remains, showing that plants 
grew in the interval between advances. As far north as Toronto, 
the interglacial beds contain remains of plants that do not now live 
so far north. 

The Glacial and Interglacial Stages. — Most glacialists are now 
convinced that there were two or more ice advances, separated by 
long intervals, or interglacial stages. It is thought by some observers 
that the older drift is 25 times as old as that of the latest glaciation. 
A full discussion of this subject does not fall within the province of 
physical geography, and it may be left here with the statement that, 
both in Europe and America, there is evidence which convinces many 
students of the subject that there were 4 or 5 stages of advance and 
recession, and some are not convinced of more than two notable 
advances, with one interglacial stage. The following table gives the 
names usually applied in the United States to the several glacial and 
interglacial stages, the Wisconsin being the latest. There is not yet 
complete agreement about the relationships of the Iowan, Illinoian, 
Peorian, and Sangamon. 



Glacial 


Interglacial 


Wisconsin 






Sangamon, Peorian 


Iowan, Illinoian 






Yarmouth 


Kansan 






Aftonian 


Nebraskan, or pre-Kansan or Jerseyan 





THE GLACIAL PERIOD 



299 



Former Glacial Periods 

The Glacial Period whose effects have been discussed in this chapter 
is not the only one of which there is record. In South Africa, for 
example, there is an extensive bed of till, consolidated to a hard rock, 
called tillite. It contains scratched stones, rests on glacially grooved 
bed-rock (Fig. 211), and bears evidence of being a deposit of a conti- 
nental glacier. Yet it occurs in the tropical zone, and the ice move- 
ment was from the equatorial regions ! This glacial period occurred far 




Fig. 211. 



Roche moutonnee made by the Permian ice sheet in South Africa. 
(R. B. Young.) 



back in geological time, during the period of the earth's history known 
as the Permian ; during the same period there was similar glaciation 
in India, Australia, and Brazil (Fig. 212). Evidence of still earlier 
glaciations is reported from Canada, from northern Norway, from 
China, and from Australia. It seems, therefore, that glaciation has 
been one of the phenomena of the geological past in regions far 
outside the range of possible glacial advance under present climatic 
conditions. 



Hypotheses to Account for Former Glaciation 

The Problem a Difficult One. — While the fact of the advance and 
retreat of the continental glaciers of the Glacial Period, and of earlier 
times, is firmly established, and most of the glacial phenomena are 



30o 



COLLEGE PHYSIOGRAPHY 



explained, science has so far failed in the establishment of any satis- 
factory explanation of the cause of the glaciation. It is not known 
either why the ice invasion came, or how long it lasted, or why, or 
exactly when, it disappeared. That it lasted for a long time, and had 




Glacial boulder from the Permian tillite of Brazil. (Woodworth.) 



a complex history of advance and recession, is absolutely proved ; 
and it is also clear that the recession was exceedingly recent, as geo- 
logical time goes. Some estimates place the vanishing of the ice from 
the United States at from 5000 to 10,000 years ago, while others place 
the recession four or five times as far back in the past. None of the 



THE GLACIAL PERIOD 301 

estimates are based upon sufficiently accurate data to warrant their 
acceptance. The best that can be said is that the recession of the 
ice was a recent event ; indeed, there is reason for believing that it is 
still in progress, for ice sheets in Greenland and the Antarctic, and 
mountain valley glaciers, show evidence of fairly continuous shrinking. 

While science has so far failed to demonstrate the cause for the 
continental glaciation, scientific men have been active in proposing 
hypotheses for consideration. In view of the fact that none of these 
is generally accepted, it does not seem worth while here to enter into 
a thorough consideration of them; but a brief statement of some of 
the hypotheses that have been put forward will serve to show the wide 
range of possibility. 

Hypothesis of Geographical Changes. — It has been proposed that 
geographical changes are sufficient explanation, such as change in 
ocean currents, or change in the relative distribution of land and water, 
or change in the elevation of the land. These do not seem adequate 
explanations for the development of vast ice sheets, even though it 
be recognized that increase in snowfall, rather than excessive cold, 
is the important factor in the generation of an ice sheet. With the 
discovery of ice sheets of former ages within the tropics, these geo- 
graphical explanations seem even less probable. 

Hypothesis of Decreased Carbon Dioxide. — Another hypothesis 
is that the content of carbon dioxide and water in the atmosphere has 
varied in some stages, much of the former being given to the air by 
the exposure of sea beds and the disintegration of the limey animal 
remains, while at other times the carbon dioxide has been largely 
withdrawn in the processes of oxidation. Carbon dioxide interferes 
with radiation, and, therefore, an abundance of it tends to keep the 
earth's surface warm. When the carbon dioxide content is slight, 
radiation proceeds more rapidly and the land is colder. According 
to this explanation, glacial periods come when the carbon dioxide and 
water content of the air is sufficiently reduced in amount. Whether 
this theory would account for the development of glaciation within 
the tropical zone is doubtful, even with the assumption of elevation 
or other geographical changes. 

Croll's Hypothesis. — The earth in its journey around the sun fol- 
lows an elliptical path with the sun at one of the foci. In one part 
of the course, perihelion, the earth is nearer the sun than at the other, 
aphelion. The ellipticity of the orbit undergoes a slow variation, 
as that in the course of long periods of time the orbit becomes more 
elliptical or, in the opposite direction, more circular. This is called 
the eccentricity of the earth's orbit. When most eccentric the dis- 
tance between the sun and earth is over 14,000,000 miles greater in 
aphelion than in perihelion, and the season is longer. 

The earth's axis is inclined to the plane of the ecliptic so that we 
have winter and summer during every revolution. If the winter 
comes during aphelion, the sun is nearer and the season shorter than 



3 02 COLLEGE PHYSIOGRAPHY 

if it comes during perihelion. As a matter of fact, there is a constant 
change owing to the motion of the earth's axis, known as precession of the 
equinoxes. By this motion the axis swings so that each pole passes 
through a complete circle during a period of 21,000 years. Thus 
at intervals of 10,500 years the winter occurs alternately in aphelion 
and perihelion. 

CrolPs hypothesis was built upon these astronomical facts as a 
basis, the underlying idea being that during a period of great eccen- 
tricity, glaciation would alternately develop in each hemisphere as the 
precession of the equinoxes brought winter in the perihelion stage; 
while interglacial conditions would develop during the intermediate 
periods. This hypothesis was brilliantly presented, much discussed, 
and found many adherents, but, even though numerous geographical 
conditions were invoked as accessory to the main theory, it has been 
quite generally considered as an inadequate hypothesis and now finds 
few supporters} 

Other Astronomical Hypotheses. — Much more vague astronomical 
hypotheses have been proposed, such as the hypothesis that the sun 
is a variable star, and others much less probable. Naturally such 
hypotheses cannot be tested, for we have not the data upon which to 
investigate them. It is surely possible that the heat emitted from 
the sun varies from time to time, and that it may be sufficiently vari- 
able to account for the development of ice sheets ; but as yet there 
are no facts which can be considered as proof of this. 

Hypothesis of a Shifted Axis of the Earth. — Finally, the hypothesis 
has been proposed that the axis of rotation of the earth changes posi- 
tion under the effect of influences as yet unknown, and that the posi- 
tion of the poles themselves is, therefore, changed. If the pole were 
shifted to South Africa, an ice sheet would naturally develop ; and 
so would ice sheets develop on the two sides of the North Atlantic if 
the pole were shifted to a point between northern Europe and America. 
This hypothesis meets with the serious objection that there is no known 
cause for such a change, and, therefore, that it is improbable. Fur- 
thermore, it fails to explain the widespread distribution of areas where 
mountain valley glaciers were more extensive than now, some of them 
farther away from the position inferred for the changed pole than 
they are from the present pole. 

Explanation of Glacial Periods Unsettled. — From this multitude of 
hypotheses it has not yet seemed possible to select one that has a suffi- 
cient body of fact in support of it to lead to its general adoption. There- 
fore the cause of the Glacial Period must be reckoned as one of the 
unsolved problems of science, the existence of which is one of thegreat 
incentives to scientific research. With the success that has in the past 
attended the patient investigation of the problems of nature, we may 
look forward hopefully to a time when this phenomenon will also pass 
into the realm of the known. 



THE GLACIAL PERIOD 303 



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3 o4 COLLEGE PHYSIOGRAPHY 

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Lake Region, ibid., Vol. 16, 1908, pp. 527-548. 



THE GLACIAL PERIOD 305 

I.-.C. Russell. A Geological Reconnaissance in Central Washington, Bull. 108, 
U. S. Geol. Survey, 1893, PP- 87-96; A Geological Reconnaissance along 
the North Shore of Lakes Huron and Michigan, Geol. Survey Michigan, 
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and Frank Leverett, Folio 155, U. S. Geol. Survey, 1908. 

I. C. Russell and Others. For maps showing former extent of mountain 
glaciation in America, see the following U. S. Geol. Survey and other 
maps: Russell, PI. 29, 8th Ann. Rept., and PL 18 in Part 2, 20th Ann. 
Rept. ; Weed, PL 1, Bull. 104; Lindgren, Folios 39, 31; Lawson, PL 31, 
p. 306, Vol. 3, Univ. Cal. Publications; Willis, PL 8, Prof. Paper 
19; Tarr and Martin, Map 1, Alaskan Glacier Studies; Calhoun, PL 1, 
Prof. Paper 50; Bastin and Blackwelder, Folios 141, 142, and PL 
28 in Prof. Paper 51; Atwood, Pis. 4, 10, Prof. Paper 61, and Figs. 1-4, 
pp. 390-398, Vol. 20, Journ. Geol. ; Capps, PL 1, Bull. 386 ; Ball, Pis. 4, 5, 
Prof. Paper 63; Cross and Howe, Folio 153; Ransome, PL 1, Prof. 
Paper 75; Hole, PL 1, p. 502, Vol. 20, Journ. Geol. ; Alden, PL 13, Vol. 
24, Bull. Geol. Soc. Amer. 

R. D. Salisbury. The Glacial Geology of New Jersey, N. J. Geol. Survey, 
Vol. 5, 1902, 802 pp.; Articles on the Drift, Journ. Geol., Vol. 1, 1893, pp. 
61-84; Vol. 2j 1894, pp. 613-632, 708-724, 837-851; Vol. 3, 1895, pp. 
70-97; Vol. 4, 1896, pp. 948-970; Vol. 8,, 1900, pp. 426-432 ; Vol. 17, 
1909, pp. 589-599; Glacial Work in the Western Mountains in 1901, 
ibid., Vol. 9, 1901, pp. 718-731; Prof. Paper 51, U. S. Geol. Survey, 
1906, pp. 71-91. 

R. D. Salisbury and W. W. Atwood. Geography of the Region about Devils 
Lake and the Dalles of the Wisconsin, Bull. 5, Wis. Geol. Survey, 1900, 

151 PP- 

E. H. L. Schwarz. The Three Paleozoic Ice Ages of South Africa, Journ. Geol., 

Vol. 14, 1906, pp. 683-691. 

N. S. Shaler. Description of glacial features in U. S. Geol. Survey publica- 
tions : Marthas Vineyard, 7th Ann. Rept., 1888, pp. 297-363 ; Nantucket, 
Bull. 53, 1889, pp. 601-653; Cape Cod, 18th Ann. Rept., Part 2, 1898, 
pp. 503-593; Narragansett Basin, Monograph $^, 1899, pp. 64-76. 

B. Shimek. The Loess and the Lansing Man, Amer. Geol., Vol. 32, 1903, 

PP- 353-369- 

G. H. Stone. The Glacial Gravels of Maine, Monograph 34, U. S. Geol. 
Survey, 1899, 499 pp. 

A. Strahan. Glacial Phenomena of Paleozoic Age, Quart. Journ. Geol. Soc, 
Vol. 53, 1897, pp. 137-146. 

R. S. Tarr. The Central Massachusetts Moraine, Amer. Journ. Sci., 3d series, 
Vol. 43, 1892, pp. 141-145; Glaciation of Mount Ktaadn, Maine, Bull. 
Geol. Soc. Amer., Vol. 11, 1900, pp. 433-448; Evidence of Glaciation in 
Labrador and Baffin Land, Amer. Geol., Vol. 19, 1897, pp. 191-197; 
The Origin of Drumlins, Amer. Geol., Vol. 13, 1894, pp. 393-407; Physi- 
cal Geography of New York State, New York, 1902, Chapters IV, VII, 
VIII, IX, and XII ; Glacial Erosion in the Scottish Highlands, Scottish 
Geog. Mag., Vol. 24, 1908, pp. 575-587 ; Papers on the Finger Lake Region 
of Central New York, — Bull. Geol. Soc. Amer., Vol. 5, 1894, pp. 339-356; 
Bull. Amer. Geog. Soc, Vol. 37, 1905, pp. 193-212 ; Amer. Geol., Vol. 33, 
1904, pp. 271-291; Journ. Geol., Vol. 14, 1906, pp. 18-21; ibid., Vol. 
12, 1904, pp. 69-82; Bull. Geol. Soc. Amer., Vol. 16, 1905, pp. 215-228; 
Watkins-Catatonk Folio, U. S. Geol. Survey, No. 169, 1909. 

F. B. Taylor. Glacial and Postglacial Lakes of the Great Lakes Region, 

Smithsonian Rept. for 1912, No. 2201, pp. 291-327 ; The Moraine Systems 
of Southwestern Ontario, Trans. Canadian Institute, Vol. 10, 191 2, pp. 
1-23 ; Correlation and Reconstruction of Recessional Ice Borders in 
Berkshire County, Massachusetts, Journ. Geol., Vol. n, 1903, pp. 2> 2 2r 
364; The Richmond and Great Barrington Bowlder Trains, Bull. Geol. 
Soc Amer., Vol. 21, 1910, pp. 747-752. 
x 



306 COLLEGE PHYSIOGRAPHY 

W. G. Tight. Drainage Modifications in Southeastern Ohio, Prof. Paper 13, 

U. S. Geol. Survey, 1903, in pp. 
J. E. Todd. The Moraines of the Missouri Coteau, Bull. 144, U. S. Geol. 

Survey, 1896, 71 pp. ; The Moraines of Southeastern South Dakota, ibid., 

Bull. 158, 1899, 171 pp. 
C. F. Tolman, Jr. The Carbon Dioxide of the Oceans and its Relations to the 

Carbon Dioxide of the Atmosphere, Journ. Geol., Vol. 7, 1899, pp. 585-618. 
J. B. Tyrrell. The Glaciation of North Central Canada, Journ. Geol., Vol. 6, 

1898, pp. 147-160. 
Warren Upham. Glacial Lake Agassiz, Monograph 25, U. S. Geol. Survey, 

1896, 658 pp.; On the Cause of the Glacial Period, Amer. Geol., Vol. 6, 

1890, pp. 327-339. 
Warren Upham, Frank Leverett, N. S. Shaler, and W. O. Crosby. Climatic 

Conditions of the Glacial Period, Proc. Bost. Soc. Nat. Hist., Vol. 24. 

1889, pp. 450-467. 
U. S. Geological Survey Folios. The following folios of the Geologic Atlas 

of United States are among the best of those which have special maps 

and texts dealing with the glacial features. 
Rockland, Me. Niagara, N.Y. Aberdeen-Redfield, S.D. 

Holyoke, Mass. Ann Arbor, Mich. Jamestowi'-Tower, N.D. 

New York City, N.Y. Chicago, 111. Cloud Peak-Fort McKin- 

Passaic, N.J. Milwaukee, Wis. ney, Wyo. 

Watkins-Catatonk, N.Y. Tallula-Springfield, 111. Tacoma, Wash. 

A. C. Veatch. Diversity of the Glacial Period in Long Island, Journ. Geol., 

Vol. n, 1903, pp. 762-776. 

F. Wahnschaffe. Die Oberflachengestaltung des Nord Deutschen Flach- 

landes, Forschungen zu D. Landes und Volkskunde, Vol. 6, 1891, 166 
pp. ; ibid., 3d edition, 1909. 
David White. Carboniferous Glaciation in the Southern and Eastern Hemi- 
spheres, Journ. Geol., Vol. 3, 1889, pp. 299-330; ibid., Vol. 15, 1907, 
pp. 615-633. 

B. Willis. Changes in River Courses in Washington Territory due to Glacia- 

tion, Bull. 40, U. S. Geol. Survey, 1887, 10 pp. ; Drift Phenomena of Puget 
Sound, Bull. Geol. Soc. Amer., Vol. 9, 1898, pp. 111-162; Research in 
China, Publication 54, Carnegie Institution, 1907, pp. 267-269. 

J. H. Wilson. The Glacial History of Nantucket and Cape Cod, Columbia 
Univ. Press, Geol. Series, Vol. 1, 1906, 90 pp. 

J. B. Woodworth. An Attempt to Estimate the Thickness of the Ice Blocks 
which gave rise to Lakelets and Kettle Holes, Amer. Geol., Vol. 12, 1893, 
pp. 279-284; Nantucket, A Morainal Island, Journ. Geol., Vol. 7, 1899, 
pp. 226-236 ; Ancient Water Levels of the Champlain and Hudson Valleys, 
Bull. 84, N. Y. State Museum, 1904, pp. 65-265; Pleistocene Geology of 
the Mooers Quadrangle, ibid., Bull. 83, 1905, 60 pp.; Permian Glacial 
Deposits of South Brazil, Bull. Mus. Comp. Zool., Vol. 56, I9i2,pp. 52-91. 

G. F. Wright. Ice Age in North America, New York, 1889, 191 1 ; Man and 

the Glacial Period, New York, 1892; The Glacial Boundary in Western 
Pennsylvania, Ohio, etc., Bull. 58, U. S. Geol. Survey, 1890, 112 pp. 

TOPOGRAPHIC MAPS 

Driftless Area 
Cross Plains, Wis. Lancaster, Wis. Sparta, Wis. 

Drumlins and Ground Moraine 

Hartford, Conn. Boston Bay, Mass. Weedsport, N.Y. 

Oswego, N.Y. Watertown, Wis. Waterloo, Wis. 

Auburn, N.Y. ' Sun Prairie, Wis. Fond du Lac, Wis. 



Baldwinsville, N.Y. 



THE GLACIAL PERIOD 

Glacial Lake Overflow Channels 
Lacon, 111. 



307 



Ottawa, 111. 



Northville, S.D. 
St. Croix Dalles, Wis. 
Eagle, Wis. 
St. Paul, Minn. 



Moraines 

Marthas Vineyard, Mass. 
Tower, N.D. 
Edgeley, N.D. 
Minnetonka, Minn. 



Whitewater, Wis. 
Ann Arbor, Mich. 
Gloversville, N.Y. 
Easthampton, N.Y. 



Elmira, N.Y.^ 
Janesville, Wis. 



Outwash Plains 

Huron, S.D. 

Mt. Sterling, Ohio 



Brooklyn, N.Y. 
Wyndmere, N.D. 



CHAPTER X 

LAKES AND SWAMPS 

Characteristics of Lakes 

General Features. — A lake is a body of standing water on the 
land. Lakes occur, therefore, where there is an obstruction to the 
free run-off of surface water. If the obstruction be slight, as on level 
ground, a swamp may result ; but if it be sufficient to give rise to a 
depression, standing water may accumulate. There is every grada- 
tion from swamp to lake ; and among lakes, there is every gradation 
from very shallow to very deep bodies of water. There is also great 
variation in size. Some are so small that they are called ponds, 
others are so large that they are often called inland seas. From the 
standpoint of physiography there is no distinction to be drawn be- 
tween lakes and ponds ; and even in popular usage, there is no definite 
distinction excepting that ponds are small lakes ; but sometimes fair- 
sized lakes are called ponds, and much smaller bodies of water are 
called lakes. 

The Variety of Lakes. — The accompanying table gives the area, 
altitude, and depth of some of the best-known lakes of the world. 

From it one sees that far the largest lake is Caspian Sea, while the 
largest fresh-water lake is Lake Superior. There is great range in 
altitude of the surface, for lakes may lie at any level up to the snow 
line, and their surface may even be below the sea level in arid interior 
basins. In depth there is also a great range, and the bottoms of 
many lakes lie below sea level. It is, perhaps, needless to say that 
the myth of " bottomless lakes " is wholly without foundation, and 
many such lakes are in reality quite shallow. Every lake is a reservoir 
in which a considerable volume of water is stored. It is estimated that 
the volume of water in the deep basin of Lake Superior is about 2800 
cubic miles, while in the much shallower and smaller Lake Erie are 
about 130 cubic miles of water. The total volume of water in the 
five Great Lakes is about 5500 cubic miles. The water of these lakes 
comes from the rainfall, some of it entering by direct run-off, some from 
underground sources, as in the case of rivers. Because of the vast 
volume, large, deep lakes are not subject to very great fluctuations in 
level with variations in rainfall. They serve, therefore, as regulators 
of streams that issue from them. 

308 



LAKES AND SWAMPS 



309 



Name of Lake 



Aral Sea . . 
Argentino . 
Athabasca . 
Baikal . . 
Balkash . . 
Bangweolo . 
Buenos Aires 
Caspian Sea 
Cayuga . . 
Chad . . . 
Chelan . . 
Como . . . 
Crater . . 
Dead Sea 
Erie . . . 
Eyre . . . 
Garda . . 
Great Bear . 
Great Salt . 
Great Slave 
Huron . . 
Iliamna . . 
Ladoga . . 
Manitoba . 
Michigan . 
Nicaragua . 
Nyassa . . 
Ontario . . 
Pontchartrain 
Salton Sea . 
Superior . . 
Tanganyika 
Titicaca . . 
Van . . . 
Victoria . . 
Winnipeg . 



Area in 


Elevation 


Square Miles 


in Feet 


26,900 


160 


590 


613 


2,850 


690 


12,500 


i,3 12 


7,800 


780 


i,67o 2 


3>76o 


780 


745 


169,000 


-85 1 


67 


381 


10,000 2 


850 


85 


1,079 


60 


650 


25 


6,239 


37o 


-1310 1 


9,990 


573 


4,000 2 


70 


189 


215 


11,200 


39i 


2,360 


4,218 


10,100 


520 


22,322 


582 


1,100 


50 


7,000 


60 


1,850 


810 


21,729 


582 


3,600 


no 


14,000 


1,500 


7,104 


247 


600 


5 3 


247 


-253 x 


30,829 


602 


12,650 


2,560 


3,300 


12,875 


1,400 


5,214 


30,000 


4,000 


9,400 


710 



Greatest 

Depth in 

Feet 



225 
122 -f 

4,997 
70 



2,400 

435 
20 
1,500 
i,340 
2,000 
i,330 

210 

i,i35 

270 + 
30-50 

650 + 

750 

600 + 

730 

870 

83 
600 + 

738 
16 

34 4 
1,008 
4,188 
700 

590 + 
70 



1 Below sea level. 



2 Variable with the season. 



3 Or less. 



4 In 1906. 



Variations in Lake Level. — The levels of lakes will, however, slowly 
rise and fall as the precipitation varies. This is especially noticeable 
in small lakes, as a direct result of run-ofT ; in large lakes it is more 
noticeable as an effect of seasonal variations, a dry season being fol- 
lowed by a lowering of the lake level, a wet season by a rise. This is in 
large part a response to variations in the amount of underground 
water contributed. There are also variations in the levels of large 
lakes as a result of wind direction, for, when the wind blows steadily 
for a long enough time, water is drifted from one end of the lake to 
the other, causing a rise of the surface at the end to which the water 
drifts. This may be seen on Lake Cayuga in central New York. 



310 COLLEGE PHYSIOGRAPHY 

Ordinarily lakes lie below the zone of permanent saturation or the 
water table, and their volume is being augmented by the movement 
of underground water toward them ; but some lakes, especially small 
ones, lie above the permanent water table. Such lakes suffer leakage, 
and if the drainage area is small and the surrounding rock porous, 
such as gravel or sand, they may entirely disappear during periods 
of drought. Another cause for variation in lake level is evaporation, 
to which all lakes are subjected. In an arid climate evaporation may 
exceed supply, and then fluctuation of level follows. It is for this 
reason, together with seepage, that shallow Lake Chad fluctuates 
so in area, becoming greatly expanded during the seasons of rains, 
and shrinking during the dry season. In desert regions there are 
many basins with no water, or with water only for a part of the year. 

Lakes without Outlets. — Lakes in which evaporation exceeds 
water supply will have their surfaces lowered, so that they cannot 
overflow the lowest point in the rim of the basin ; that is, they have 
no outlet. In that case, they soon become transformed to salt lakes, 
like Aral Sea, Caspian Sea, Dead Sea, and Great Salt Lake. The 
reason for this is that the mineral substances which the surface and 
underground waters bring to the lake in solution cannot escape by 
evaporation. They, therefore, become more and more concentrated ; 
and, since salt is one of the substances carried, in small quantities, 
in solution the lake water gradually grows saline. A lake with out- 
let suffers no such concentration of dissolved mineral matter ; but we 
may be certain that if such a lake, say Lake Superior, were lowered 
by evaporation so that it no longer had outflow, and its water had no 
underground escape, as by seepage, it would become a salt lake. It 
would in time become even much Salter than the sea itself, as the Dead 
Sea and Great Salt Lake are. 

The Streams Related to Lakes. — Most lakes have outlet streams, 
though this is not true (a) of lakes in arid regions where evaporation 
exceeds supply, or (b) of lakes where seepage exceeds the supply. 
There are also inlet streams, usually at the heads of lakes ; but such 
streams may also be absent in lakes of small size, or lakes of small 
drainage area. Some lakes of this sort receive practically their en- 
tire supply from the rainfall and slight contributions of run-off from 
their narrow rim. This is true, for instance, of even so large a body 
of fresh water as Crater Lake, Oregon, whose main supply is from direct 
rainfall and from an exceedingly limited drainage area, while it dis- 
charges, not through an outflow channel, but by seepage. 

Besides rainfall, rain-born rills, underground supplies, and an inlet 
stream, most lakes receive water from tributaries. Large lakes, like 
one of the Great Lakes, receive water from hundreds of such trib- 
utary streams, each contributing sediment as well as water. Some- 
times the mouths of the tributary streams are at the heads of bays, 
sometimes they project as delta points into the lake. There is, in 
fact, great variety in form of lake, some being long, narrow, and 



LAKES AND SWAMPS 311 

straight-walled, some circular, some notably irregular. The form of 
the lake varies with the origin, and with the topographic features 
of the region in which the lake basin is developed. 

The outlet streams from lakes are well regulated as to volume. 
The St. Lawrence, flowing from the Great Lakes, has a fairly steady 
volume and no floods, while the Ohio, with no lakes, has floods which 
rise 50 to 60 feet. The St. Lawrence is clear, while the Missouri, 
without headwater lakes, is extremely muddy. 

Causes for Lake Basins 

Davis's Classification. — There have been numerous classifica- 
tions of lakes, but the one that seems to possess the most philosophical 
basis is the one proposed by Davis. This considers them a phase of 
drainage, their basins being local depressions obstructing the free 
run-off of water. Lakes are episodes in the history of valley develop- 
ment by the action of running water, their basins serving as the stor- 
age places for some of this water, as the seat of deposit for river sedi- 
ment, and as temporary baselevels for the inlet and tributary streams. 
Since the lakes are an integral part of the land drainage, and their 
basins are parts of the valleys of river systems, it seems proper to 
consider lakes and lake basins as phases of river valley development. 

Upon this basis, lakes may be classified as (1) lakes consequent upon 
new land surfaces, (2) lakes formed in the course of the normal develop- 
ment of river valleys, and (3) lakes due to accidental interruptions to 
normal development. 

Consequent Lakes. — Upon any new land surface, such as a sea 
bottom raised above sea level to form a coastal plain, there may be 
depressions, in which standing water will gather. Lakes thus formed 
are consequent upon original irregularities, and may, therefore, be 
classed as consequent lakes. Some of the shallow lakes of the southern 
part of Florida are of this origin. Any new land surface, such as a 
lava flow, or a sheet of till, or a moraine, may have irregularities in 
which consequent lakes form. Thousands of such lakes are dotted 
over the surface of the glaciated country, most of them so small as to 
be commonly called ponds. Consequent lakes also occur in depres- 
sions on the bottoms of extinct lakes such as Lake Agassiz. Even 
so large a lake as Winnipeg is at least partly of consequent origin. 

Lakes of Normal Development. — As we have seen, lakes are de- 
veloped upon floodplains during the normal meandering of rivers. 
Such lakes are narrow and shallow, usually with curved outline, and 
in their most typical form are ox-bow lakes. The upward growth of 
a floodplain may pond back the mouths of tributaries, making their 
lower courses broad, lake-like expanses, as in the case of the tribu- 
taries of the Amazon. Or low-grade streams may be ponded by the 
growth of vegetation, or by the accumulation of " rafts " of tree trunks 
(Fig. 213), as in the case of the tributaries to the Red River of 



312 



COLLEGE PHYSIOGRAPHY 









c3 

s 



oo to 



LAKES AND SWAMPS 



313 



Louisiana. Or a tributary stream, depositing sediment in the main 
river may pond it back, forming lake-like expanses. This is illustrated 
in the Colorado River (Fig. 66), where there are lake-like stretches 
above the coarse deposits made by steep-grade tributaries. It is 
also illustrated in the Mississippi River above the mouth of the Chip- 




FlG. 214. 



Lake Pepin, where a tributary dams back the main stream of the Mississippi. 



pewa, where the river expands into what is called Lake Pepin (Fig. 
214). Glaciers may also pond back a river, as the Copper River, 
Alaska (Fig. 377). 

A river bed is really a succession of basins of small size, and, as such 
a stream dries up, a chain of tiny lakes succeeds the stage of running 
water. An abandoned stream course may give rise to more per- 
manent lakes of this sort. Thus, south of Syracuse, N.Y., there is 
a small body of water called Jamesville Lake, in a pot hole excavated 
at the base of a waterfall which was probably as large as Niagara, 
in a marginal channel that flowed along the edge of the receding ice 
sheet. There are similar lakes near Coulee City, Washington, at the 
base of an abandoned waterfall of the Columbia River which was 
diverted southward during the Glacial Period and formed the Grand 
Coulee (Fig. 337), a dry stream course in whose bottom are many 
saline lakes. Small lakes in depressions on the beds of abandoned 
marginal channels are not uncommon in formerly glaciated regions. 

Lakes develop at the mouths of rivers, both where they enter the 
sea through bays, and where they enter it over deltas. In the former 
case sand bars are often thrown up across the mouth of the bay, either 
partly or completely imponding the waters. Lakes and ponds of this 



314 



COLLEGE PHYSIOGRAPHY 




Coastal lakes in Russia near Odessa. 



origin are common along the shores of the Great Lakes, and along the 
ocean, as on Martha's Vineyard, Mass., and along the Black Sea south- 
west of Odessa (Fig. 215). Delta lakes develop by the irregular 
growth of the delta, combined with the action of the waves. Thus 






LAKES AND SWAMPS 315 

the outward growth of the Mississippi delta has left an unfilled de- 
pression called Lake Pontchartrain (Fig. 90), and there are partly or 
completely formed lakes of similar origin on this and other deltas, 
such as the Danube (Fig. 92). 

On the land the growth of alluvial fans may establish a dam across 
a stream and thus form a lake, as King River has done in the Valley 
of California, forming Tulare Lake. 

In the course of the development of underground drainage the set- 
tling of the surface gives rise to depressions, or sink holes, which, 
though normally open at the bottom, are sometimes rilled either 
naturally or by man, and they become the seat of sink hole ponds. 
Such lakes abound in limestone regions. 

Lakes due to Accident. — There are a multitude of causes by which 
basins may be formed, usually by the development of a dam across 
a preexisting drainage line. There are, for example, thousands of 
dams of organic origin. Man is now one of the most effective lake 
makers, placing dams across streams to impound water for his service 
for many uses, such as irrigation, water-power supply, and municipal 
drinking supply. Thousands of lakes and ponds have been made 
by man. In North America, before white men entered to destroy 
it, the beaver was a lake maker of great importance. In more remote 
regions, or in places where protected from the hunters, it still builds 
its dams of sticks, and lives in the quiet waters of the pond above. 
The growth of plants may so check the run-off as to cause shallow 
lakes, as in the case of Lake Drummond in Dismal Swamp. 

Landslides and avalanches sometimes cause lakes by forming dams 
across streams, especially in lofty mountains, but on a smaller scale 
even in hilly regions. Sand carried by the wind is often deposited 
across streams, especially along the coast line, forming small ponds. 
Lava flows also dam some streams and form lakes, such as Snag 
Lake, California, the Sea of Tiberias in the Jordan valley, and nu- 
merous small lakes in the central plateau of France and other volcanic 
regions. In 1783 a lava now in Iceland dammed back side streams, 
making lakes which covered villages and destroyed much life. The 
craters of volcanoes are basins and in these lakes often occur after the 
volcanic activity ceases. These are normally circular in outline, and 
are sometimes of great depth. Crater Lake in Oregon is an instance 
of such lakes, and there are others in the Auvergne region of central 
France, in the Eifel region of western Germany, in Italy, as at Lake 
Nemi near Rome, and Lake Avernus near Naples, and in many other 
places. 

The level of the land is subject to change by uplift or by depres- 
sion, and changes of this sort have been in progress during the geo- 
logical past. During such changes basins may be formed by the 
down-warping of a portion of the surface, or by the uprising of a 
region. Basins of this sort are a natural result of the changes of level 
that occur during mountain growth. Thus, there are extensive basins 



316 COLLEGE PHYSIOGRAPHY 

among the mountains of western United States, though, owing to the 
aridity of the climate, the lakes in the lower portions of these basins are 
small and shallow, and usually without outlets. Caspian Sea is in 
a basin that was probably formed by a down-warping of the surface, 
and it is thought by some geologists that change of level may be one 
of the causes for some of the Great Lakes of North America. 

The formation of basins by crustal movement may be due either 
to folding or faulting (Fig. 216). Of the latter, Lake Warner and 



LAKE SOTTERH 




Swedish lakes in fault block depressions, (de Geer.) 



other lakes in southern Oregon are examples, as is the Dead Sea, 
which lies in the bottom of a basin whose bottom has sunk between 
two faults. The chain of lakes in east-central Africa — including 
Tanganyika and Albert Nyanza — has been explained also as a result 
of down-faulting, forming what is called a rift valley lake. 

Such movements of faulting and folding often give rise to large 
and deep basins ; but there are also small ones of the same origin. 
Thus, near San Francisco there are small lakes and artificial reser- 
voirs in a rift valley, where faulting occurred in the earthquake of 
1906. Small lakes and ponds are not uncommonly caused by move- 
ments along fault lines during earthquakes or by the settling of por- 
tions of the surface. For instance, during the earthquake of 1819 in 
India, a portion of the delta of the Indus River settled, forming an 
inland sea 2000 square miles in area ; and during the earthquake of 
181 1 in the Mississippi valley there was sinking of the bottom lands 
in northern Arkansas and neighbouring states, causing a number of 
lakes. One of these, Reelfoot Lake in Tennessee, is 20 miles long and 
7 miles broad ; it is said that here the fisherman's boat to-day floats 
over the submerged tops of cypress trees. 

Without doubt, the most common cause for lakes is glacial action, 
for wherever glaciers have been there have been two processes in 
operation, as a result of which basins may be produced : {a) irregular 
erosion, (b) irregular deposition. We have already seen how important 
this cause for lakes has been, both in mountain regions where glaciers 
have recently been more extensive, and in areas of former continental 
glaciation. By erosion rock basins have been formed ; by irregular 
deposition dams have been raised across stream courses ; by irregu- 
larities in the moraine, the till sheet, and other glacial deposits basins 
have been made ; and by the ice itself temporary dams have been 
formed, behind which lakes, often of large size, have gathered. There 
are a' multitude of variations in the conditions under which such lake 
basins have been formed ; and very often a combination of two of 
these causes has operated to form a basin. One of the most common 
conditions is that of erosion forming a rock basin and deposition raising 






LAKES AND SWAMPS 317 

the dam higher (Fig. 217), as in Seneca and Cayuga lakes in central 
New York, and in Lake Ontario. In the latter case warping may 
be a third cause cooperating to form the- lake basin. One-twelfth 
of the surface of Sweden is covered by glacial lakes. Finland and 




Fig. 217. — Lake Cayuga, New York, whose basin is due to glacial broadening and 
deepening of a river valley and glacial deposition at one end. 

Canada are similar. In the lake district of northern Wisconsin 15 
per cent of one county, which is five-sixths as large as the state of 
Rhode Island, is occupied by the waters of 346 small glacial lakes. 

Another Classification of Lakes. — Lakes may be classified in other 
ways than the above. They may, for instance, be classified according 
to the cause which produced the dam, and this is a common classifi- 
cation. There are, for instance, (1) lakes due to land movements, or 
diastrophism, (2) lakes due to volcanic action, (3) lakes due to river 
processes, (4) lakes due to wave and tide work, (5) lakes due to wind 
action, (6) lakes due to glacial action. Each of these could be sub- 
divided, but we will not follow classification further. 

Stability of Lake Dams 

The Destruction of Dams. — A lake dam may be of such loose 
material that it is easily washed away by the outlet current, and then 
the lake is lowered, and perhaps drained in a brief interval of time. 
It may happen that the dam is so weak that it is removed abruptly 
enough to precipitate the water of the lake suddenly into the outflow 
stream. Then even though the lake is of no great size, an appalling 
flood rushes down the stream. The bursting of artificial dams, as in 
the Johnstown flood (p. 107), has caused great destruction of life and 
property. A lake five miles long and seven hundred feet deep was 
formed in the upper Ganges in 1893 by an avalanche falling across 
a valley, and one year later the dam gave way and a flood of great 
destructiveness swept down the valley. Such dams are sometimes 
cut through by the streams, sometimes undermined by seepage, or, 
where made of soluble rock, by solution. In building artificial dams 



3 i8 COLLEGE PHYSIOGRAPHY 

care must be taken to avoid the danger of wear at the top, seepage, 
solution, and erosion by pot hole action at the base of the dam. 

Removal of Ice Dams. — Similar floods are caused by the sudden 
drainage of lakes held in by ice dams. This has been illustrated in the 
Alps, where small lakes thus impounded have found an outlet beneath 
the ice, and several instances are known in Alaska. Doubtless as 
the continental glaciers were receding there were numerous instances 
of floods, as the vast ice-dammed lakes fell from one level to another. 

Erosion of Lake Outlets. — Where the dam is of greater stability, 
as where it consists of solid rock, the removal is far slower, and, were 
the destruction of lakes dependent upon the removal of the dam, it 
would be a far slower process than it is. Even though the volume 
of water is large, and the velocity is rapid, a lake outlet has little power 
of erosion, for it has been robbed of its cutting tools by sediment de- 
posit in the quiet lake waters. Thus Niagara has done almost nothing 
toward lowering the level of Lake Erie, except to cut away uncon- 
solidated drift; and the same is true of the St. Lawrence where it 
flows out of Lake Ontario. Here the current divides and subdivides 
among the Thousand Islands, not having cut deeply enough to estab- 
lish a single channel. It is difficult to tell where the lake ends and 
the river begins. This is exceedingly immature drainage. 

The Filling of Lakes 

Rivers the Mortal Enemies of Lakes. — While some lakes are 
destroyed by the removal of the dam, and most lakes are lowered 
somewhat by cutting down at the outlet, it is not this action of the 
outlet stream that led Gilbert to state that " rivers are the mortal 
enemies of lakes." So long as a lake exists it is a temporary baselevel 
below which the inlet and tributary streams cannot cut their beds ; 
it is also the receptacle for the sediment which the inflowing streams 
bear. Given time, even the deepest and largest lake will be exter- 
minated by the deposit of the sediment that the streams bring into it. 

Delta Growth in Lake Cayuga. — The rapidity with which this 
work is progressing is often easily inferred from the visible deposits 
of the inflowing streams. This may be illustrated by a specific case — 
Lake Cayuga in central New York. At the head an inlet and several 
tributary streams have built a delta three miles long, and a mile wide, 
filling the valley at the lake head from side to side, and extending out 
beneath the lake water till it ends in an abrupt slope. Each tribu- 
tary stream that enters the sides of the lake is likewise building a 
delta, and the shoreline, therefore, has numerous projecting points, 
some of the largest being from a quarter to a half mile in the longest 
direction. Since its formation at the close of the Glacial Period, the 
area of Lake Cayuga has been diminished certainly by more than 5 
square miles and perhaps twice that amount as a result of delta growth. 

A larger illustration of the same thing is found in Lake Geneva, 



LAKES AND SWAMPS 319 

Switzerland. The muddy Rhone, fed by glacial streams, has a delta 
20 miles long and built outward a mile in the 1900 years since Roman 
times. At the outlet of Lake Geneva the Rhone is clear, having had 
its sediment strained out in the delta-building of the inlet stream and 
the settling of the finer sediment on the lake bottom. 

Lakes Bisected by Deltas. — Other lakes illustrate the same pro- 
cess, and in various stages, some, very recently formed or with little 
drainage, having but little filling ; others partly filled ; and some 
completely destroyed by filling. An intermediate stage of some in- 
terest is where deltas from opposite sides of a lake grow out toward 
each other and, finally meeting, divide a single lake into two lakes 
connected by a river-like channel. An early stage in this process is 
illustrated a few miles from the head of Lake Cayuga, where two deltas 
have each advanced a quarter of a mile or more toward each other. 
The completed stage is illustrated at the St. Mary Lakes of Glacier 
National Park, at Buttermere and Crummock Water in the English 
Lake District, and at Interlaken in Switzerland, where two lakes — 
Thun and Brienz — have been made by delta division of a former 
single lake. 

Lake-bottom Deposits. — The visible delta is but a part of the 
process of lake filling by stream-borne sediment. Using Lake Cayuga 
again as illustration, after each period of heavy run-off, as when the 
winter snow melts rapidly, the lake water is discoloured with sediment 
far off shore from the stream mouth. Evidently, therefore, sediment 
is finding its way to the lake bottom beyond the delta front. Thus 
the lake is being shallowed, as well as narrowed by delta growth. 
We have no data for estimating the rate of shallowing, and in any 
event it would vary greatly from lake to lake. The steep front of the 
deltas indicates that the delta growth is the more rapid of these two 
causes for lake filling, though it must be remembered that the coarse 
delta deposit is localized, while the finer, suspended sediment is spread 
over a much wider area. 

Other Mechanical Deposits. — Without doubt, stream-borne sedi- 
ment is the chief factor in the filling of most lakes of sufficient size 
to have inlets or permanent tributaries ; but it is not the sole cause. 
Rain wash drags sediment down slopes bordering lakes, as it does down 
other slopes ; and from precipices weathering loosens fragments for 
gravity to pull down. Sediment from these sources is added to the 
accumulations that are filling lakes. The wind is also an agent of 
transportation of sediment to lakes. Another source of sediment is 
the beating of waves against the coast. This may cut the coast back 
and therefore enlarge the area of the lake. This is the case along 
parts of the southern shore of Lake Ontario, where the waves are 
rapidly cutting into drumlins that rise along the shore. But most 
of the material removed finds deposit on the lake bed, and therefore 
the process shallows the lake at the same time that it enlarges its area ; 
and the volume of lake water displaced by deposit greatly exceeds 



32o COLLEGE PHYSIOGRAPHY 

that which spreads over the wave-cut bench, for the waves are con- 
suming land that rises a hundred feet above lake level and depositing 
the debris off shore. 

Organic Accumulations. — The filling of lakes is also aided by or- 
ganisms. There are many shell-building animals and some plants 
that secrete lime or silica. The remains left upon the death of these 
organisms contributes materially toward the deposits that are rilling 
the lakes. They help to fill the lakes by removal of some of the dis- 
solved mineral load brought to the lakes by the rivers and under- 
ground water. In the shallow waters, and especially in the protected 
places, and in small lakes where good-sized waves cannot be formed, 
a luxuriant plant life thrives, including a variety of water-loving species. 
The remains of these plants, protected from rapid oxidation by the 
water, accumulate to form beds of plant remains. 

Transformation to Swamps. — The last stage in the filling of a lake 
is often that of organic deposit, and in some cases where streams bring 
little or no sediment this is the main cause for the extinction of lakes. 
The lake water becomes more and more occupied by growing plants 
and plant remains, finally becoming a swamp ; and this may later 
become high and dry enough for tree growth. Thousands of shallow 
lakes of glacial origin have thus been transformed to swamps in north- 
ern United States, Canada, and Europe. In some of the filled 
lakes occur layers of marl — : calcareous remains of organisms and in- 
fusorial earth — silicious remains, both useful to man ; and in the 
swamps are sometimes beds of bog iron ore, a deposit precipitated 
from percolating water by the influence of decaying vegetation. 
Though used in early days, and a possible reserve for the future, this 
source of iron is now of little use. 

The Brief Life History of Lakes. — Since lakes are the depository 
of the sediment borne by the inflowing streams, their life history is 
necessarily brief, as geological time goes. They are, therefore, to be 
considered as recent phenomena of drainage. The length of time 
required for their extinction varies with the rate of accumulation and 
with the size of the lake. Already many small, shallow lakes formed 
by the continental ice sheet have been filled, and lava-dammed lakes 
near Mt. Shasta have been converted to meadows. Many others, like 
the Great Lakes, have gone but a short way on the road to extinction. 
Even the largest lakes, however, are doomed to ultimate destruction 
by filling, or by removal of the barrier, or by both combined ; and the 
complete filling of even the Great Lakes would be reckoned as a brief 
task from the standpoint of geological time, or from the standpoint 
of the life history of a river valley. Lakes are merely episodes. 
Consequently, there are no lakes at present occupying basins which 
originated in early geological times ; and where lakes exist we may be 
certain that they indicate either a youthful stage of drainage, or an 
accident to existing drainage, or the result of recent aggradation by 
the streams. 



LAKES AND SWAMPS 321 



Removal of Lake Deposits 

The Change in Stream Efficiency. — As long as a lake exists in a 
stream course it serves as a temporary baselevel ; but when the lake 
is filled, or when it is drained by removal of the barriers, the streams 
can then flow across it as over any other land form. No longer being 
robbed of its sediment, the stream can more effectively cut into its 
bed along the course of the former outlet ; and being no longer limited 
in its downcutting by the level of the standing lake water, the stream 
can sink its bed into the lake sediments. It will then proceed to 
remove the sediment burden temporarily deposited in the lake. 

Meanders and Terraces on Lake Bottoms. — The removal of the 
lake sediments may be a very long process, — far longer than re- 
quired to make the deposit. In fact, we have numerous cases of lake 
deposits of earlier geological ages, now transformed to solid rock and 
only partly removed. There are other cases where the streams have 
not yet begun the removal, but flow in meandering course over the 
filled lake. In the process of removal of these sediments the streams 
may develop a series of fine terraces as they swing back and forth, 
during downcutting in the unconsolidated lake deposits. Such ter- 
races are to be seen at Bozeman, Mont. In the removal of lake 
sediments the streams behave as they do in other rock of similar 
texture and position. . 

Salt Lakes 

Desiccation. — The removal of lakes by filling or cutting down of 
the barrier is a common mode of extinction of lakes ; but, under some 
conditions, there is extinction by other processes, as by the recession 
of the dam, as a result of which marginal glacial lakes are sometimes 
exterminated. Another mode of extinction is by evaporation, or 
desiccation. 

In all the continents there are areas where there is too little rainfall 
to fill basins to the point of outflow. There, as we have seen, salt 
lakes necessarily develop as the result of concentration of dissolved 
mineral matter. The regions where such lakes exist are regions of 
interior drainage ; but if the climate becomes more moist, the basins 
may rise to overflow, or, if aridity sets in, the water level may sink 
below the outflow. 

The Great Basin. — This succession of events has occurred in 
various parts of the earth, but the evidence of such changes has been 
most thoroughly worked out for parts of the Great Basin region of 
western United States. The Great Basin has an area of about 200,000 
square miles of interior drainage, but it is not a single basin in any 
other sense than that it is, on the whole, a region lower than the moun- 
tains and plateaus round about. There are upwards of 60 separate 
basins in the Great Basin, each with streams entering it, and each 

Y 



322 



COLLEGE PHYSIOGRAPHY 



without outlet. Some of these are high above sea level, and one, 
Death Valley, lies below sea level. In some of these basins salt lakes 
lie ; in others there is standing water only at intervals. 

The Region near Great Salt Lake. — In one of these basins lies 
the shallow Great Salt Lake, on a broad desert plain, with moun- 
tains rising above it. It has an area of about 2000 square miles, and 
an average depth of about 15 feet. The plain is evidently made of 
lake sediment, though its surface is in places crusted with salt and 
alkali. On the mountain sides is a succession of wave-cut cliffs and 
beaches, with such perfection of form that even the settlers recognized 
them as old shorelines (Fig. 218) before they were so interpreted by 




— Beaches of Lake Bonneville, Oquirrh Range, Utah. (Gilbert.) 



scientific study. It is evident that lake waters have risen to these 
levels ; and an old outflow channel at Red Rock Pass is proof of former 
outflow. This channel, several hundred feet deep and a third of a 
mile wide, was occupied by a large-volumed stream. 

Lake Bonneville. — At the time of overflow this great lake, called 
Lake Bonneville, had an area of 19,750 square miles and a depth at 
the deepest point of 1050 feet. Where the Mormon Temple stands, 
in Salt Lake City, the water was so deep that the temple would be 
under 850 feet of water if the old lake were restored. Not far from 
two hundred thousand people now live on the site of the extinct lake ; 
and there are over 700 miles of railway there (Fig. 219). 

A study of the physical features of the region and of the deposits, 
as interpreted by Gilbert, show that there was (1) an early period 
of long duration in which the climate was more arid than now. Then 
came (2) a period of rise of lake water, but not to the point of over- 



LAKES AND SWAMPS 



323 




Fig. 219. 



Lake Bonneville, with the present Great Salt Lak( 
northeast corner. (Gilbert.) 



itish tint near 



flow. This lasted for a very long time. Following this was (3) a 
shorter period of aridity, then (4) a second rise to the point of over- 
flow, followed by (5) the present period of aridity, the shortest of the 
three, and one that seems to be growing even more arid. During 



324 COLLEGE PHYSIOGRAPHY 

the overflow the lake was fresh water, the present saltness of the Great 
Salt Lake being the result of evaporation during the present period 
of desiccation. The basin in which Great Salt Lake is situated is 
merely a shallow depression in the sediments deposited on the bed of 
ancient Lake Bonneville. It is nowhere over 50 feet deep. 

This basin, however, has been deeply filled during and before the 
Bonneville stage. Borings on the Lucia cut-off of the Union Pacific 
Railway show nearly 800 feet of clay, gypsum, and quicksand near the 
west coast of Great Salt Lake. 

It cannot be exactly stated how long any one of the Bonneville 
stages persisted, though the evidence is clear that each had a duration 
of thousands of years. Nor can the cause for the climatic variations 
be given. There is reason for believing that the changes were accord- 
ant with the glacial and interglacial stages of the Glacial Period, the 
times of aridity coinciding with the interglacial stages, the times of 
humidity and lake rise with the glacial stages. One indication of the 
latter is the presence of moraines, showing that local glaciers extended 
into the lake during the expansion. 

Lake Lahontan and Other Extinct Lakes. — Elsewhere in the Great 
Basin there has been a similar succession of lake rise and fall, notably 
in the case of extinct Lake Lahontan in Nevada. Shore lines around 
parts of interior basins in other continents tell a similar story of cli- 
matic change. Thus the history of lakes in arid regions, as well 
as of glacial phenomena in humid regions, testify to the fact that 
the climate of the earth is subject to notable change. From this 
testimony the conclusion seems warranted that the present is a 
period of relative aridity, as well as a period of relative shrinkage of 
glaciers. 

Deposition in Salt Lakes. — When, through aridity, lakes shrink 
below the rim of the basin, the mineral load becomes more and more 
concentrated, but, with concentration, deposition necessarily follows. 
This is illustrated along the shores of Great Salt Lake, where carbonate 
of lime is being deposited in little rounded grains, called oolitic grains, 
giving the appearance of sand. In the shallow waters of Lake Mono 
calcareous tufa deposits are being made. This is because less car- 
bonate of lime can be carried in solution in salt than in fresh water, 
and the carbonate of lime carried in by the land drainage is precipi- 
tated in the saline water. Gypsum, or sulphate of lime, is also 
precipitated, and even the salt cannot be carried in solution after a 
certain stage of concentration. Accordingly the shores and bottoms 
of very salt lakes may glisten with deposits of gypsum or salt, or 
both. 

Salt lakes that have approached this stage are salter than the ocean 
waters, and their mineral-charged waters are so dense that one cannot 
sink in them. A bather in Great Salt Lake floats of necessity, and on 
emerging from the water a coat of salt crystals covers his body and 
clothes as the water evaporates in the dry air. It is estimated that 



LAKES AND SWAMPS 325 

there are 400,000,000 tons of salt dissolved in the waters of the Great 
Salt Lake, and the production of salt from its waters is an important 
industry, as it is around the shores of other salt lakes. 

Bitter Lakes. — With further concentration the salt lake may be- 
come a " bitter lake," for other salts increase in relative percentage, 
notably the chloride of magnesium. With the increase in amount 
of the latter, common salt is precipitated. Thus in the Dead Sea 
there is nearly twice as much chloride of magnesium as common salt 
(chloride of sodium) ; but in the Great Salt Lake there is about 
eight times as much common salt as chloride of magnesium. As 
evaporation continues, however, more and more of the common salt 
will be deposited, and the water will become more bitter. 

It seems possible that some lakes are fresh merely because they 
are new. Thus the Dead Sea and Great Salt Lake are known to have 
originally been fresh. If we compare the salinity of several lakes — 
(a) Great Salt Lake, 18 per cent, (b) the Dead Sea, 24 per cent, 
(c) Lake Van, 33 per cent, and (d) a saline — it is clear that these 
enclosed seas are progressively becoming Salter. Salt lakes like the 
Caspian Sea, which were originally part of the ocean, do not fall in 
this class. The latter has been freshened by river waters till it is less 
saline than the ocean. 

Formation of Salt and Gypsum. — In arid regions there are dried- 
up salt lakes where deposits of salt, gypsum, and other minerals are 
found. Similar deposits, formed in earlier geological times, are now 
found even in humid regions, stratified with the sedimentary rocks, 
and are an important source of salt, gypsum, and other mineral sub- 
stances of value. 

Playa Lakes. — Basins of salt lakes, even though the water does 
not rise to the point of overflow, are nevertheless being slowly filled. 
The streams that enter the basins leave their sediment in them, 
as we have seen. The winds add other deposits, while precipitation 
of dissolved mineral aids still further in the filling. These processes 
are very often seen where streams, extending out from bordering moun- 
tains, wither at their lower ends by evaporation and by sinking into 
the sediment that they have brought to their alluvial fans. Now and 
then the withered stream is given such a volume that its waters extend 
on to the bottom of the basin, there forming a temporary lake, or, 
as it is called in western United States, a playa lake. When the 
supply of water ceases, the playa lake shrinks and finally disappears 
by evaporation, but its site is marked by alkali deposit where the 
dissolved mineral has been precipitated and later baked and cracked 
in the sun. 

Alkaline Soil. — Alkali flats are unsuited to most kinds of plant 
growth and are, therefore, valueless for agriculture. Salt and alkali 
are sometimes disseminated through the soil of arid regions, and streams 
are even impregnated with them. Such alkaline water, spread out 
upon fields in the process of irrigation, may leave a deposit which is 



326 



COLLEGE PHYSIOGRAPHY 



fatal to agriculture ; and even where the water used in irrigation is 
free from alkali, if there is any in the soil, it may rise to the irrigated 
surface and unfit the land for raising crops. 

The Great Lakes 

The World's Largest Lakes. — The five Great Lakes of the United 
States and Canada form the greatest group of lakes in the world. 
They have had a profound influence on the development of both these 
countries. Their combined area is about 95,000 square miles, or 
more than the total area of the island of Great Britain. Various 
facts concerning these lakes are given in the accompanying table, 
compiled by Vedel. This does not include the other large bodies 
of water, Lakes Winnipeg, Athabasca, Great Slave and Great Bear 
to the northwest outside the St. Lawrence drainage system, although 
the last two are larger than Lake Erie. 









w 


in 








w 


w 
> ^ 


u 




§«« 


Name of 


B 


3 


3 


W 


5§ 


en 

W 

Q 

w 

< 
M 
w 
> 


0H 


Q 

W 
W w 

CO - 

-1 

& 


"I 

H 


p 

U 

w" 


5d 

w w 


III 

< fa W 


Division of 

Great Lakes 

System 


Q 

W 
O 
< 

W 
> 
< 


a 

y, 
< 

3 


W 
Z 

. J 

w 
a 

B 

in 


■.d& 

* 9 
w z 

H < 
< i-l 


s 0" 
S w 

a Q 

a z 

< 


si 

W w 
Q 


>j a 

w 

H 

< 

5 


O B< 

3 a 


S wo- 
0^ - 

W q Q 

Bi S W 

"^ 5 a 

33™ 


Lake Superior 


390 


70 


160 


1300 


31200 


475 


1008 602 


— 406 


2800 


51600 


82800 


St. Mary's River . 


\ 53 
1 40 
335 


»4 


ii 

85 


100 


200 


— 


53 








— 


800 


IOOO 


Lake Michigan 


58 


875 


20200 


335 


870 


58l 


- 289 


1290) 






Green Bay . 


us 


IS 


21 


260 


I700 


95 


i44 


S81 


+ 437 


30 | 


37700 


60100 


Mackinac Strait . 


30 


16 


23 


60 


500 


75 


234 


58i 


+ 347 


7 J 






North Channel . 


no 


12 


18 


220 


I400 


70 


240 


58i 


+ 341 


20 J 






Lake Huron . . 


250 


54 


100 


725 


17400 


210 


702 


58i 


— 121 


650 [ 


31700 


55700 


Georgian Bay . 


120 


40 


58 


390 


5200 


170 


462 


58i 


+ 119 


1 70 J 






St. Clair River 


35 


1 


— 


70 


30 


— 












3800 


3830 


Lake St. Clair . . 


19 


25 


29 


90 


4IO 


— 


21 


575 


+ 554 


1 


3400 


3810 


Detroit River . . 


27 


2 


s4 


54 


60 


— 










■ 


1200 


1260 




Lake Erie . . . 


25 


40 


58 


590 


I OOOO 


70 


204 


573 


+ 369 


130 


22700 


32700 


Niagara River. . 


34 


1 


2 


70 


60 


— 












300 


360 




Lake Ontario . . 


180 


40 


sS 


600 


7300 


300 


7^8 


247 


- 491 


410 


21600 


28900 


St. Lawrence River 


760 


20 


95 
























Totals . . 


1 i 


5 101 


95660 










55o8 


174800 


270460 



Water Content.— The rainfall of the region of the Great Lakes aver- 
ages about 31 inches per year, and this suffices to fill the basins and 
cause a discharge of about 86,000 cubic feet per second from Lake 
Superior, 225,000 cubic feet from Michigan and Huron, 265,000 
cubic feet from Erie, and 300,000 cubic feet per second from Lake 
Ontario. The aggregate discharge is double that of the Ohio River, 
and nearly half that of the Mississippi. 



LAKES AND SWAMPS 327 

Over half of the water in the Great Lakes is in Lake Superior, which 
is both the deepest and by far the largest of the lakes. It has been 
estimated that the amount of water in the Great Lakes is sufficient to 
sustain Niagara Falls in the present condition for about 100 years. 

The Lakes and Rapids. — The lakes are a series of boat-shaped 
basins with their long axes pointing in different directions, so that 
they penetrate a wide area of country and bring it within reach of this 
great, navigable, interior waterway. From the great deep basin of 
Lake Superior, the elevation of whose surface is 602 feet, there is a 
descent to the basins of Lakes Michigan and Huron, whose surfaces 
lie 581 feet above sea level. The main descent is in the rapids of 
Sault Ste. Marie. Huron and Michigan are on the same level, and 
the surface of Erie is but eight feet lower. Between Lake Erie, whose 
surface lies at an elevation of 573 feet, there is a great descent to On- 
tario, whose surface is only 247 feet above the sea, the greater portion 
of this descent occurring at the Niagara cataract. Below Lake Ontario 
the St. Lawrence consists of alternate, lake-like expanses and rapids, 
and the greater part of its total fall of 247 feet is accomplished in a 
few narrow stretches of rapids. The rapids and falls in this water- 
way have been serious obstacles to navigation, though the building 
of canals, especially by the Canadians, has done much to overcome 
the effects of the obstacles. Now large boats may go up the St. Law- 
rence to the western end of Lake Superior by river, canal, and lake ; 
and smaller boats may go from the Hudson to Lakes Ontario and Erie 
by canal. 

The Problem of Origin. — The question of the origin of these basins 
is one of great interest, but one which cannot as yet be answered with 
certainty. There is abundant reason for the conclusion that the lakes 
did not exist before the Glacial Period, and, as has been stated in pre- 
ceding pages, there has apparently been a combination of three causes 
operating to produce the basins : (1) local warping or tilting of the 
earth's crust, (2) glacial erosion, (3) glacial deposit across preexisting 
valleys. The relative value to be placed upon these three causes has 
not been demonstrated, and it is quite probable that a different rela- 
tive value will be found for the different basins. Nor can the relative 
value of the work of the different ice sheets be stated. There are 
buried gorges in the Great Lakes region, and these are more or less 
filled with drift. The mistake has been made of considering all 
such gorges as preglacial stream courses, whereas it is far more prob- 
able that at least some of them are interglacial gorges. 

St. Lawrence System Abnormal. — The St. Lawrence system 
seems to be not a normal stream system, but a composite of parts of 
several. A normal stream system should show a general gradation 
from source to mouth. But the St. Lawrence shows this condition for 
a distance above Montreal; then, above that, there is an absolute lack 
of it. At the Thousand Islands there is no valley, but the river flows 
on the surface of a low, hilly land, drowning the shallow valleys. 



328 COLLEGE PHYSIOGRAPHY 

Above, comes a broad, deep, boat-shaped basin, but there is no con- 
tinuation of this either toward Lake Erie or toward Georgian Bay, 
for buried gorges, probably of interglacial age, are no true continuation 
of such a basin as that of Lake Ontario. A similar statement is true 
of the relation of the other lakes to those above or below. 

The Great Lakes are anomalous forms of drainage, being a series 
of basins of different shapes and depths, connected by straits or rivers 
which are quite out of harmony with the basins in depth and width. 
Moreover, though forming a part of a great river system, the Great 
Lakes have a peculiarly narrow and irregular divide. Sometimes the 
divide comes down almost to the lake shore, and nowhere is it very 
far distant. There is no regular variation, the divide of Lake Ontario, 
for instance, being no farther away from the lake than that of Lake 
Superior. The bottoms of some of the lake basins are far below sea 
level, which is abnormal in a drainage system. 

Theories of Origin. — These anomalies have been generally recog- 
nized by all who have written on the subject, and, in explanation of 
them, one theory has been based upon the attempt to reconstruct the 
preglacial drainage along essentially the line of the present-day 
basins, assigning the anomalies primarily to warping. Another theory 
has been that before the Glacial Period streams flowed in different 
directions, some going to the Mississippi, some possibly to the Arctic 
or toward the east. By glacial erosion, glacial deposition, and warp- 
ing of the land, in relative amounts not determined, basins were 
formed, and when the ice disappeared, drainage from one to the other 
followed along lines quite independent of preglacial drainage, giving 
rise to the present peculiar lake system. That the lakes and their 
drainage were different in form and direction during interglacial time 
is probable, though not proved. 

This is as much as can at present be said upon the basis of existing 
evidence regarding the origin of these basins ; and interpretations that 
pretend to be more exact are misleading. That further careful study 
may make possible a more definite statement of the origin of the 
Great Lakes is to be expected. It will, however, in all probability 
involve four elements : (a) glacial erosion, (b) glacial deposit, (c) warp- 
ing, (d) diversion of preexisting drainage. But what relative weight 
will ultimately be placed upon each of these four elements cannot 
now be safely predicted. Some students of the subject, including the 
author, believe it probable that glacial erosion will be proved to be 
the leading factor. 

Movements of Lake Waters 

Currents near Inlets and Outlet. — Although the lake water has 
been spoken of as standing water, it is not to be considered as actually 
motionless. There is a current opposite the mouth of each inflowing 
stream, and a current must of necessity set toward the outlet ; but 



LAKES AND SWAMPS 329 

out in the central part of a large lake such movements are practically 
negligible. 

Effect of Temperature Changes. — A far more important cause for 
movement of lake water is that resulting from change of temperature, 
and, therefore, of density. As the temperature at the surface descends, 
the water becomes denser, and, being then heavier, it settles, displac- 
ing the water below. Thus a vertical circulation is set up, cold dense 
water sinking, and warm, lighter water rising. In fresh water this 
continues until 30° F. (4 C.) is reached, after which settling ceases, 
because this is the point of greatest density of fresh water. As a 
result of this circulation the temperature of the bottom water of good- 
sized lakes in regions having cold winters is low, even throughout 
the summer. It approaches 39 , though usually it is a few degrees 
above this point. 

Effect of Winds. — The winds are another important cause for a 
circulation of lake waters. They not only set the water into undula- 
tion, forming waves, but, by the friction of the moving air, a drift of 
water is started in the direction toward which the air is moving. 
After a day of steady wind there is a perceptible drift, and it may con- 
tinue for hours after the wind dies down, being noticeable by the slow 
drifting of floating bodies, and by the heaping up of water at one end 
of a long lake (p. 309). 

It has been observed on Lake Cayuga that when strong winds blow 
from the south, even though they are warm winds, the water at the 
southern end of the lake is colder than normal. This is due to the 
fact that the warm surface water is drifted northward, giving 
rise to an increase in the height of the water toward the north and a 
decrease in height at the southern end. The higher column of water 
to the north presses downward and forces a southward flow of the 
lower, colder water to equalize the pressure, and this cold water even 
rises to the surface. Thus the wind causes not only a circulation at 
the surface, but also a movement involving layers below the surface. 
This is probably the explanation of the fact that the bottom waters 
of deep lakes are not at the temperature of the maximum density of 
water. 

Lake Currents. — In very large lakes, like the Great Lakes, a fairly 
definite set of currents is .established, primarily by the wind. This is 
illustrated for the Great Lakes in the accompanying map (Fig. 220). 
The strength of the currents varies, and even their direction is not 
uniform from day to day, but there is an average set of surface waters 
approximately as indicated. Doubtless the circulation is actually 
much more complex than indicated ; and doubtless, also, there is a 
vertical circulation as well as the horizontal movement of the surface 
layers. The general eastward trend of the waters is a result of the 
fact that the wind direction averages from a westerly quadrant. As 
previously noted the drift of water toward the outlet end of a lake 
augments the volume of the outlet stream, and a movement in 



33o 



COLLEGE PHYSIOGRAPHY 




Fig. 220. — The currents in the Great Lakes. (After Harrington.) 



the opposite direction diminishes the volume. Notable fluctua- 
tions in the volume of Niagara River are sometimes caused in this 
way. 

Minute Tides in Lakes. — Minute tides are generated in very large 
lakes and, on quiet days, may be measured by delicate instruments. 
They become especially noticeable when concentrated in narrowing 
bays. A tide of about 3 inches is reported at the southern end of 
Lake Michigan and at the western end of Lake Superior. 

Seiches. — Much more noticeable than this is an irregular rise and 
fall of the lake water known as seiches, studied especially in Lake 
Geneva in Switzerland, but noticeable on most, if not all, large lakes. 
During the seiches the lake waters rise and fall in a rhythmic swing, 
with a movement somewhat like that which can be caused by tipping 
a basin of water back and forth. The phenomenon of seiches is due 
usually, if not always, to decided differences in atmospheric pressure 
on different parts of the lake. The atmosphere exerts a certain press- 
ure on the lake water, which at sea level is approximately 15 pounds 
per square inch. If a storm passes over the lake, the pressure at this 
point may be notably lessened, while all around it remains as it was. 
In that case the surface of the lake will rise beneath the area of low 
pressure ; or, if it be a high-pressure area, the lake surface will be 



LAKES AND SWAMPS 331 

lowered by the extra pressure of the air. This disturbance of the lake 
level is in the nature of a wave, and it may sweep across the lake from 
end to end, and even traverse the lake back and forth for several 
times before dying out. 

Lake Shores 

Likeness to Ocean Coasts. — In their more general features lake 
and ocean shores are so nearly alike that they may be discussed 
together. Both, for example, are modified by wave attack, with the 
resulting development of wave-cut cliffs and wave-built beaches of 
various forms. Both are influenced by currents, and in both cases 
there is a rise and fall of the water surface, though in lakes this is 
less rhythmic and less important than the tidal rise and fall of many 
oceanic coasts. Naturally the resemblances become greatest in the 
shorelines of large lakes, and a study of such a lake shore gives ample 
basis for an interpretation of many of the fundamental elements of 
ocean shore features. Therefore, in the succeeding chapter (p. 342), 
where shorelines are treated specifically, both lake and ocean shore- 
lines are included. 

Contrasts with Ocean Coasts. — There are, however, some direc- 
tions in which lake shorelines differ from ocean coast lines to a 
greater or less degree. Ordinarily lake shorelines are less intensively 
developed, for the agents are less intense ; but there are many bays 
along the ocean in which there is far less intense work than along the 
coast of the Great Lakes. Tidal currents, locally important along 
ocean shores, find no equivalent in lakes. 

The work of organisms is wholly different in the two bodies of water. 
There are, for instance, no coral reefs along lake shores, and the effect 
of fresh-water plants is quite different from that of salt-water plants. 
Very often the shores of small lakes and ponds are completely under 
the domination of plant growth. By different species of plants this 
condition is imitated in protected bays along ocean coasts though with 
different kinds of plants. 

Larger Deltas in Lakes. — Lake shores are commonly the seat of 
more pronounced deposition than ocean shores, for the sediment 
poured into them is less widely distributed. For this reason delta 
points project from lake shorelines opposite the stream mouths far 
more commonly than on the ocean coast, and their form is not the 
same. Here again, however, this is less true of the very large lakes, 
and there is a close resemblance between lakes and ocean bays in this 
respect. 

Ice Ramparts and Boulder Pavements. — The work of ice along 
lake margins is different from that along most ocean coasts, though 
resembling that of the frigid zones and of bays that freeze over in the 
cold temperate regions. The effect of ice along the shores of ponds is 
often very pronounced. By it ridges of gravel are often piled up along 



33* 



COLLEGE PHYSIOGRAPHY 



the coast, forming ice ramparts (Fig. 221), and boulders are shoved 
slowly up the beach, often forming a boulder pavement. 

Some of this ice work is performed by the ice hurled against the 
coast by waves, but much of it is the direct result of the powerful shove 







i ^ k 










Fig. 221. — Ice ramparts in Lake 
Mendota, Wisconsin. (After Buck- 
Icy.) 

Since the process is repeated 
movement may eventually 



of the ice cover of the frozen pond or 
lake. As the ice is formed it expands, 
and a lateral thrust may result. 
When the temperature descends, the 
ice contracts, and cracks open in 
which freezing takes place, exerting a 
further lateral thrust. If then the 
temperature rises, the ice expands and 
other thrust is applied. By these 
thrusts sufficient force is applied to 
push along even good-sized boulders, 
winter after winter, a considerable 
result. 

Lake Shores more like Ocean Bays. — From this it is clear that 
lake shorelines bear a close resemblance to ocean shorelines, but that 
there is closer resemblance between the shores of ocean bays and 
lakes. Only in relatively unimportant details are there differences. 
That this is true is indicated by the fact that the shoreline of the 
extinct glacial lake south of Lake Ontario and the one in the Lake 
Superior basin has been interpreted as an ocean shoreline by at least 
one observer. 

Importance of Lakes 

The Water covers Arable Land. — Whether lakes really return 
more to man in the various uses to which they are put than would 
be returned if their area were dry land, upon which farming and other 
industries could be carried on, is an academic question, and one upon 
which no certain answer can be given. Doubtless the answer would 
be different in different cases. In Sweden, for instance, where one- 
twelfth of the surface is lake, this is undoubtedly true, for many of 
the lakes are now of little service. The site of a large lake at Ragunda, 



LAKES AND SWAMPS 333 

accidentally drained in 1796 by the diversion of the outlet, has become 
the seat of many valuable farms, and, without doubt, the products 
of these farms far exceed the value of the lake. Whether the drainage 
of the Great Lakes, even though they cover an area of 95,000 square 
miles of possibly arable land, would be an economic advantage is 
exceedingly doubtful. 

Man's Uses of Lakes. — Lakes are of service to man in numerous 
ways. (1) They store water useful for (a) regulating stream volume; 
(b) supplying water for city drinking supply, as at Chicago ; (c) supply- 
ing water for factories ; (d) furnishing water for irrigation. (2) They 
are an important source of food fish. (3) From the surfaces of lakes 
ice is cut for many uses. (4) As resorts, for health and pleasure, lakes 
possess a high value to mankind. They may even be sought for 
protection, as by the ancient lake dwellers of Venezuela. (5) The 
larger lakes are highways of navigation, especially well illustrated in 
the case of the Great Lakes, one of the world's busiest highways, and 
so important that they have been a large factor in the location and 
growth of a number of large cities, — Buffalo, Cleveland, Detroit, 
Chicago, Milwaukee, Duluth, Superior, and others. (6) Lakes 
exert a powerful influence upon local climate. 

Influence on Climate. — The influence of lakes upon local climate 
is illustrated even by small lakes, which cool the air in summer and 
warm it in winter if their surface is not frozen over. Where lakes are 
numerous, as in Sweden or Finland, the large expanse of lake water 
must exert a very notable influence, both upon the temperature and 
the humidity of the air. Large lakes are still more important. This 
is well illustrated by the Great Lakes, from whose vast expanse the 
winds must receive much vapour, and must have their temperature 
greatly modified. It is due to the influence on temperature that fruit 
raising is such an important industry in the neighbourhood of some of 
the Great Lakes. On the peninsula of Ontario, for instance, between 
Lakes Ontario and Erie on the one side and Lake Huron on the other, 
grapes, peaches, and other fruits, and even tobacco, are extensively 
raised. The peninsula of Michigan, between Lakes Huron and Michi- 
gan, is a noted fruit region, but the west shore of Lake Michigan in 
Wisconsin is less favourable because the prevailing winds are toward 
the lake, not from it. The shores of both Lakes Erie and Ontario 
are favourable to fruit raising. 

In the grape district along the southern shore of Lake Erie the grow- 
ing season is lengthened by the effect of the lake waters in retarding 
the cooling of the air during times of frost. Thus late frosts in spring 
and early frosts in autumn are less common near the lake than at a 
distance from it. In lake valleys, such as those of Seneca and Keuka 
lakes, the air temperature is greatly modified by the water, and grape 
raising is an important industry. This influence is often clearly illus- 
trated in the neighbouring Lake Cayuga valley, when the snows com- 
pletely melt from the lower valley slopes while the hills are still white ; 



334 



COLLEGE PHYSIOGRAPHY 



and farmers, coming from the upland to the valley in sleighs, find bare 
ground when they are only two or three hundred feet above the city 
in the valley bottom. 

Lakes as Barriers. — Great lakes are often barriers, resulting in the 
necessity of man making detours in road and railway building, al- 
though the lake may be used as a highway of commerce by steamer, 

freight by water being cheaper. 




The railways are forced to swing 
far southward through Chicago, 
because Lakes Erie and Michigan 
make a direct line from Buffalo 
to St. Paul impossible. Long, 
narrow lakes, however, may be 
bridged. Car ferries are oper- 
ated on Lake Michigan between 
Wisconsin and Michigan. Lake 
Baikal on the Trans-Siberian 
Railway was crossed by a line 
laid over the ice in winter and 
by car ferry in summer before 
the expensive line around its south end was built. Great Salt Lake 
was crossed by a line, the Lucin cut-off, built on piling and filling for 
over 25 miles in this broad, shallow lake to avoid the long, crooked, 
original line around the north end of the lake (Fig. 222). 



ENGRAVING CO., 



Fig 



-The railway crossing Great Salt 
Lake at Lucin cut-off. 



Extent and Value of Swamps 

Nature of Swamps. — A swamp is a part of the surface of the land 
which is wet and saturated with moisture, though not usually covered 
with standing water. Some swamps are called marshes, bogs, or 
muskegs. At least a part of the Arctic tundra is also swamp, especially 
in summer when the frozen surface soil thaws. There is every gra- 
dation between swamps and lakes and between swampy and dry 
land. In this intermediate class are areas which are swampy only 
during a part of the year ; and many swamps are covered by a thin 
sheet of standing water during seasons of heavy rain or rapidly melt- 
ing snows. 

Uses to Man. — Swamps are of great importance to man, for they 
cover enormous areas, and, until drained, are of little value. It is 
estimated that there are in the United States 79 million acres of swamp 
and marsh land, equal to the combined areas of the states of Ohio, 
Indiana, and Illinois, or of Great Britain and Ireland. This is largely 
waste land in its present state, though some of it has timber growth, 
some is utilized for rice or cranberry culture, and some is occupied by 
cattle for a part of the year. When drained, however, such land 
often makes excellent farm land, for the surface is level, and the abun- 
dant humus in the swamp deposit favours the growth of many crops. 






LAKES AND SWAMPS 335 

Drainage of Swamps. — Some swamp land is readily drained by 
simple means, such as the use of tiles for underground drainage, and 
that method has been employed extensively in the reclamation of the 
swampy prairie areas of the Central States. Here the fertile soil owes 
its black colour to the swamp vegetation that nourished before the 
artificial drainage. In some cases only the removal of vegetation is 
necessary to drain a swamp, for the dense growth of grass or forest 
may so interfere with the run-off of water from level land as to make it 
swampy land, at least for a part of the year. 

Reclaimed Swamps in Europe. — Other swamps can be drained 
only by means of ditches, and sometimes by a complex and expensive 
system of ditches and drainage canals. This has been much done in 
Europe, where for many generations the land has been so fully occupied 
that it has paid to reclaim waste land, even at the expense of much 
labour. There are tracts of such reclaimed swamp land in England; a 
part of Holland is reclaimed noodplain swamp ; and there are exten- 
sive tracts of reclaimed swamp land in Germany. An instance of 
the latter is in the Spreewold district, not far from Berlin, where a 
swampy tract along the Spree River has been reclaimed by the digging 
of a multitude of canals and ditches, and the quaint people who dwell 
there go to their fields, to church, and to school, not over the land, 
but by boat along the drainage canals. 

The Swamp Resource in United States. — In the United States 
little has so far been done toward drainage of the more expensive type, 
for as yet land has been plentiful and cheap. But the time is at 
hand when the reclamation of some of these waste areas must be 
undertaken. This is important not alone from the standpoint of the 
increase in the area of farm land, but also from the standpoint of 
health. The damp swamp lands are unhealthful, and often the 
breeding place of mosquitoes, which spread malaria well beyond the 
limits of the swamp. Along the swampy bottom lands of the Arkan- 
sas and lower Mississippi rivers, for example, the people are cursed 
with " fever and ague " ; and many a town in the north also suffers 
from malaria as a direct result of the close neighbourhood of mosquito- 
breeding swamp tracts. 

Much fertile land can be added to the farm acreage of the United 
States by adequate drainage and protection from overflow, and a vast 
saving be made in human life also. It has been estimated that 77 
million acres of the swamps in the United States can be drained at 
nominal cost, resulting in an increase in value of 2849 million dollars. 

Kinds of Swamps 

Influence of Levelness and Impervious Soil. — The cause of swamps 
is the inability of water to run off or percolate into the ground rapidly 
enough to drain the land. This implies levelness of surface and either 
a state of permanent saturation of the ground or such a degree of 



336 COLLEGE PHYSIOGRAPHY 

imperviousness as to interfere with rapid percolation. Thus a sandy 
area of level ground will not become a swamp as readily as a level clay 
area ; but even a sandy tract of level land below the water table will 
become a swamp. 

Swamps on Coastal Plains. — Any cause which will produce a level 
surface introduces the prime condition necessary to swamp develop- 
ment when water is added. Such a cause is the uplifting of a level sea 
bottom, forming a coastal plain, and this accounts for the great de- 
velopment of swamps along the southern coast of the United States, 
from Virginia to Texas, including the Everglades of Florida (5000 
square miles) and the Dismal Swamp of Virginia and North Carolina. 
There are thousands of square miles of swamp land on this coastal 
plain, mostly now waste land, but much of it capable of drainage. 

Swamps in River Valleys. — Another important cause for level 
land is river deposit, forming floodplains and deltas ; also, by natural 
levees, or by deposit of silt at river mouths, making obstacles to the 
run-off of water. Consequently rivers on floodplains and deltas are 
commonly bordered by swamps (PL III) . It is estimated that along 
the lower Mississippi there is a tract of land subject to overflow 
equal in area to the entire state of South Carolina or 30,000 square 
miles. 

Influence of Vegetation. — Swampiness depends upon the rates of 
(a) run-off, (b) percolation, (c) evaporation, and (d) on the volume of 
water supplied. Given a certain volume, the rate of run-off is governed 
by the slope and the amount of vegetation. The influence of vege- 
tation in checking run-off is very great, and many swamps are due to 
this influence alone, while the area of others is enlarged by it. This 
is true, for instance, of Dismal Swamp in Virginia and North Carolina, 
and doubtless of many other of the coastal plain swamps. It is one 
of the reasons for swampiness in the tropical zone, where heat and 
dampness encourage luxuriant plant growth. With the development 
of the swampy condition, plant growth is encouraged by the dampness, 
and hence swamp development is still further aided. Reeds, cane, and 
other plants thrive, and even trees adapt themselves to growth in 
persistently wet lands. Notable among these are the Cyprus and 
black gum, the former sending projections from their roots upward 
above the swamp level in order to insure the necessary air, the latter 
having arches in the roots which accomplish the same purpose. 

Percolation is governed by the porosity of the soil and the level of 
the water table. Evaporation varies with the dryness of the air and 
the amount of wind to remove the vapour. A vegetation cover checks 
evaporation and in that way also encourages swampiness ; but it 
operates in the opposite direction by removal of water to build into 
the plant tissue, and to give out to the air by transpiration. Water 
volume depends solely upon the rainfall in many swamps, but in river 
swamps it is partly, or even largely, supplied by river flow. This 
cause greatly increases the area of permanent swamps along rivers, 



LAKES AND SWAMPS 337 

and even further increases the area of temporary swampiness which 
succeeds each overflow until run-off, evaporation, and percolation 
can dry out the wet soil. 

Swamps in Arid Lands. — Arid lands and deserts are not notable 
for swamp areas, for although there is level land, there is light rain- 
fall, rapid percolation, rapid evaporation, and such an absence of vege- 
tation that run-off is little checked by that cause. Swamps in such 
regions are mainly confined to the river courses and to the evaporated 
lakes at the terminus of intermittent streams. Alkali flats and salinas 
are a desert form of swamp. 

Swamps in the Tropics. — Tropical countries of heavy rainfall are 
especially favourable to swamp development, for there is abundant 
water supply, the water table is high in so damp a climate, and per- 
colation is, therefore, reduced. The air is so humid that evaporation 
is at the minimum, and vegetation growth is so luxuriant that the 
interference with run-off is at the maximum. This swampiness • is 
one of the prime reasons why malaria and other diseases make living 
in the tropical zone so hazardous. 

Swamps due to Glaciation. — Glaciated regions are the seat of 
innumerable swamps, partly because level tracts were made by glacial 
deposit, and partly because shallow lakes of glacial origin have been 
partially or completely filled since formation. In the course of lake 
filling, as we have seen, vegetation is of much importance in the final 
stages, and ultimately the site of the lake becomes a swampy plain. 

Swamps in Cool, Northern Lands. — In the cool, northern climate 
the assemblage of plants growing in the shallow lakes and swamps is 
different from that of the warmer, southern regions. Among the 
plants there is one of such dominant importance as to call for special 
mention, namely, the sphagnum moss. It grows luxuriantly in northern 
Europe and United States, and in Canada, and takes an active part 
in the late stages of lake filling, forming sphagnum bogs. 

Quaking Bogs. — Sphagnum will grow outward, even on the sur- 
face of shallow ponds, and sometimes cover the surface, while beneath 
is a miry liquid, 

part water, part ^ _ „ a 

decaying vegeta- 
tion (Fig. 223). 
Walking upon 

Such a surface ^ IG " 223 ' — Cross-section of a lake being filled by sphagnum moss, 
. - 1 bb, and muck from its decay, cdc, converting the lake into a 

results m a Shak- quaking bog. (Shaler.) 

ing like jelly, 

giving rise to the name quaking bog. From the bogs of Ireland and 
other regions the bodies of men and animals are sometimes ex- 
cavated, showing the danger that may result in trusting to such an 
unstable surface. The acids of the decaying vegetation have a pre- 
servative effect, and such remains are often in a remarkable state of 
preservation. 




338 COLLEGE PHYSIOGRAPHY 

Climbing and Bursting Bogs. — Sphagnum is sponge-like, and it is 
able to grow even on slopes, taking up and retaining water. In damp 
climates, like Ireland, it may grow even on hillslopes, forming climbing 
bogs, and similar bogs occur on slopes in the United States where 
springs, emerging from the hillside, supply the necessary water. In 
Ireland they grow to such size and on such slopes that, becoming 
charged with water, they sometimes slide down the slope. Such 
bursting bogs sometimes destroy both life and property. 

Relation of Swamps to Formation of Coal. — In Ireland, Scotland, 
Scandinavia, north Germany, and other parts of northern Europe, the 







'JSg****-; ...... ^ 

! 



Fig. 224. — An Irish bog, where peat is being excavated for fuel. 

sphagnum bogs are an important source of fuel for local use (Fig. 224) ; 
in North America, although there are hundreds of square miles of 
sphagnum bog, it is as yet practically unused. In the sphagnum bogs, 
and in the swamps of the more southern regions, such as Florida, we 
see a first stage in coal formation, though with entirely different plant 
assemblages. The coal swamps probably developed on level coastal 
plains, the vegetation grew luxuriantly, and extensive deposits of 
plant remains accumulated, protected from decay by the dampness, 
and, as in Florida, there was little admixture of sediment. Then came 
submergence and deposit of sediment, and the layer of plant remains 
became incorporated in the strata and started on the slow series of 
changes by which it changed to the mineral coal. Submergence of 
Florida, or of Ireland, beneath the sea would carry the swamp deposits 
one step farther toward the stage of mineral coal, and after the 
lapse of sufficient time they would become seams of coal, bedded 
between other kinds of rocks, just as is the case in the coal beds now 
mined. 



LAKES AND SWAMPS 339 

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pp. 189-235; Lake Lahontan, Monograph 11, U. S. Geol. Survey, 1885, 

288 pp.; Mono Lake Region, 8th Ann. Rept., U. S. Geol. Survey, Part 1, 

1889, PP- 267-319. 
E. H. Sellards. The Florida Lakes and Lake Basins, .id Ann. Rept., Florida 

Geol. Survey, 1910, pp. 47-76. 



34o COLLEGE PHYSIOGRAPHY 

N. S. Shaler. On the Origin of the Excavated Lake Basins of New England, 
Proc. Bost. Soc. Nat. Hist., Vol. 10, 1866, pp. 358-366; Sea Coast Swamps 
of the Eastern United States, 6th Ann. Rept., U. S. Geol. Survey, 1885, 
PP- 353 - 3o8 ; General Account of the Fresh Water Morasses of the United 
States, 10th Ann. Rept., U. S. Geol. Survey, Part 1, 1890, pp. 255-339; 
Beaches and Tidal Marshes of the Atlantic Coast, National Geographic 
Monographs, New York, 1896, pp. 137-168. 

R. S. Tarr. Physical Geography of New York State, Chapter VI, New York, 
1902. 

J. B. Tyrrell. The Genesis of Lake Agassiz, Journ. Geol., Vol. 4, 1896, 
pp. 811-815. 

A. C. Veatch. Formation and Destruction of Lakes of Red River Valley, 
Prof. Paper 46, U. S. Geol. Survey, 1906, pp. 15, 60-66. 

P. Vedel. Facts about the Great Lakes, Amer. Geol., Vol. 18, 1896, p. 196. 

T. L. Watson. Lakes with More than One Outlet, Amer. Geol., Vol. 19, 
1897, pp. 267-270. 

TOPOGRAPHIC MAPS 

Crater Lakes 
Lassen Peak, Cal. Ashland, Oreg. Crater Lake Special 

Delta and Coastal Plain Lakes 
Point a. la Hache, La. Norfolk Special Salton Sink, Cal. 

Glacial Lake Plains 
Casselton, N.D. Fargo, N.D. Tower, N.D. 

Finger Lakes 

Watkins, N.Y. Skaneateles, N.Y. Ovid, N.Y 

See also maps showing forms below, as well as above, lake level in Birge and 
Juday's paper, Bull. U. S. Bureau of Fisheries, Vol. 32, 191 2, Washington, 1914. 

Glacial Lakes and Swamps 

Becket, Mass. Monadnock, N.H. Ashby, Minn. 

Mt. Lyell, Cal. Minneapolis, Minn. Madison, Wis. 

Oconomowoc, Wis. Newcomb, N.Y. Ann Arbor, Mich. 

Hydrographic maps of 12 Wisconsin lakes, by L. S. Smith, Wis. Geol. Survey; 
see also the 29 maps in Juday's Bull. 27, Wis. Geol. Survey, 1914. 

Great Lakes 

Charts of the whole lake, of parts, and of harbours, for example : Lake 
Superior, S., Sf., Sf.8, etc., and similar charts of the other Great Lakes — Survey 
of the Northern and Northwestern Lakes, U. S. War Dept., Detroit, Mich., 
or Buffalo, N.Y. 

Ox-bow Lakes 

Miss. R. Commission Charts, 1 : 20,000, Nos. 23, 24, 52, 55, etc. 
Butler, Mo. Junction City, Kan. Elk Pt., S.D. 

St. Louis, Mo. Marshall, Mo. Millikin, La. 

Alkali Flats and Playas 
Sierraville, Cal. Van Horn, Tex. Disaster, Nev. 



LAKES AND SWAMPS 



34i 



Glassboro, N.J. 



Donaldsonville, La. 



Pulaski, N.Y. 



Glassboro, N.J. 



San Francisco, Cal. 
New Haven, Conn. 
Brooklyn, N.Y. 



Coastal Plain Swamps 
Hempstead, N.Y. 

Delta Swamps 
Point a la Hache, La. 

Lake Swamps 
Plattsburg, N.Y. 

River Swamps 
Marysville, Cal. 



Salt Marshes 

New London, Conn. 
Atlantic City, N.J. 
Boston Bay, Mass. 



Norfolk Special 



Oswego, N.Y. 
Minneapolis, Minn. 



Norfolk Special 

Stonington, Conn. 

New York City and Vicinity 



CHAPTER XI 

SHORELINES 

Factors Involved 

The Ever Changing Coast. — The contact between the sea and the 
land, and to a lesser degree between lake water and land, is a zone of 
active change, and, as a result of such change, topographic forms of 
great diversity have been caused. The nature of the larger elements 
of a coast is dependent upon the factors discussed below. 

Rock Structure and Attitude. — Along the coast, as on the land 
back from the coast, there is great variety among the rocks, from the 
standpoint both of position and condition ; and, as these are attacked 
by agents of denudation, the resulting topographic form of the coast 
varies with the nature of the coast line rock. 

Crustal Movements. — Crustal movements are readily noticeable 
along the sea coast, for the sea level is a delicate register of even 
slight change. These movements consist of (a) elevation, (b) depres- 
sion, (c) mountain growth. 

Activity of Agents of Land Denudation. — The agents of denudation 
on the land — weathering, wind, rain wash, rivers, and glaciers — 
affect the coast line, either by erosion, or by deposition, or by both 
combined. 

Activity of Organisms. — In the ocean itself there are organisms, 
both plant and animal, which either aid or retard erosion, and which, 
by their abundant growth, aid in coast line deposit. 

Erosive Agents of the Sea. — There are also movements of the water 
which work effectively in erosion, transportation, and deposition. 
These movements are (a) waves, (b) tides, (c) currents, the waves 
being far the most important. By the waves the land contact is being 
incessantly attacked, and a large part of the detail of shore form is 
the result of this attack, either by a direct cutting or by deposit of the 
rock fragments removed. But it is not wave work alone, for all the 
other factors mentioned above are in operation, and the coast form is 
the outcome of the complex interaction and interrelation of a number 
of these. The nature of this complexity of processes, activities, and 
conditions will appear as the subject is developed. 

Effect of Submergence 

Relation to Topography. — Either a lowering of the land, or a rise 
of the water level, drowns a portion of the land. The new coast line 

342 



SHORELINES 343 

will be a horizontal line, traced at the contact of sea and land. If the 
land is perfectly level, this line will be straight ; but if the land surface 
is irregular, the new coast line will be sinuous ; and, since most land 
has been subjected to denudation, a sinuous coast line will ordinarily 
result from depression. The degree of sinuosity will vary with the 
degree of dissection of the sunken land ; but, in all cases, the water 
will extend up valleys, forming bays or harbours, while divides between 
valleys will project, forming points, capes, or peninsulas. Hills com- 
pletely submerged will form shoals, and hills partly submerged may be 
entirely surrounded by water, forming islands, separated from other 
islands or from the mainland by straits. 

The Submergence of Rugged Land. — If the sunken land is rugged, 
the bays will be long, the promontories high, the coast line bold and 
irregular, and the depths of the water over the sunken land extremely 
variable. Since rugged lands are commonly underlaid by consolidated 
rock, the coast line will, in all probability, consist of resistant rock, 
perhaps varying greatly in kind and position from place 'to place. 

The Submergence of Plains. — If the sunken land is a plain, or 
only gently undulating, the bays will be small, the promontories 
low, the water off shore shallow, and the variations in depths only 
moderate in amount. Such a coast line may be fairly regular, or it 
may be very irregular, according to the topography of the sunken 
land ; but it cannot be a bold coast. Whether it is also a rock coast, 
or is one of unconsolidated material, will depend upon the nature of 
the submerged land ; but unconsolidated strata form a great majority 
of such coast lines in the world. 

Coast of Norway. — Northwestern Europe and northeastern North 
America have a coast line whose major features seem to be due to 
sinking of the land. In Europe the Scandinavian peninsula is the 
higher part of a mountainous land, partly submerged. Off the western 
coast are many shoals, on which food fish live in great abundance, so 
that the fishing banks are an important source of food. The coast 
itself is exceedingly irregular, with a maze of rocky islands, mountain- 
walled fiords, and passageways between the islands and the mainland. 
It is one of the grand scenic spots of the world, and each summer the 
coast of Norway is visited by a stream of tourists. For centuries it 
has been the home of hardy mariners, trained to a sea-faring life by 
the forbidding nature of the land itself, by the invitation offered by 
the quiet waters of the fiords and sounds, and by the supply of food 
which the water contains. 

Coast of Scotland. — A similar coast is found in Scotland, but 
submerged valleys separate the British Isles from one another and 
from the mainland. The North Sea and the Baltic Sea are shallow 
bodies of water, spread over a submerged plain, a part of which rises 
above the sea in southern England and along the coast of the mainland 
of Europe. Here the coast is much less irregular, and far less bold ; 
in fact, for much of the distance it rises almost imperceptibly out of 



344 COLLEGE PHYSIOGRAPHY 

the sea, and shallow water extends far off shore. The river mouths 
are broad bays or estuaries, with low-lying shores, contrasting strik- 
ingly with the rugged, irregular, rock-bound coast of Scotland and 
Norway. Where the level of the plain was higher, as in Denmark, 
low islands and peninsulas rise above the sea. The coast of the low, 
hilly land of Sweden and Finland is intermediate in form between 
that of the level plain and the rugged mountainous land. Here are 
found a maze of small rock islands and promontories, and partly 
enclosed bodies of water ; but, in spite of the irregularity, the relief 
is slight. 

Northeast Coast of North America. — Eastern North America illus- 
trates the same condition. Labrador, Nova Scotia, Hudson Bay, 
Gulf of St. Lawrence, Newfoundland, the Grand Banks of Newfound- 
land, and the islands of the Arctic are all the result of the subsidence 
of an irregular mountainous land. The coast is prevailingly rock- 
bound, it is notably irregular both in general features and in detail, 
with a multitude of fiords, bays, straits, islands, and promontories, 
and it is, on the whole, a rugged and bold coast. 

Coast South of New York. — South of New York the coast is still 
irregular, for the latest movement has been downward ; but here the 
land is a plain. Throughout, this coast is of unconsolidated strata, 
and the river mouths are all drowned, so that the tide enters into them 
and in places transforms them to estuaries or bays. These arms of 
the sea increase in breadth and depth toward the north, partly because 
the land surface was more irregular there, and partly because subsi- 
dence was greater. Accordingly, there are numerous, broad, shallow 
bays, with low-lying coast, such as Delaware Bay, Chesapeake Bay 
and its branches, Mobile Bay, and Galveston Bay. 

Between the partly drowned southern plains and the sunken 
mountainous lands of the north is an intermediate area of low, hilly 
land, also partly drowned, resembling somewhat closely the coast of 
Sweden. Thus from New York to New Brunswick there is a low, 
prevailingly rock-bound coast, with a multitude of islands, promon- 
tories, bays, harbours, and straits (PL X). 

Other Submerged Coasts. — Similar drowned coasts are found in 
other parts of the world : in northwestern United States, on the 
Dinaric coast of the eastern shore of the Adriatic, and in many other 
places. Drowned coasts are also common in lakes, whose waters are 
often forced to rise over an irregular land whose topography was 
developed before the lake came into existence. The eastern end of 
Lake Ontario at the Thousand Islands, and the islands of Georgian 
Bay in Lake Huron, are illustrations ; and the bays along the south 
shore of Lake Ontario and other of the Great Lakes are also instances 
of the drowning of land by the rise of lake water. Many irregular 
lakes, such as abound in Maine, Canada, Finland, and Sweden, owe 
their irregular shoreline to the fact that the lake waters are spread 
over an irregular land surface. 



SHORELINES 345 



Effect of Elevation 

Relation to Sea Bottom. — Uplift brings the sea bottom into the 
air ; and the coast line has a form dependent upon the outline of the 
sea bottom. This outline may be irregular, as it would be, for example, 
if there should be an uplift along the northeastern coast of New Eng- 
land ; but more commonly it would be regular, for the deposit of sedi- 
ment in the sea tends to level the bottom. Thus in time the sea 
bottom off New England, whose irregular form is due to the fact that 
it is a recently submerged, hilly land, will become smoothed over by 
sediment deposit. Because of sediment deposit the greater portion 
of the ocean floor is a plain ; and it is, moreover, made of unconsoli- 
dated rock. 

By the uplift of such a sea bottom a straight coast line is established, 
and the land rises gently out of the sea, while shallow water exists off 
the coast. At the coast line itself the waves come in contact with 
unconsolidated sediments, and these also lie off shore on the shallow 
sea bottom, and form the land back of the coast. Such a coast is 
difficult to approach because of the shoal water, and there are few 
harbours in which a vessel can anchor, though there may be some 
indentations where there were depressions in the sea bottom. 

Illustrations from North and South America. — From New York 
southward to Central America there is such an uplifted sea bottom, 
though subsequent to its uplift there has been slight subsidence, as 
noted above. In general, therefore, it fits the case fairly well, though 
there are more irregularities than normal, because of later, slight sink- 
ing, especially toward the north. On the whole, the coast is straight, 
the sea is shallow off shore, there are few good harbours, and the sea 
bottom, the land, and the shore line are all unconsolidated rock. The 
peninsulas of Florida and Yucatan are higher portions of the sea floor, 
the cause for which is not certainly known. A similar uplifted coast 
is found in eastern Argentina, and there are strips of upraised sea 
bottom, forming narrow coastal plains with straight shorelines, 
along the coast of Africa and other continents. Very often such up- 
lifted coasts are only local and connected with mountain growth. 

Effect of Mountain Growth 

Uplifted Mountain Coasts. — Mountain uplift, either by folding, 
or by faulting, sometimes occurs along the sea coast, as in western 
South America. This gives rise to a fairly regular coast line, with 
few harbours, capes, and peninsulas (PI. VI). Back of the coast the 
mountains rise steeply, and the sea bottom slopes rapidly away 
from the continent. Thus, west of South America the sea bottom 
lies 15,000 to 20,000 feet below sea level a short distance off shore; 
and in a distance of 75 miles there is a difference in elevation of 40,000 
feet between the sea bottom and the lofty Andean peaks. The coast 



346 COLLEGE PHYSIOGRAPHY 

ranges of western United States give rise to a similar, though less 
regular, coast line. During the mountain uplift, narrow strips of sea 
bottom have been raised, so that there is often a belt of coastal plain 
between the mountains and the sea, but the coast is essentially a 
straight mountain coast. Because of the few harbours, the narrow 
strip of level land, and the lofty mountains, cutting off communica- 
tion farther inland, such a coast is not suited to dense population 
and high development of industries. 

Mountain Ranges in the Sea. — Elsewhere mountain ranges rise 
out of the sea, their crests forming chains of islands, such as the 
West Indies, the East Indies, New Zealand, and the Japanese and 
Philippine Islands. Such islands are usually elongated in the direc- 
tion of the mountain chain ; but volcanic eruption often gives rise to 
roughly circular islands in such mountain chains ; and sometimes the 
volcanic peaks are the only portions of the mountain chain that rise 
above the sea. This is especially true of mountains rising above the 
floor of the deep ocean far from land, as in the Hawaiian Islands, and 
many others in the open Pacific and Indian oceans. 

Seas between Continents and Off-lying Islands. — When moun- 
tains rise off the mainland coast the island chains to which they give 
rise often partly enclose arms of the sea, such as the Caribbean Sea, 
the Gulf of Mexico, Japan Sea, and China Sea. And since mountain 
uplift is usually, if not always, accompanied by neighbouring sinking 
of the land, the beds of such enclosed seas are often very deep as a 
result of subsidence. 

Mountain Peninsulas. — Mountains often extend as spurs from the 
mainland out into the sea, thus forming peninsulas, as do the Coast 
Ranges in lower California, the Alaskan Range in the Alaska Peninsula, 
the Atlas Mountains in Tunis, the Apennines (Fig. 225), and the Balkan 
Mountains in the Balkan Peninsula. Bays, such as the Gulf of Cali- 
fornia, the Adriatic Sea, and others are formed in this way, often owing 
a part of their depth, however, to subsidence. 

Plateau Peninsulas. — Great crustal movements, such as plateau 
and mountain uplift, give rise to large peninsulas, and subsidence 
forms great seas and gulfs. In such ways were formed the peninsulas 
of Indo-China, India, Arabia, and Spain, with their associated seas 
and gulfs. 

Mediterranean Seas. — The Mediterranean occupies a sunken 
portion of the earth's crust between the mountains of Europe and 
Africa. The Caribbean Sea and Gulf of Mexico are of similar origin. 
The bed of the Mediterranean Sea of Europe and Africa lies over 14,000 
feet below the level of the sea in places. It is almost divided in two 
where the mountains of Italy and of northern Africa approach each 
other, and are connected by a submarine ridge. The mountains 
almost come together again at the Straits of Gibraltar. Its coast is 
very irregular, owing to the projection of mountainous peninsulas, 
and mountainous islands and volcanic peaks rise above its surface. 



SHORELINES 



347 




348 COLLEGE PHYSIOGRAPHY 

Some of the islands and mountainous coasts are parts of the earth's 
crust that have not sunk below sea level, others are raised by moun- 
tain uplift. Along the coast of Italy are strips of coastal plain, with 
straight coast line uplifted above the sea ; and in Greece, and on the 
eastern shore of the Adriatic, there are drowned coasts, where sinking 
of the land has admitted the sea into the mountainous valleys. Here, 
as in other mountain regions, the adjustment of the earth's crust is 
not complete, and subsidence and uplift are still in progress in places. 

Complexity of Crustal Movements 

Simple Coasts Rare. — In some coasts the recent changes, by which 
the present coast line has been determined, have been rather simple. 
Such is the case, for example, along the mountain coast of western 
America, and in the sunken coasts of northwestern Europe and north- 
eastern North America. But even in such cases there are evidences 
of more than one movement. For example, in western America, while 
the main movement has been upward, there has been recent down sink- 
ing at the northern and southern ends and still more local subsidence 
in places, as at San Francisco Bay. The sunken coast of northwestern 
Europe and northeastern America has risen somewhat since its greatest 
subsidence, and beaches, wave-cut cliffs, and marine clays are found 
on the land, well above sea level. It has already been stated that the 
uplift of the coastal plain of southern United States was followed by 
a slight subsidence. 

Complex History the Rule. — Some coasts have had an exceedingly 
complex history, and there are notable differences from place to place. 
This is well illustrated in the Mediterranean, where there are straight 
mountain coasts, irregular mountain coasts due to uplift, irregular, 
drowned mountain coasts, uplifted coastal plains, volcanic coasts, and 
coasts where uplift and subsidence have succeeded one another. 

Present Coast Lines Unstable. — Whether the sea coast of any 
considerable part of the earth has ever stood for long periods at one 
level in relation to the sea cannot be stated ; but at present the coast 
line is one of great instability, having risen or been depressed, or both, 
within very recent periods ; and many coast lines are known, even 
now, to be rising or sinking. Ever so slight a change of level swings 
the zone of wave work up or down, and only a moderate change is 
necessary to completely alter the form and condition of the sea coast. 

The Effect of Land Agents of Erosion 

Effect of Weathering. — Weathering operates upon the sea coast, 
as it does on all other exposed land surfaces, and is an effective aid to 
wave work in supplying rock fragments, for use as tools and for deposit 
in the sea. Three factors tend to make weathering active along the 
shoreline strip, (i) There is much steep rock slope, where wave 



Plate VI 






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COAST OF CALIFORNIA 

/Tap to show the harbor of Los Angeles, wave-cut cliffs, sandy deposits near shore (brown stipple), kelp 
at greater depths (blue pattern), and sand or mud in various depths of water. Contour interval 20 
feet. Elevations on the land in feet; depths within brown stipple in feet; other depths in 
fathoms. (After chart of Santa Monica Bay, No. 5144, United States Coast and Geodetic Survey.) 



SHORELINES 349 

attack is active. (2) Vegetation cover is absent or sparse on many 
rock coasts to which the salt spray reaches. (3) The rocks are fre- 
quently wet, thus aiding rock disintegration, and the salt water is 
more favourable to mineral change than rain water, owing to its chemical 
composition. But neither weathering nor rain wash give rise to any 
notable shore forms, or changes different from those of the land back 
of the shore. 

Effect of Wind Work. — The work of the wind along coasts has 
already been discussed (pp. 59-64), and will need no further consider- 
ation than incidental mention in connection with the coastal forms 
upon which it is especially active. It may perhaps be stated that the 
wind drives sediment into the sea, and that, upon desert coasts, this 
may be a very important aid in the supply of sediment to the waves 
and currents. 

Effect of Rivers. — Rivers contribute far more sediment than the 
wind does, and at times much more than the waves and currents can 
dispose of. This is one of the reasons for the presence of sand bars 
along some coasts, and it has already been shown that it is the explana- 
tion of deltas at river mouths. Where waves and currents are least 
effective, as in lakes and bays, delta deposits are most common. In 
bays, as in lakes, the tendency of the inflowing streams is to fill them 
with sediment, both by the growth of deltas and by the deposit of 
sediment upon the bottom of the bays. Many coasts have been 
partly straightened by the filling of bays, and many others have allu- 
vial flats at the bay heads or bay margins. 

Illustrations from Italy, California, and Elsewhere. — This is true 
in Italy, for example, where the Po has filled a broad valley, and the 
river sediment forms a plain along the Adriatic coast far to the south. 
In Greece the heads of many bays are alluvial flats, and there are 
similar river-filled bays and bay heads in Asia Minor, the Persian Gulf, 
western United States, and many other places. An excellent illus- 
tration of this is at the mouth of the Colorado River at the head of the 
Gulf of California. Here the head of the gulf is a broad alluvial 
flat, really the Colorado delta, bordered by mountains. The growth 
of this delta has cut off the upper part of the gulf, leaving a basin whose 
bed is 300 feet below sea level at the lowest point. The climate is 
so arid that this basin is not filled with water, though a shallow lake, 
Salton Sea, now occupies a small part of it. Now and then, as in 
1905, the Colorado sends some of its water down into this basin, caus- 
ing Salton Sea to rise and doing much damage to the irrigated lands 
in this low-lying area. 

Estuaries and Allied Forms. — Rivers affect coast lines also by the 
formation of valleys, into which the ocean water may enter when 
there is subsidence of the land, forming bays, estuaries, and harbours. 
Even the river mouth itself, without subsidence, may make a harbour, 
for the river water scours out a channel slightly below sea level. Some 
of the larger rivers are navigable by ocean-going vessels, as the Missis- 



35° 



COLLEGE PHYSIOGRAPHY 



sippi is as far as New Orleans, ioo miles from the river mouth. The 
mouths of smaller streams are apt to be too shallow, or are too much 
obstructed by sand bars for use as harbours, unless, by subsidence, a 
broader opening is made. 

Fiords Mainly Produced by Glacial Erosion. — It is only recently 
that it has become generally recognized that glaciers have had a truly 




Fig. 226. — A fiord on the coast of Norway. 



important share in the shaping of some coast lines. Glaciers are able 
to erode their beds even well below sea level, for their ends will not 
float until a depth of 600 to 800 feet is reached in an ice front 100 feet 
high, and 1200 to 1400 feet in the case of the much commoner glacier 
terminus, which is 200 feet or more in height. Thus erosive power can 
extend that far at least. Back from the glacier terminus it may extend 
even lower, for glaciers do not need an even grade for their beds, but 
are capable of eroding basins so long as an adequate surface grade is 
maintained. The evidence is now deemed by most glacialists to be 
conclusive that glaciers cut deeply into their beds, lowering them 
1000 feet, 1500 feet, or even more in favourable situations. 

Among the places where there is clearest evidence of such erosion 
are the horded coasts, as Norway (Fig. 226), Greenland, Alaska, British 
Columbia, Patagonia, and New Zealand. Here the evidence indicates 
that valleys whose beds were above sea level before the Glacial Period 
were eroded below sea level by glacial sculpture alone, during the ice 
occupation. When the ice disappeared, the sea flooded the valleys and 



SHORELINES 



35i 



the fiord coast came into existence. In Alaska it has been shown that 
submerged hanging valleys are common (Fig. 227). Fiords (PI. V), 
with their deep waters, steeply rising walls, and hanging valleys above 
and below sea level do not require for their explanation anything 




GRAVWA 



Fig. 227. — An Alaskan fiord and submerged hanging valleys. 

further than this, — though in some cases there is independent evi- 
dence that there has been slight subsidence in addition. The fiord 
characteristics are mainly the result of glacial erosion, and the subsi- 
dence, if present, has been merely an incident of secondary importance. 



The Agents of Erosion along Coasts 

Waves, tides, and currents, singly or combined, are ceaselessly at 
work along the margin of the land, modifying the coast line. The sea 
coast, especially that exposed to the vigorous waves of the open ocean, 
is the seat of some of the most active changes on the earth. Coast 
lines are worn back, or built forward, as the case may be, at so rapid 
a rate that their effects became noticeable in some cases in the course 
of only a few years, while during the centuries of historic time striking 
alterations in the coast line have taken place. 



352 



COLLEGE PHYSIOGRAPHY 



Wind Waves. — Of the oceanic agencies the wind waves are by far 
the most important. They are generated by the friction of the mov- 
ing air upon the mobile water surface, as we may easily illustrate by 
blowing upon a basin of water. With steady strong winds blowing 
over a great expanse of water, waves of large dimensions may be 
generated. The discussion of the nature of wind waves, and the pro- 
duction of ground swell, white caps, and wind drift currents, is, for 
the present, postponed (see Chap. XXI). 

Breakers, Surf, and Alongshore Currents. — In the open ocean 
wind waves are important to navigation, being a source of danger 




-A» DIRECTION OF WAVE MOVEMENT „ 



Fig. 228. — Diagram of a wave approaching shore and forming surf, through interference 
of the bottom with the motion of the water particles. (See Fig. 412.) 

to small or weak craft ; but the ocean is so deep that they produce no 
effect upon the solid earth. As they approach the land, however, all 
this is changed, and even the condition of the wave is altered. Near 
the coast, the motion of the particles of water (Fig. 228) is interfered 




Fig. 229. — Powerful waves breaking on the coast of New England. 

with by friction along the bottom, the wave becomes steep-sided 
toward the land, and finally topples over, and a huge volume of water 
rushes with resistless force up on the shore. Such a wave is a breaker, 



SHORELINES 



353 



and a succession of them forms surf. When waves break diagonally 
on a coast, they set up a movement of the water known as the 
alongshore current. 

Wave Erosion. — The breakers exert enormous force as they 
strike blow after blow against the coast (Fig. 229). Thus it is 
recorded that a 300 pound bell, 100 feet above high water mark, was 
wrenched off by the waves on the west coast of England. Breakwaters 
are torn to pieces, and stones ten to fifty tons in weight are moved 
about by the waves. In its powerful action the breaker works (1) by 







Fig. 230. — Beach of fine sand at Atlantic City, N.J., with very moderate surf. 

its mechanical force as it rushes along, (2) by alternate compression 
and expansion of air in crevices in the rock, (3) by hydraulic pressure 
as the water is driven into the crevices, (4) by hurling rock against 
rock, that is, by using rock fragments as tools. 

It is by such attack, repeated at intervals of a few minutes, some- 
times with great violence, sometimes with less vigour, but rarely quiet 
for any length of time, that exposed coasts, even though made of the 
hardest of rocks, are being worn away at rapid rate. In this attack the 
rate varies (a) with the exposure, (b) with the kind of rock. Weak rocks, 
or rocks with abundant joint planes, fall ready prey to the waves, but 
none are exempt. Combined with the mechanical work of the waves 
is some chemical action and weathering ; and modifying the rate of work 
is the influence of animal and plant life, and, on some coasts, of ice. 



354 COLLEGE PHYSIOGRAPHY 

Disposal of Wave-eroded Material. — The materials obtained by the 
waves, or turned over to them from the land, must be disposed of, or 
else the debris will accumulate. There are six processes by which the 
materials are disposed of. (i) Some is dissolved by the sea water and 
therefore removed in solution. (2) Much is ground up by the constant 
beating of the waves, and thus reduced to such fine form that it is 
readily removed in suspension (Fig. 230). (3) A very large portion, 
both large and small fragments, is driven along the coast by the diagonal 
approach of the waves. A wave that strikes the coast at exact right 
angles to its course will rise and fall without any lateral component ; 
but, owing to the irregularity of the coast and to the fact that waves 
will only occasionally approach exactly normal to the coast, the waves 
commonly strike the coast diagonally, and there is thus a tendency to 
push rock fragments along the coast in one direction or the other, 
according to the direction from which the waves most frequently come. 
(4) There is a similar tendency for the smaller fragments to drift 
along the coast in the wind drift current, which, upon reaching the 
coast, is deflected along it. (5) An outward movement of water 
along the bottom, the undertow (Fig. 228), occurs along wave-beaten 
coasts, and, in this, rock fragments, especially those of small size, 
are moved away from the coast. (6) Tidal and other currents remove 
fine-grained rock fragments. 

The Tidal Currents 

Twice each day the ocean surface, in most places, rises and falls, as a 
result of the tidal wave which the pull of sun and moon generates in the 
hydrosphere (Chap. XXI). Throughout most of the ocean the move- 
ment is unnoticeable, and, even on most coasts, it is a factor of slight 
importance in modifying the shore current, as when it passes through 
narrow straits, or is otherwise influenced by irregularities of the coast 
line or of the sea bottom. 

Ordinarily the tidal currents move so slowly that they can do no 
more than transport the finest sediment, but locally they attain suffi- 
cient velocity to move sand. Along some narrow channels, they are 
effective agents, not merely of transportation, but of scouring as well. 
It is, however, in transportation and deposition that the tidal currents 
are most effective in modifying the shorelines. 

Ocean Currents 

In addition to the wind drift and tidal currents, there are larger 
movements of ocean water, flowing with a fair degree of permanency. 
Ocean currents are important in modifying temperature, and in influ- 
encing the distribution and abundance of marine organisms; but 
excepting as they encourage the work of organisms they are only of 
minor importance in shoreline development. Undoubtedly they aid 



SHORELINES 



355 



in the distribution of sediment ; but, if they scour at all, it is only 
locally and mainly on the bottom offshore. 

Work of Erosive Agents 

Upon all coasts the agents of erosion are at work, with a vigour that 
varies from place to place, and with a result that is also widely vari- 
able, according to conditions. Whether the initial form be that of 
a drowned coast, or a mountain coast, or an uplifted sea bottom, the 
waves and currents are active, and change is in progress. Roughly 
we may divide the result of this activity into two categories, (i) de- 
structional work, (2) constructural work ; but it must be borne in 
mind that the two grade into one another, and overlap. 

Destructional Work 

Formation of Sea Cliffs. — In destructional work the waves are the 
main agents. The zone of most vigorous wave attack extends through 
but a few feet vertically, and along this narrow horizontal zone the 




Fig. 231. — Wave-cut cliff on Lake Superior, near Marquett 



rock is planed away. The tendency is to undercut the coast along this 
horizontal plane, and in some cases this is actually accomplished ; 
but by weathering, by the fall of rock under the pull of gravity, and 
by the attack of waves at levels above the zone of greatest activity, 
there is usually such active removal of the rock, that the overhanging 
condition is not common. The result is a precipitous sea cliff (Fig. 231). 
It may be 600 to 1000 feet high, though sea cliffs over one or two 
hundred feet high are not common. It is sometimes vertical, or over- 



356 



COLLEGE PHYSIOGRAPHY 



hanging, but usually some 
degrees less than vertical. 
The angle of slope varies 
with the rock and the vigour 
of the waves, being steepest 
in resistant massive rocks, 
and where the waves are 
most active. 

In unconsolidated strata 
the sea cliff commonly re- 
mains approximately at the 
angle of rest of unconsoli- 
dated material, from 30 to 
35 , and there is little vege- 
tation, since there is frequent 
sliding of the cliff face as 
the waves move material 
from its base. A similar 
slope may be maintained in 
consolidated rock where the 
wave attack is slow, or where 
the rock is much jointed, and, 
therefore, easily weathered 
away. Such sea cliffs are 
characteristic features of 
headlands or other exposed 
parts of the coast, but they 
can be present only where 
the waves are able to remove 
the material that comes to 
them. If they fail in this, 
they then cease their attack 
on the cliff and it wastes 
away. 

The rate of recession of 
sea cliffs in unconsolidated 
strata is indicated by Shaler's 
computation that the Nash- 
aquitsa Cliffs on Martha's 
Vineyard Island receded 220 
feet from 1846 to 1886, or 
about 5J feet a year, and by 
Roorbach's studies of drum- 
lins in Boston harbour, 

Fig. 232. — Block diagrams to show (1) a moun- 
tainous region of resistant rock ; (2) the bays, 

peninsulas, and islands resulting from sinking of the land ; (3) the cliffs and spits result- 
ing from wave work ; (4) the features revealed by subsequent elevation. For the life 
history of a coast in weak rock see Fig. 246. 




SHORELINES 357 

where a cliff retreated 9 inches a year from i860 to 1908. On the 
coast of England the average annual rate is said to be from 9 to 
20 times this latter amount. 

Where the waves have planed far back into the land, or where they 
are attacking a highland coast, the cliff is high; on the other extreme, 
where the relief of the land is low, or the extent of cutting has been 
slight, the sea cliff may be but a few feet in height. The irregularity 
of the cliff is caused by (1) irregularities in the land (Fig. 232), 
(2) variations in wave force, (3) differences in character of the rock 
(Fig. 246), (4) the effect of weathering. 

Much depends upon the nature of the rock. If it is massive, the 
cliff approaches greatest regularity, but if it is jointed, or varies in 
resistance, due to stratification ,or other causes, the cliff may become 
quite irregular, and the irregularities vary according to the inclina- 
tion of the layers as well as to their resistance. A sea cliff in horizontal 
strata has a notably different form from one in vertical strata ; there 
is a different form produced when the strata dip toward the sea than 
that in strata dipping toward the land ; and the degree of inclination 
adds further cause for variation in cliff form. An analysis of sea cliff 
form, therefore, would need to include a complex series of factors. 
A majority of sea cliffs are sloping rather than vertical or overhanging, 
from which it may be inferred that wave attack is less effective than 
the work of subaerial agents, though the relationships of joints, of 
bedding, and of the influence of gravity must needs be considered in 
connection with each individual cliff. 

Spouting Horns or Blow Holes. — Among the irregularities are 
some sufficiently characteristic and common to be given names. One 
of these is the spouting horn, or blow hole, a place where, when the 
waves break, either air or water is forced out of a cavity in the rock, 
perhaps at some distance from the place where the wave is breaking. 
It is due to the presence of an opening, often along a joint plane, into 
which the wave enters, forcing air or water out of the other end of the 
cavity either by compressing air in the cavity or by passing through 
it. Sometimes the water spouts, fountain-like, with the incoming 
of each large wave ; at other times the air is alternately sucked in or 
forced out as the wave recedes or advances. 

Sea Caves. — ■ Where the rock varies in resistance, or the direction 
of wave attack favours, the cliffs are locally undercut, forming pockets 
or arches, called sea caves (Figs. 233, 234). Once these are started, the 
swirling wave may tend to enlarge them, much as the falling water of 
rivers gives rise to pot holes. Recession may even proceed so far as 
to develop natural bridges beneath which the waves rush. Sea 
caves may develop in any rock, but they are most common in lime- 
stone, doubtless partly because of its softness and solubility in the 
ocean water, but very often as a result of the discovery by the 
waves of subterraneous caverns, previously developed by land 
drainage. 



358 



COLLEGE PHYSIOGRAPHY 



Chasms. — Vertical weakness in the rock leads to the excavation 
of chasms, or narrow indentations. These may be due to the presence 
of a soft, or soluble, or jointed layer. They occur most commonly in 
vertical sedimentary layers, or along narrow dikes of igneous rock. 
When the indentation is begun, the wave attack increases in vigour, 
because of the increased intensity of the breaking wave when thus 
directed. This leads to a rapid extension of the chasm; but this 





^ Mm^ JH 


jt jtJ^~^j|juj|5BiB 









Fig. 233. 



Sea cave of elevated shoreline on coast of California. 
Survey.) 



(Arnold, U. S. Geol. 



extension is limited, for soon such a length is reached that the wave 
wears itself out by friction against the sides and bottom. Then the 
headward extension of the chasm must await either the cutting back 
of the headland, at the chasm mouth, or the widening of the chasm 
mouth so as to admit a greater volume of water. The latter action 
may result in the production of a small bay or cove. 

Large bays cannot result from wave work, for if there is, for any 
reason, a concentration of wave activity on a part of the coast, or if 
there is an area of weak rock, the indentation that would naturally 
result soon becomes a place where the waves lose force by friction. 
After a certain size is reached, there comes a balance in which the 
slackened wave attack at the bay head no longer exceeds the rate of 
attack at the headland. Thus a cliffed coast line is commonly 
sinuous, with shallow indentations and slightly projecting headlands ; 



SHORELINES 



359 



but if there are large bays, or harbours, we may be certain that they 
are due to other causes than wave erosion. 

Stacks. — As the coast wears back, certain parts, especially at the 
headlands, are temporarily left unconsumed, forming small rock 



















..lA'.'Si 


■-■■■": ?B. 


v.,::—; :;,:a^ iV :^, 


■ 


1 


:'-}J^~M^-l 




y^-P. 




mm 

lit it 




*% 


lis. 

















Fig. 234. — A stack and a wave-cut arch in a sea cliff on the coast of France. 



islands, or stacks, or skerries, often conspicuous and striking features 
of the coast, and sometimes pierced by sea caves, or partly divided 
by chasms. Such remnants of the worn-back coast may be due to 
some peculiarity of the rock, or 
to some deflection of the wave 
attack ; but their duration can- 
not ordinarily be long in the 
face of the oceanic forces oper- 
ating round about them (Fig. 
234). 

Offshore Benches. — As the 
waves plane back the coast 
line, they leave a shallow 
of shore bench or shelf (Fig. 235). 
Slowly the water becomes deeper 
on the offshore bench, for the 

waves and currents gradually wear the rock away ; if they did not, 
the bench would in time become so broad an area of shallow water 
that the waves would wear themselves out in passing over it, and 




A sea cliff and rock bench. 
(Gilbert.) 



360 COLLEGE PHYSIOGRAPHY 

reach the shore without power to further cut the sea cliff back. Even 
when the average depth of water on the offshore bench has become 
deep enough so that the waves do not break in passing over it, there 
may be a shoal near, and hidden reefs, not yet consumed. 

Dangers to Navigation. — Because of the offshore bench, approach 
to a cliffed coast is dangerous for vessels, and many a wreck has 
occurred upon it. Very often the disaster has been complete, for 
the storm waves in such a place are commonly high, owing to the influ- 
ence of the shallow water. The vessel receives the full force of the 
breakers, launching small boats is difficult or impossible, and the cliff- 
bound shore offers no safe haven for them, even if they are success- 
fully launched. 

Constructive Work 

Formation of Beaches. — A sea cliff base is commonly strewn with 
rock fragments, wrenched loose by the waves, or fallen from the cliff 
(Fig. 236). These are used by the waves as tools for further attack 
against the land, and, as they are washed about, they are ground down 
and the fragments either carried offshore or driven along the coast. In 
places of especially active supply, or in indentations where wave force 
is diminished, these fragments may accumulate, forming beaches. The 
accumulation in the indentation is encouraged by the driving of 
fragments along the coast by the diagonal wash of the waves. 

The material in the beach varies with the source of supply, both in 
kind of work, and in size of fragment. Upon surf-beaten coasts the 
beach may be of boulders or of large cobblestones ; where the supply 
is extensive, even on exposed coasts, the beach may be of pebbles; 
or it may be of sand. Even with moderate supply, sand and pebble 
beaches commonly develop in indentations or in other situations 
where the waves are not of great size. 

Pocket Beaches and Crescent Beaches. — Along rock-bound coasts, 
beaches of boulders or pebbles are commonly formed at the head of 
minor indentations, into which they have been driven by the waves. 
Small patches of this nature are called pocket beaches, and large ones 
often form crescent beaches. Such beaches are really mills, in which 
the rock fragments are ground down to such a state of fineness as to 
permit their removal from the pocket in which they have become 
lodged. As the surf rolls up and down on the beach, the pebbles, 
cobbles, and even boulders are rolled back and forth, soon assuming 
a rounded form, and rapidly diminishing in size. Were it not for 
this, the indentations would become filled with fragments wrested 
from the headlands, and then the headlands themselves would be 
littered by a protective sheet of rock fragments, thus putting an effec- 
tive check upon the recession of the shoreline. 

The crescentic form of beaches is characteristic of indentations, for 
it is the result of a tendency toward equilibrium between supply and 
removal. If waves of a given average velocity enter the indentation 



SHORELINES 



361 




Fig. 236. — Beach gravels at base of sea cliff, Hinchinbrook Island, Alaska. (White.) 



362 



COLLEGE PHYSIOGRAPHY 



with a straight crest, they begin to bend, owing to the less rapid 
motion along the margins of the indentations where retarded by 
friction, and they reach the head of the indentation with greater 
vigour in the centre than on the sides. As a result, the beach deposit 
has a similar curved, or crescentic form. The size of the crescent 
depends upon the size of the indentation ; the extent to which the 
curve develops depends upon (a) the force of the waves, (b) the depth 
of water in the indentation, (c) the amount of material supplied to the 
beach. When in perfect equilibrium, a wave from the open ocean will 
break upon all parts of the crescent beach at the same instant. 

Movement of Material Along Shore. — On a very irregular coast, 
such as a drowned coast, the exposed headlands are commonly cliffed 
by wave attack ; but the indentations are in many cases, and perhaps 
in most, quite free from the attack of the open ocean waves, though 
modified to some extent by waves that develop within their own 
confines. The rock fragments wrested from the headlands are in 
part driven along the coast by waves and alongshore currents, and, 
coming to the opening of the indentation, are driven into it. How 
far in they may be driven, will depend upon (1) the size of the frag- 
ments, (2) the abundance of the material, (3) the force of the waves, 




Fig. 237. — Head of a bay in Lake Mendota closed by a barrier bar and converted into a 
bog. A second bar has subsequently been built up part way to lake level. (Wisconsin 
Geological Survey.) 



(4) the size, shape, and depth of the indentation, and (5) the nature 
of the currents, tidal or otherwise. 

Whatever the relative value of these several factors, there will 
come a place in the rather slightly disturbed water of the indenta- 
tion where much or all of this material will come to rest. Some will 
lodge upon the shore, some upon the bottom, and some may go far 



SHORELINES 



363 



into the indentation. But the greatest part will come to rest at the 
point where the average transporting power of wave wash or current 
is so checked that it can no longer move the average-sized fragments. 



>M 






.:v^nv... 
J§K -; ai 



B fli\ 



^ 



s '%f 








Fig. 238. — Four stages in the cutting of cliffs, the destruction of drumlins, and the build- 
ing of beaches which tie islands to the mainland at Nantasket, Mass. (Johnson.) 

This point is usually not far from the mouth of the indentation, but 
slightly within the mouth. 

Bays Closed by Bars. — It is as a result of this process that the 
mouths of bays and other indentations on an irregular coast are being 



364 COLLEGE PHYSIOGRAPHY 

closed by deposit. In some cases, bay mouths are completely shut 
in by bars of sand, or pebbles, or boulders ; in others the process is 
only partially completed. On some coasts, especially sandy coasts 
where waves and currents have abundant material, the bars across 
bays are a serious obstacle to navigation, and much expense is required 
to keep a ship channel open. Most good-sized bays cannot be com- 
pletely closed to tidal currents, and the outflow of water from the land 
must maintain an opening ; but such an opening may be shifted both 
in position and depth. 

Tied Islands. — Rock fragments wrested by the waves from the 
shores of an island are driven along the coast, and often come to rest 
in the quieter water in the lee of the island. There a bar is built, 
which ultimately may rise above high tide level. If the island is not 
far from another island, or from the mainland, the bar may extend 
to the neighbouring land, tying the island to it. Such tied islands 
abound along drowned coasts, as in Maine. Sometimes the island 
is tied by a single bar, perhaps partly submerged, sometimes by two 
bars, one from either side of the island, enclosing a lagoon between ; 
with the filling of the lagoon the connecting bar becomes broad and 
the island forms the tip of a peninsula, whose rock is made of marine 
sediment, — sand, pebbles, or boulders. Gibraltar is an instance of 
such a tied island, forming a peninsula projecting from the Spanish 
mainland, at first as a broad, low, flat bar and at the end rising abruptly 
as a rocky hill. 

Irregular coast lines abound in instances of tied and partly tied 
islands, and of bars across bays or other indentations in various stages 
of development (Figs. 237, 238). 

Spits and Cuspate Forelands. — At times conditions exist as a 
result of which sediment is driven outward from the coast in such 
amount as to form a sand or gravel point, called a spit. At a bend 
in the coast, for example, wave wash from opposite directions may be 
so nearly in balance as to cause deposit at the bend, and then the coast 
grows outward. This outward growth may give rise to a pointed spit, 
or to a rounded point, or to two bars meeting either in a point or in 
a curve, forming a cuspate foreland (Fig. 239). These shoreline fea- 
tures may also develop as a result of the action of currents ; and they 
may form where there is excessive sediment supply from the land, 
as on delta margins. They are an expression of the activity of the 
waves or currents in disposing of the sediment load consigned to them. 

Small spits develop in many lakes, as at Crowbar Point in Cayuga 
Lake ; in indentations of the coast line as in the so-called Bras d'Or 
Lakes of Cape Breton Island (Fig. 240) ; and also along sand coasts. 
Cuspate forelands are also found in the Bras d'Or Lakes ; and Capes 
Hatteras, Fear, and Canaveral on the eastern coast of the United 
States are large instances on an exposed ocean coast. 

Offshore Bars. — From headlands supplying abundant sediment, 
or from river mouths which pour much sediment into the sea or lake, 



SHORELINES 



3^5 



bars often extend in either direction, or in only one if the waves or 
currents come from a single direction. This is well illustrated on the 
New Jersey coast, where, from the cliffs of unconsolidated deposits, 
the waves are receiving so great a supply of sediment that the surplus 
is driven along the coast, both southward and northward, forming 
of shore bars. The one extending northward is called Sandy Hook, 



Currituck C.H.. 




5 W 1'5 20 35 



B.D.S«ri>oj». .ST. 7. 



Fig. 239. — Cuspate forelands of Capes Hatteras and Lookout. 



and it has grown part way across the mouth of New York harbour. 
Much expense in dredging is necessary to maintain a passageway across 
the submarine extension of this deposit, and the ship channel winds 
in a devious course across it. 

A similar condition exists along the eastern shore of Cape Cod. 
The waves are eating back into the unconsolidated deposits at and 
near Highland Light, and the debris is driven both northward and 



366 COLLEGE PHYSIOGRAPHY 

southward. That to the north forms the rounded end of the cape. 
That to the south extends out under water, supplying sand to the 
shifting Nantucket Shoals, one of the most dangerous parts of our 
eastern coast. 

By the growth of such bars, bay mouths are enclosed and lagoons 
are formed between the mainland and the coast. On the land side 
of these lagoons an old sea cliff may sometimes be seen, as in New 
Jersey, formed when the waves beat against the mainland, before the 
protecting bar was built offshore. The bars are the seat of steady 
wave work and change, for the waves and currents are engaged in the 
task of perfecting them ; and if this were the only work, the bar would 
be continuous. As a matter of fact it is broken, sometimes into linear 
islands, sometimes merely by the gaps where the land water escapes 
from the enclosed bays or lagoons. In the latter case waves and 
currents work steadily to completely seal the bay ; while the inflowing 
and outflowing currents work to maintain a passageway across the 
bar. As a result of these opposing tendencies, the form of the bar, 
and the form and depth of the breach in it, are subject to notable 
change. This is well illustrated along the New Jersey coast, where 



Fig. 240. — A hooked spit or hook in one of the Bras d'Or Lakes in Nova Scotia. 

the sites of houses and hotels of a few decades ago are now occupied 
by an inlet ; while the former site of inlets is now occupied by bars. 
Hooks. — One result of the conflict between the advance of sedi- 
ment in a bar or spit, and the currents is the turning, or curving of 
the end, forming what is called a hook, such as Sandy Hook and the 



SHORELINES 



367 



hooked end of Cape Cod. Similar hooks are common in lakes, and 
at the ends of bars partly enclosing bays. The hook form is the result 
of the inability of the transporting agent to drive forward the end of 





Fig. 241. — The beach at Rockaway, Long Island, where a hooked spit advanced west- 
ward over three miles between 1835 and 1908. (Putnam.) 



the spit or bar as fast as cross currents or waves can push the sedi- 
ment in another direction. The tendency to curve the end of the bar 
may be present all the time during the bar .growth ; but it becomes 



3 68 



COLLEGE PHYSIOGRAPHY 



effective in actually developing the hook form (a) when the distance 
of the bar end from the source of supply becomes so great that the 
cross currents or waves can dominate, or (b) when the size of fragment 
diminishes to a size which the cross currents can more easily handle, 




Fig. 242. — Shoreline changes at the Haulover, Nantucket Island. (Putnam.) 



or (c) when, by the constriction of an inlet, the cross current is given 
sufficient velocity to dominate in sediment movement (Figs. 241, 242). 
Barrier Beaches and Lagoons. — Some low-lying coasts are bor- 
dered by fairly continuous offshore sand bars or barrier beaches, or 
sand reefs. This is true, for instance, of a large proportion of the coast 
south of New York, and is particularly well illustrated along the Texas 
coast, where there is a continuous sand bar from the mouth of the Rio 
Grande fully 100 miles northeastward (Fig. 243) ; beyond this the bar 
continues, though broken here and there by inlets. A part of the 
supply for such beaches comes from river sediment ; and, as we have 
seen along the New Jersey coast, a part may come from cliffs against 
which the waves are cutting. But a barrier beach may develop with- 



SHORELINES 



369 



out any such source of sediment if the sea bottom is shallow and sandy. 
This is the case on the coast south of New York. 

Along such a coast the waves come in contact with the shallow 
bottom and push the sand before them, finally raising it in a barrier 
beach at the appropriate distance offshore. If no additional supply 
is obtained, the barrier beach will slowly 
migrate landwards ; but if sediment is 
supplied by rivers, it may grow outward. 
The pushing back of the barrier beach 
is due partly to the fact that the sand 
is ground finer and carried away by the 
waves or currents, and partly to the 
action of the wind, which drives the sand 
from the beach into dunes back of the 
beach or into the lagoon behind the 
beach. Ultimately the beach might be 
pushed back to the land margin and the 
waves then attack the land itself. 

The process of pushing back the 
barrier beach is, however, a slow one, 
and, where sediment is supplied by 
rivers, the rate is still further decreased. 
It therefore is commonly the case that 
the shallow lagoon behind the barrier 
is slowly rilled with sediment, some from 
the land streams, some brought from 
the beach by the wind, and some the 
remains of plants and animals living in 
the lagoons. 

On the coast of Brazil, Branner has 
described stone reefs, which represent 
another termination of the history of 
the offshore bar. In this case they are 
converted into reefs of solid rock by the 
carbonate of lime from calcareous skele- 
tons of animals and plants buried in 
the sand, which cements the sand grains 
of the upper 10 to 12 feet of the reef 

into sandstone or quartzite. These stone reefs follow the shores 
of northeastern Brazil for about 1250 miles (Fig. 244). They are 450 
feet or less in width and are interrupted by channelways at distances 
of from a few hundred feet to 8j miles. They have been modified by 
the waves, which have swept away the loose unconsolidated material, 
making the borders more broken and angular than in ordinary offshore 
bars. There is a similar reef at Jaffa in the Mediterranean ; but the 
lack of similar climatic conditions seems to have prevented the forma- 
tion of stone reefs elsewhere. 




12 3 4 5 



Fig. 243. — Offshore bar and lagoon 
on the coast of Texas. 



37° 



COLLEGE PHYSIOGRAPHY 



Uses of Barrier Beaches by Man. — The barrier beach is built to 
the height reached by the highest waves, and then, by wind action, 
still higher, sometimes 50 to 100 feet above mean sea level. Fisher- 
men and summer residents build homes on the sand ; in the sea 
islands of southern United States cotton is raised ; at Galveston 
there is an important seaport city, and at Atlantic City, a popular 
summer resort (Fig. 230). 

The open coast is straight and smooth as a result of the 
diagonal reach of the waves and the currents, and it is surf-beaten, 

as the waves of the open ocean 
break upon it. The lagoon 
coast is far more irregular, and 
the waters there are both 
shallow and protected. Being 
commonly no more than 5 to 
10 feet in depth, the lagoons 
are not navigable to large 
boats ; but they deepen where 
they merge into bays indent- 
ing the land ; and opposite the 
mouths of such bays there is 
commonly a break in the bar, 
or an inlet. It is on the margin 
of such an inlet that Galveston 
is located ; and by building 
jetties, thus further confining 
the current of water that 
flows through the inlet, the entrance to the harbour has been 
deepened so as to admit large ships (Fig. 245). The harbour of 
Pernambuco, Brazil, is protected by a stone reef. 




Fig. 244.- 



Stone reef off the coast of Brazil. 
(Branner.) 



Development of a Coast Line 

Structure, Process, and Stage in Shorelines. — As in the case of 
every land form, the evolution is influenced (a) by the material worked 
upon, (b) by the forces in operation, (c) by the time element, or, 
as put by Professor Davis, by -structure, process and stage. We have 
already seen that, given variation in the forces, there results difference 
in the form ; and that there is variation in the shoreline according 
to the material worked upon. It is equally true that there is a great 
difference in shoreline form according to the stage of development, 
for shorelines, like other land forms, pass through a life history. 
This could be illustrated by considering the shore forms in detail — 
the cliff, the sea cave, the hook, etc. ; but we will go no further than 
to consider it in its more general application to a coast line as a whole. 

Young Consequent Coasts. — At its beginning a shoreline will 
have that form which is the consequence of the line of contact of sea 




Fig. 245. — Changes in the form of the barrier beach at Galveston as a result of improve- 
ments by man. (Putnam.) 

371 



372 



COLLEGE PHYSIOGRAPHY 



and land — a consequent form (Fig. 246). Conceivably the conse- 
quent shoreline might be straight, but it would be far more likely to be 
irregular, and, on a drowned hilly land, it may have an extraordinary 

degree of irregularity. With 
the passage of time there is 
the tendency toward the de- 
velopment of regularity both 
by cut and by fill. The 
headlands are cut backward, 
the bay mouths have de- 
posits made across them 
with materials derived from 
the waste of the sea cliffs, 
and the inlets tend to be- 
come filled by deposits from 
the land, from the sea, and 
from organic remains. Lo- 
cally minor irregularities 
may result, such as spits, 
hooks, and cuspate fore- 
lands, but these are only 
the temporary exceptions. 

Mature Coasts are 
Straight. — Given time, the 
most irregular coast line 
would become straightened 
under these influences ; but 
then irregularity might be 
introduced by the outward 
projection of deltas at the 
stream mouths. These 
would form actual points 
or peninsulas if deposit ex- 
ceeded the power of waves 
and currents to remove ; or 
rounded points if the excess 
of sediment were slight ; or 
offshore sand bars if the 
waves and currents were 
able to give it wide distribu- 
tion. 
Factors Influencing Rate of Development. — There will be a not- 
able difference in the rate and in the nature of the development of 
coast lines according to (1) the nature of the rock, (2) the direction 
of waves and currents, (3) the intensity of waves and currents, (4) the 
depth of water, (5) the height of the land, (6) the amount of sediment 
supplied from the land. It may even happen that the initial coast 




Fig. 246. — Block diagrams to show life history of 
a coast in weak rock, as in Maryland and New 
Jersey : (1) coastal plain with shallow valleys and 
small deltas ; (2) embayed coast produced by sink- 
ing of the land; (3) low cliffs and short bars pro- 
duced by wave work ; (4) offshore bar, resulting 
from later wave work and salt marsh in lagoon. 
A fifth stage for old age of this coast would show 
the offshore bar pushed back, the inner bays 
sealed by bars, and the coast straight and simple. 
For the life history of a coast in resistant rock 
see Fig. 232. 



SHORELINES 373 

line is rapidly straightened, the old land being faced by a barrier 
beach of new land, as along the southern coast of the United States 
— such a straight coast, backed by an unfilled lagoon, is really a young 
coast, although straight where the waves have thrown up an offshore 
bar of new land. 

Where wave action is vigorous, and supply is not excessive, there 
may be very notable change in the coast, even in very brief periods 
of time. This is well illustrated at Cape Cod, where the waves have 
actively cut back the cliff near Highland Light, and a straight beach 
has been made, both to the north and south, along which the sedi- 
ment is driven. As the cliff has moved backward, the bar to the 
north has moved outward as from a fulcrum at Highland Light cliffs, 
and the outward movement has also extended to the hooked end of 
the bar. The present curved outline of the north end of Cape Cod 
represents the form of equilibrium assumed between present supply 
and transporting forces from the cliff of to-day. In earlier times, when 
the cliff was farther out, other curved outlines of smaller radius were 
developed and can still be traced; they represent the equilibrium 
of forces, supply, and cliff position of former days. The whole north 
end of Cape Cod has been made by the transportation of sediment 
from the receding cliff, some of it blown inland to form sand dunes. 

Old Coasts Rare. — As a whole the coast lines of the world are in 
a stage of youth, for, as has been already pointed out, the relation of 
land and sea is not long maintained without change. An old coast 
line would be a straight one, no matter what its initial irregularity, 
with delta projections opposite the stream mouths. 

The Influence of Animals and Plants 

Constructive, Destructive, and Protective Effects. — Both animals 
and plants exert an influence upon coast lines either through (1) con- 
structive, (2) destructive, or (3) protective effects, or (4) by a com- 
bination of two or all of these. 

The constructive effect is the result of the deposit of more or less 
indestructible organic remains, sometimes merely as parts of deposits 
of other origin, as in the case of sand beaches and mud flats, in which 
shells and other organic remains are included. In other cases certain 
types of organisms are so abundant that they give rise to purely or- 
ganic deposits, for example in coral reefs. 

The destructive work of organisms is an aid to the modification of 
coast lines. In sand beaches and mud flats, for example, there are 
burrowing animals which make and leave openings in the sediment, 
and even on rocky coasts there are organisms which eat directly into 
the rock, or aid in its destruction by indirect means. As an illus- 
tration of this type of work may be mentioned the burrowing shells 
which excavate cavities in the limestone rock, of which the shell 
lithodomus in the Mediterranean is an instance (Fig. 262). 



374 COLLEGE PHYSIOGRAPHY 

Many coasts receive effective protection from organic life. This is 
true in tropical waters where fringing coral reefs receive the blow 
of the breaking wave, while the coast of the mainland itself is faced 
by the quiet lagoon. It is also illustrated on many rocky coasts where 
seaweed and other organisms cling to the rock, both in the zone of 
the breaking wave and on the shallow offshore platform. On the 
rocky coasts of temperate latitudes the seaweed forms a mat of tough, 
rubbery, organic tissue, against which a large measure of the wave- 
blow is expended. It is probable that without this protective influ- 
ence the rate of wearing back of such coast lines would be far more 
rapid than is the case. 

While there are a multitude of ways in which organisms aid in a 
constructive, destructive, or protective way, sometimes with local 
effects of marked importance, there are three kinds of organic work 
which give rise to important and well-recognized coast forms. Two 
of these, salt marshes and mangrove swamps, are the result of the 
influence of plant growth, while the third, coral reefs, are due to the 
effect of animal life. 

Salt Marshes. — Salt marsh plants cannot grow where the waves 
break, but in the protected lagoons and estuaries of the cool temperate 




Fig. 247. — A salt marsh in eastern Massachusetts at mid-tide. At high tide it is 
completely submerged. 

region there are extensive plains called salt marshes. Their surfaces 
rise about to the level of the high tide, and over them the salt water 
flows at intervals. These marshes are traversed by a series of mud- 
walled channels, into which the tide rises, and out of which the salt 
water is drained from the marsh area at low tide. Upon the surfaces 
of the marshes (Fig. 247) there is vegetation, consisting of a variety of 
plants adapted to life in a salt soil. The marshes are growing and 



SHORELINES 375 

filling the estuary or lagoon by deposit of sediment brought by the 
tidal currents, and they are growing upward by deposit during the 
periods of overflow and by the accumulation of organic remains. 

These salt marsh plains, therefore, consist of inorganic sediment, 
of the remains of the salt marsh grasses, and of animals that live in 
this habitat. In the deposit of sediment the marsh plants aid indi- 
rectly in so checking the currents as to induce deposit. Ultimately 
by such accumulation a lagoon or estuary may become filled from side 
to side, with perhaps the single exception of one or more channelways 
through which the tide passes. Ultimately the surface will be raised 
to, or even above, the level of the tide. On extensive salt marshes 
there are areas so low that they are covered by every tide, others 
which are reached only by the highest spring tides, and even sections 
to which the tide no longer reaches. 

Utilization of Salt Marshes. — By a slight uplift of the land such a 
marshy plain may, after proper drainage, become good agricultural 
land; or, by building embankments to shut out the tide, man may 
reclaim marsh lands which have not yet risen into the dry land con- 
dition. In this way extensive tracts of salt marsh have been reclaimed 
in England, in Holland, and in Nova Scotia, the so-called land of 
Evangeline in NoVa Scotia being salt marsh reclaimed by the French 
Acadians. As yet little has been done toward the reclamation of salt 
marshes in the United States, but, in the thousands of square miles 
of sand marshes along the eastern coast, there are many areas which 
will doubtless be reclaimed when land values are sufficient to warrant 
the expenditure. 

Mangrove Swamps. — In the warm waters of the tropical and sub- 
tropical lands the mangrove tree, represented by many species, has 
adapted itself to life in the quiet waters of lagoons and estuaries. In 
such climates, therefore, the mangrove replaces the swamp grasses, 
forming mangrove swamps. The mangrove tree rests upon a branching 
base with roots extending through the marine soil (Fig. 248) ; from the 
branches of the tree other roots descend to the sea floor, thus giving 
rise to an almost impenetrable thicket of branching tree roots, and 
furnishing to the tree great stability, although growing in unstable 
soil. The cigar-shaped fruit, floating with the root-end downward 
and germinating when in contact with the bottom, sends up a shoot, 
which adds to the root tangle of the mature trees. 

In such a tangle the currents of ocean water are checked and sedi- 
ment deposit is assisted ; while to this deposit is added the decaying 
remains of the mangrove itself and the shells and other durable parts 
of marine organisms. Thus the mangrove swamp extends its area, 
giving rise to a characteristic coast form in the quiet tropical waters. 

Coral Reefs. — A multitude of marine animals abstract mineral 
matter from the sea water and incorporate it in their shells or skeletons, 
which, upon their death, remains as a part of marine deposits. In some 
situations along coast lines the deposit of sediment is so limited and the 



376 



COLLEGE PHYSIOGRAPHY 



abundance of shell-building animals is so great that shell deposits are 

formed. Thus there are shell banks and oyster beds along some coasts. 

In tropical waters the abundance of shell-building organisms is 

far greater than in temperate latitudes, and there are some species 





f*'Vi£i.> >, \ 


'"•SH 


n 


V' 










*r$tl 


r 






••*~*0 " 


Ah 


1% 






*4 ll 


^ 



Fig. 248. — Mangrove swamp on the coast of Florida. 

which thrive in such abundance that their remains form extensive 
deposits. Of these the reef-building corals are the most noteworthy, 
though upon coral reefs there are, besides corals, a multitude of other 
shell-making animals, and also lime-secreting plants, notably the 
calcareous algae. 

Conditions Requisite for Coral Growth. — The reef-building corals 
and their associates are not uniformly deposited throughout the warm 
ocean waters, for their growth in sufficient abundance to give rise to 
coral deposits depends upon a delicate balance of favourable conditions, 
among the most important of which are the following: (1) The 
temperature of the water must be high and in no case less than 68° F., 
even this temperature being too low for the most abundant coral 
growth. (2) The water must be shallow, with a depth not exceeding 
90 to 120 feet. Although it is true that shell-building animals live 
at greater depths, the reef-building corals do not thrive. They grow 
best in 35 to 50 feet of water. (3) The water must be normally saline, 
and, therefore, along the ocean margins where the water is freshened 
by the inflow of rivers, reef-building corals do not thrive. (4) The 



SHORELINES 



377 




Fig. 249. — Barrier reef in the Marshall 
Islands, Pacific Ocean. 



water must be clear and free from abundant sediment, therefore, where 
muddy rivers enter the sea or where wave work causes muddy coastal 
water, coral reefs cannot develop. (5) There must be sufficient food 
supply to nourish the abundant life of the coral reef. The most 
favourable condition for this purpose is the presence of steadily flowing 
ocean currents, which are ever sweeping up to the stationary organ- 
isms the needed food supply. Coral reefs are extensive on the east 
coasts of Africa, central America, and Australia, which are bathed by 
warm currents, while on the west 
coasts of these lands the corals 
occur only in scattered patches. 
Abundance of Life on Coral 
Reefs. — Where all these con- 
ditions are met, the abundance 
of coralline and other marine 
life is so great that rapid growth 
occurs and reefs develop. The 
abundance and variety of life on 
such a coral reef is almost in- 
conceivable, for each branching 
coral or each coral head is the 
home of scores of hundreds of in- 
dividuals, or polyps, each spread 
out flower-like, beyond its stony 
home, exhibiting a surprising variety of form and colour. Each ani- 
mal is engaged in the double process of seizing food as it passes and 
abstracting carbonate of lime to build the cells in which it lives. Asso- 
ciated with corals are variously coloured sponges, calcareous algae, and 

a great variety of mol- 
lusks and crustaceans. 
Every square inch of 
surface is inhabited by 
some form of life, and 
often there are two or 
three tiers of organisms, 
the vast majority of 
which have, as a part 
of their structure, 
either carbonate of lime 
or silica which is aiding 
in the upbuilding of 
the reef. It has been 
estimated that the 
corals have built up the reefs in Florida over 40 feet in 1000 or 1200 
years. 

Fringing Reefs. — Along some coasts there are extensive fringes 
of coral reef known as fringing reefs. These reefs parallel the coast 




Fig. 250. 



An atoll in the Carolina Islands, Pacific 
Ocean. (E. S. Holden.) 



378 COLLEGE PHYSIOGRAPHY 

at a variable distance, according to the depth of the water, and 
around them the ocean waves break, while between the reef and the 
land is a protected, shallow lagoon, in which the growth of the lime- 
secreting organisms is less rapid because of the more limited food 
supply. There is such a reef along the coast of southern Florida. 

Barrier Reefs and Atolls. — Fringing reefs are relatively close to 
the coast, but islands in the open waters of the warm oceans are sur- 
rounded at a little greater distance by a barrier reef which has grown 
outward upon its own talus. The greatest of all the reefs is the Great 
Barrier Reef, which extends along the eastern coast of Australia for 
a distance of 1250 miles with a width of from 10 to 90 miles. There 
are also reefs upon shallow banks, like the Florida Keys. In the 
tropical Pacific and Indian oceans are circular reefs known as atolls 
(Fig. 250). 

Darwin's Theory of Atoll Formation. — There has been much dis- 
cussion concerning the origin of atolls, and as yet it cannot be said 
that their cause is definitely established. Both Darwin and Dana 



\r. 



} L 



Fig. 251. — Block diagrams to show fringing reef (left), barrier reef (middle), and atoll 
(right) around sinking volcano, as proposed by Darwin. 

put forward the theory that these atolls are the descendants first of 
fringing, then of barrier reefs around oceanic islands, which have 
disappeared by slow subsidence while the fringing reef continued 
to grow. The difficulty of believing in such widespread sinking of 
the sea bottom at so slow a rate as the upward growth of the reef 
would demand has, to many, seemed very great (Fig. 251). 

Daly has suggested a relationship between the coral reefs found on 
flat ocean platforms less than 300 feet below sea level and the lowering 
of sea level by the temporary locking up of water in the ice of the con- 
tinental glaciers. This involves marine planation of the platforms, 
a slow increase in depth of water, but no change of level of the land. 

An 1114-foot boring in the coral reef of Funafuti does not seem 
to give conclusive proof of the correctness of the Darwin-Dana 
hypothesis, which, however, is supported strongly by the presence 
of drowned valleys at the borders of the island inside certain barrier 
reefs (Fig. 249). 

Murray's Theory of Atoll Formation. — The rival theory, proposed 
by Murray, has seemed to many a more probable explanation of the 
peculiar atoll form. This theory is that while some atolls may be the 
result of slow subsidence with accompanying upgrowth of fringing 
reefs, others, and perhaps the majority, have developed upon sub- 



SHORELINES 



379 




pwj^^p^ 



marine shoals, such, for example, as a volcanic peak which did not 
rise to the surface, or upon the platform of an island destroyed by wave 
erosion. The circular form of the atoll and the lagoon which it en- 
closes are upon this theory explained as a result of the more rapid 
growth of reef-building corals on the exposed outer side, and of the 
more rapid solution of calcareous remains in the lagoon than the 
growth of organisms can counterbalance. Some atolls occur where 
uplift rather than sinking has taken place. 

Evidence from Serpula Atolls. — The process by which this devel- 
I opment of atolls is said to have proceeded is illustrated on the shores 
of Bermuda, where 
planed-off stacks have 
furnished platforms upon 
which shell-building ma- 
rine organisms have taken 
hold, especially the genus 
serpula, which lives in 
the calcareous tube that 
it secretes. In these 
situations the serpula 
grows in the zone of 
wave attack over the 
entire platform, but it 
grows more abundantly 
on the outer side than 
on the inner, with the 
result that, by its 

growth, a platform is built into a saucer shape with an atoll-like 
rim, enclosing a small lagoon, the diameter of the entire area of 
the serpula atoll being only a few yards (Fig. 252). 

The Life History of Coral Reefs. — When a coral reef rises into the 
zone of vigorous wave attack, it is itself subjected to partial destruc- 
tion by solution and by mechanical erosion. It may, therefore, 
happen that the balance between upward growth and destruction 
by wave attack will be about equal and the upward rise of the coral 
reef be checked. It cannot in any event be built above the level of 
the lowest tide, for the coral animals cannot stand exposure to the air. 
The fragments torn off by the waves may either be driven into the 
lagoon behind the reef, or upon the inner shore of the lagoon, or it 
may even be raised to form a coral beach upon the reef crest itself. 
In the Bermuda Islands fragments of coral torn from the fringing reef, 
together with shells driven in from the shallow offshore waters, ac- 
cumulate upon the beaches of the islands and are there ground up 
into coral and shell sand. Some of this is then drifted inland by the 
winds, forming shell and coral sand dunes, as already described. On 
the atolls, and on the other coral reefs, the beaches that are made 
upon the reef crests likewise serve as a source of supply of coral sand 



Fig. 252. — Serpula atolls, Bermuda. 



3 8o COLLEGE PHYSIOGRAPHY 

which the winds drive above the reach of waves. It is because of 
this cooperation of the wind that many coral islands have been made 
habitable. Some of those in the south Pacific Ocean support a large 
native population. The people live chiefly on fruits, especially the 
cocoanut, and fish for pearls in the lagoons of the atolls. In other 
cases uplift has raised the coral above sea level. 

Islands 

Constructional Islands. — Islands are sometimes classified as con- 
tinental and oceanic; but this does not take into consideration any 
essential element either in origin or form. The better classification 
is into the two divisions : (a) constructional and (b) destructional. 
It would be possible to subdivide each of these two divisions into a 
great variety of kinds according to origin, but it will serve our purposes 
to merely illustrate the two major divisions. Coral islands are con- 
structional, and so also are volcanic islands and the higher parts of 
growing mountains, such as the West Indies, the Philippines, and the 
East Indies. Islands resulting from deposits at river mouths or in 
connection with the growth of sand bars are also constructional. 

Destructional Islands. — Islands of destructional origin include 
those which are formed by the subsidence of the land, leaving the 
higher parts isolated. In this class are also included those islands 
which develop as the waves cut back coast lines, leaving insular 
stacks. 

Life History of Islands. — In the life histoiy of an island there is 
always involved the attack of the agents of erosion which are engaged 
in an effort to remove it. If, however, it is of constructional origin, 
the attacks of these agents may be less effective than the operation 
of the processes which are forming it. Thus, for instance, a coral 
island may grow faster than the waves can remove the coral fragments, 
or a sand bar may continue to grow in the face of continuous and 
vigorous wave attack, or a volcanic island may steadily extend its 
area by eruption of lava or ash, although exposed to the full violence 
of open ocean waves. But if the constructional processes cease, or 
if they become so diminished that wave attack exceeds accumulation, 
the life history is then one of destruction. Subaerial agents are en- 
gaged in removing material from its surface, while oceanic agents are 
attacking its periphery. The ultimate fate of such an island would 
be reduction below the level of the sea. If near the mainland, one 
stage in the process of destruction of the island may be the tying of 
it to the mainland by a sand bar, temporarily transforming it to a 
peninsula. 

Bays and Harbours 

Bays Due to Subsidence. — The great majority of bays, harbours, 
estuaries, and other indentations of the coast line are the result of 



SHORELINES 



38i 



Ancient Shore Line 

Present Water Surface ^ 

ruled ^ — — ' p^j jf** 
Intermediate Shore Lines 
on Gulf of Latmos ^ ,. 



subsidence of the land, admitting the sea into the lower portions of 
the land. 

Bays Due to Uplift and Other Causes. — Some indentations are, 
however, also caused (a) by irregular uplift, as during mountain ele- 
vation, (b) by volcanic action, (c) by glacial erosion, or (d) by the 
development of coral reefs or sand bars. 

Variations in Form. — The form and depth of such indentations 
varies greatly, according to the surrounding conditions. Some har- 
bours are broad, branching, and irregular, as where the irregular moun- 
tain growth has been in progress, or where depression of the land has 
admitted the sea into an irregular valley. Others are long, narrow, 
and linear, as in the case of fiords whose form is due to glacial erosion, 
and in the case of river mouths. Some even are circular, as (a) where 
volcanic craters are breached so as to admit the sea, and (b) the la- 
goons of the circular atolls. Some are deep and free from shoals, as 
along the fiorded coast, while others are shallow and interrupted by 
islands and shallow patches, as in the lagoons around coral reefs and 
sand bars, and in indentations resulting from subsidence of an irregular 
land. Man often creates har- 
bours where there is no good 
protection from the waves 
(PL VI). 

The Destruction of Harbours. 
— Whatever the origin and 
form of an indentation along 
the coast line, it is subjected to 
a double action which tends 
towards this extinction, (1) 
through deposit, (2) through 
the closing of the entrance. 
The rate at which this extinc- 
tion progresses varies with the 
area and depth of the indenta- 
tion, and with the supply of 
sediment which is coming into 
it. Some shallow bays are 
soon filled, and their mouths 
quickly close ; others resist the 
processes of extinction through 
long periods of time. One can- 
not doubt, however, that the 

continuation of deposit by the many streams that are entering 
so large a body as the Gulf of Mexico would ultimately succeed 
in completely closing it. In times past, deposit has filled large areas 
of interior United States, and in more recent times the deposits of 
the Mississippi and its tributaries have filled a bay which extended 
as far up the valley as Cairo in southern Illinois. The broad, fertile 




Fig. 253. — Changes in coast of Asia Minor 
where a shoreline has advanced seaward 10 
miles since the time of Christ. 



3 82 



COLLEGE PHYSIOGRAPHY 



valley of the Po in Italy is a river-filled bay head, the normal exten- 
sion of the Adriatic Sea, and the valley of Tigris and Euphrates in 
Mesopotamia is the rilled head of the Persian Gulf. The continua- 
tion of the process of extermination of these indentations is now so 
rapidly in progress that notable changes in the position of the coast 
line have occurred since the days of the Roman Empire (Fig. 253), 
and even since the Middle Ages. That some indentations are not 
filled is an indication of their relative youthfulness, for they are the 
seat of deposit of sediment both from the land and from the agencies 
of the ocean. 

Elevated Coast Lines 

Features produced by Change of Level of the Land. — The insta- 
bility of the relation between land and sea is so great that a large part 




Mfo-M \ 11/ 

SfflfcHfoi \\ Eli 




Fig. 254. — Shorelines of Lake Bonneville. (Gilbert.) 



of the coast line gives evidence either of recent subsidence or of recent 
elevation. It has already been shown that, by subsidence, an irregu- 
lar coast line is produced. By elevation the sea bottom is raised into 



SHORELINES 



383 



the air and the coast line of that stage becomes a feature of the dry 
land. Wave-cut cliffs, beaches, sand bars, and clays with marine 
organisms entombed are then exposed to view. 

Abandoned Shorelines near the Sea. — Such elevated shorelines 
are revealed along the coast of northeastern North America, from Bos- 
ton northward to northern Labrador and to the islands further north. 
Similar uplifted shorelines are clearly exhibited along the west coast 
of Scotland and along the Norwegian shore. In Norway the uplifted 
strip of sea bottom forms some of the best farm land along the horded 
coast, and back of 
the farm lands rise 
wave-cut benches 
in which wave- 
eroded chasms and 
sea caves are still 
preserved, showing 
the recency of the 
uplift (Fig. 233). 

Abandoned 
Shorelines in the 
Interior of the 
Continent.— Shore- 
lines of similar 
character are found 
around the mar- 
gins of the Great 
Lakes, where they 
were formed on 
the coast of the 
temporary lakes 

during the closing stages of the Glacial Period. Among the most 
perfect abandoned shorelines are those which lie above the Great 
Salt Lake, formed during the expanded stages of Lake Bonneville. 
From a study of such shorelines a clear idea of the nature and origin 
of shoreline features can be gained, and it was from a study of the 
Bonneville beaches (Fig. 254) that we have obtained the best study 
of shorelines that has ever been published — the classical work of 
G. K. Gilbert. 




Fig. 255. — Map showing four stages in the destruction of the 
island of Heligoland off the coast of Germany. The figures 
give the circumference in miles at various dates. 



Sea Coasts and Man 

Aside from such obvious relationships of man to sea coasts as have 
been already alluded to, his use of harbours, his fishing in shallow 
arms of the sea, and the perils to navigation through the wrecking 
of vessels upon reefs, the contact of sea and land touches his activi- 
ties at many other points. The erosion of the coast may cut away 
his land, as in southeastern England, where whole farms and villages 



3§4 



COLLEGE PHYSIOGRAPHY 



have been washed away in the last few centuries, the sea cliffs re- 
treating from 7 to 15 feet a year, or on the coast of Holland, where 

a Roman castle, built on dunes 
1 J miles back from the sea, was 
in 1694 a half mile out in the 
sea, or in Heligoland, where an 
island has been tremendously 
reduced in area (Fig. 255), or 
Sharp's Island in Chesapeake 
Bay, which was reduced by 
wave erosion from an area of 
438 acres in 1848 to 53 acres 
in iqio. Man's greatest de- 
fiance of the sea is probably 
the building of the Florida 
coast railway which traverses 
coral reefs and the intervening 
stretches of open water with a 
concrete causeway. It is over 
100 miles long, extending from 
the mainland of Florida to Key West (Fig. 256). 




Fig 



256. — Map showing the railway to Key 
West. 



References to Literature 



C. Abbe, Jr. The Cuspate Capes of the Carolina Coast, Proc. Bost. Soc. Nat. 

Hist., Vol. 26, 1895, pp. 489-497. 
A. Agassiz. Notes from the Bermudas, Amer. Journ. Sci., Vol. 147, 1894, 

pp. 411-416 ; Coral Reefs of the Tropical Pacific, Memoir 28, Mus. Comp. 

Zool., 1903, 410 pp.; The Elevated Reef of Florida, ibid., Vol. 28, 1896, 

pp. 29-62. 
W. W. Atwood and J. W. Goldthwait. Physical Geography of the Evanston- 

Waukegan Region, Bull. 7, 111. Geol. Survey, 1908, 102 pp. 
J. A. Bancroft. Geology of the Coast and Islands in British Columbia, 
Canadian Geol. Survey, 1913, pp. 31-51, 1 19-123. 
Stone Reefs, Bull. Geol. Soc. Amer., Vol. 16, 1905, pp. 



Memoir 23 
J. C. Branner. 

1-12. 
A. P. Brigham 



The Fiords of Norway, Bull. Amer. Geog. Soc, Vol. 38, 

1906, pp. 337-348. 
H. M. Cadell. The Story of the Forth, Glasgow, 1913, 299 pp. 
T. C. Chamberlin. The Attitude of the Eastern and Central Portions of the 

United States during the Glacial Period, Amer. Geol., Vol. 8, 1891, pp. 

266-275. 
G. H. Cook. On a Subsidence of the Land on the Seacoast of New Jersey and 

Long Island, Amer. Journ. Sci., 2d series, Vol. 24, 1857, pp. 341-355. 
V. Cornish. Sea Beaches and Sand Banks, Geog. Journ., Vol. n, 1898, pp. 

528-543, 628-658. 
R. A. Daly. Geology of the Northeast Coast of Labrador, Bull. Mus. Comp. 

Zool., Vol. 38, 1902, pp. 205-270; Pleistocene Glaciation and the Coral 

Reef Problem, Amer. Journ. Sci., Vol. 30, 1910, pp. 297-308. 
J. D. Dana. Corals and Coral Islands, New York, 1872, 398 pp. 
Charles Darwin. Structure and Distribution of CorSl Reefs, London, 1842, 



SHORELINES 385 

1874; Geological Observations, London, 1846; ibid., 3d edition, New York, 

1897. 

C. A. Davis. Salt Marsh Formation near Boston and its Geological Signifi- 

cance, Econ. Geol., Vol. 5, 1910, pp. 623-639. 

W. M. Davis. Dana's Confirmation of Darwin's Theory of Coral Reefs, 
Amer. Journ. Sci., Vol. 35, 1913, pp. 173-188; The Outline of Cape Cod, 
Geographical Essays, Boston, 1909, pp. 690-724; Der Marine Zyklus, 
Erklarende Beschreibung der Landformen, Leipzig, 1912, pp. 463-554; 
The Home Study of Coral^ Reefs, Bull." Amer. Geog. Soc, Vol. 46, 19 14, 
PP- 56i-577, 641-654. 

J. W. Dawson. On a Modern Submerged Forest at Fort Laurence, Nova 
Scotia, Quart. Journ. Geol. Soc, Vol. 11, 1855, pp. 119-122. 

H. W. Fairbanks. Oscillations of the Coast of California during the Pliocene 
and Pleistocene, Amer. Geol., Vol. 20, 1897, pp. 213-245. 

N. M. Fenneman. Development of the Profile of Equilibrium of the Sub- 
aqueous Shore Terrace, Journ. Geol., Vol. 10, 1902, pp. 1-32. 

G. de Geer. On Pleistocene Changes of Level in Eastern North America, 
Amer. Geol., Vol. 11, 1893, pp. 22-44. 

A. Geikie. The Scenery of Scotland, London, 1887, pp. 46-89. 

G. K. Gilbert. The Topographic Features of Lake Shores, 5th Ann. Rept., 
U. S. Geol. Survey, 1885, pp. 69-123; Lake Bonneville, ibid., Monograph 
1, 1890. 

J. W. Goldthwait. The Abandoned Shore-lines of Eastern Wisconsin, Bull. 
17, Wis. Geol. Survey, 1907, 134 pp.; The Twenty-foot Terrace and Sea- 
cliff of the Lower St. Lawrence, Amer. Journ. Sci., Vol. 32, 191 1, pp. 
291-317. 

J. P. Goode. The Development of Commercial Ports, Chicago Harbor Com- 
mission, 1908, 103 pp. 

J. W. Gregory. The Nature and Origin of Fiords, London, 1913, 542 pp. 

F. P. Gulliver. Cuspate Forelands, Bull. Geol. Soc. Amer., Vol. 7, 1896, pp. 
399-422; Shoreline Topography, Proc. Amer. Acad. Arts and Sci., Vol. 
34, 1899, pp. 149-258; Nantucket Shorelines, Bull. Geol. Soc. Amer., 
Vol. 14, 1903, p. 555; Vol. 15, 1904, pp. 507-522; Vol. 20, 1910, p. 670. 

F. G. Hahn. Inselstudien, Leipzig, 1883. 

L. M. Haupt. A Menace to the New York Harbor Entrance, Bull. Amer. 
Geog. Soc, Vol. 37, 1905, pp. 65-77. 

G. J. Hinde and Others. The Atoll of Funafuti, Report on the Materials of 

the Borings, Royal Society of London, 1904, 428 pp. 
G. D. Hubbard. Fiords, Bull. Amer. Geog. Soc, Vol. 33, 1901, pp. 33°-337> 

401-408. 
J. F. Hunter. Erosion and Sedimentation in Chesapeake Bay Around the 

Mouth of Choptank River, Prof. Paper 90-B, U. S. Geol. Survey, 1914, 

pp. 7-15. 

D. W. Johnson. The Supposed Recent Subsidence of the Massachusetts and 

New Jersey Coasts, Science, N. S., Vol. 32, 1910, pp. 721-723 ; Fixite de la 
Cote Atlantique de l'Amerique du Nord, Annales de Geographie, Vol. 31, 
1912, pp. 193-212; Beach Cusps, Bull. Geol. Soc. Amer., Vol. 21, 1910, 
pp. 599-624; Botanical Phenomena and the Problem of Recent Coastal 
Subsidence, Botanical Gazette, Vol. 56, 1913, pp. 449-468. 

D. W. Johnson and W. G. Reed. The Form of Nantasket Beach, Journ. Geol., 
Vol. 18, 1910, pp. 162-189. 

A. C. Lawson. The Post-Pliocene Diastrophism of the Coast of Southern 
California, Bull. Dept. Geol. Univ. Cal., Vol. 1, 1893, pp. 1 15-160; Geo- 
morphogeny of the Coast of Northern California, ibid., Vol. 1, 1894, pp. 
241-272. 

J. Le Conte. Tertiary and Post-Tertiary Changes of the Atlantic and Pacific 
Coasts, Bull. Geol. Soc. Amer., Vol. 2, 1891, pp. 3 2 3~33°- 

A. Lindenkohl. Submarine Channel of the Hudson River, Amer. Journ. Sci., 
3d series, Vol. 41, 1891, pp. 489-499; ibid., Vol. 29, 1885, pp. 475-480. 

2C 



386 COLLEGE PHYSIOGRAPHY 

J. O. Martin. The Ontario Coast between Fairhaven and Sodus Bays, New 

York, Amer. Geol., Vol. 27, 1901, pp. 331-334. 
Lawrence Martin. Some Features of Glaciers and Glaciation in College Fiord, 

Prince William Sound, Alaska, Zeitschrift fur Gletscherkunde, Vol. 7, 

1913, pp. 289-333. 
J. Murray. Structure and Origin of Coral Reefs and Islands, Proc. Roy. Soc. 

Edinburgh, Vol. 10, 1880, pp. 505-518; Nature, Vol. 39, 1888, pp. 424- 

428; Vol. 40, 1889, p. 222. 

F. Nansen. Oscillations of Shore Lines, Geog. Journ., Vol. 26, 1905, pp. 604- 

616. 
O. P. Phillips. How the Mangrove Tree adds New Land to Florida, Journ. 

Geog., Vol. 2, 1903, pp. 10-21. 
S. Powers. Floating Islands, Pop. Sci. Monthly, Vol. 79, 191 1, pp. 303-307. 

G. R. Putnam. Hidden Perils of the Deep, Nat. Geog. Mag., Vol. 20, 1909, 

pp. 832-837. 
C. Reid. Coast Erosion, Geog. Journ., Vol. 28, 1906, pp. 487-495. 
H. Reusch. Norges Relief, Norges Geologiske Undersogelse, No. 32, Aarborg 

for 1900, pp. 124-217, English summary, pp. 239-263. 
G. B. Roorbach. Shoreline Changes in the Winthrop Area, Mass., Bull. 

Geog. Soc. Phila., Vol. 8, 1910, pp. 46-64. 
W. Saville-Kent. The Great Barrier Reef of Australia, London, 1893, 387 pp. 
E. C. Semple. Coast Peoples, Influences of Geographical Environment, New 

York, 191 1, pp. 242-291. 
N. S. Shaler. Postglacial Erosion of Martha's Vineyard, 7th Ann. Rept., 

U. S. Geol. Survey, 1888, pp. 347-351; Nantucket, Bull. 53, U. S. Geol. 

Survey, 1889, pp. 11-15, 47-52; Mt. Desert, 8th Ann. Rept., U. S. Geol. 

Survey, Part 2, 1889, pp. 1009-1034; Sea and Land, New York, 1894; 

The Geological History of Harbors, 13th Ann. Rept., U. S. Geol. Survey, 

Part 2, 1893, pp. 93-209 ; Evidences as to Change of Sea Level, Bull. 

Geol. Soc. Amer., Vol. 6, 1895, pp. 141-166; Beaches and Tidal Marshes 

of the Atlantic Coast, National Geographic Monographs, New York, 

1896, pp. 137-168. 

T. Sheppard. Changes on the East Coast of England within the Historical 

Period, Geog. Journ., Vol. 34, 1909, pp. 500-513. 
W. J. Sollas. Funafuti, The Story of a Coral Atoll, Ann. Rept., Smithsonian 

Institution for 1898, pp. 389-406. 
J. W. Spencer. The Submarine Great Canyon of the Hudson River, Amer. 

Journ. Sci., Vol. 169, 1905, pp. 1-15 ; ibid., pp. 341-344. 
Eduard Suess. The Face of the Earth, Part 3, The Sea, Vol. 2, London, 1906, 

556 pp. 
R. S. Tarr. Changes of Level in the Bermuda Islands, Amer. Geol., Vol. 19, 

1897, pp. 293-303; Wave-formed Cuspate Forelands, Amer. Geol., Vol. 
22, 1898, pp. 1-12; Chapter X, Physical Geography of New York State, 
New York, 1902; Postglacial and Interglacial (?) Changes of Level at 
Cape Ann, Mass., Bull. Mus. Comp. Zool., Vol. 42, 1903, pp. 181-191. 

R. S. Tarr and Lawrence Martin. Recent Changes of Level in the Yakutat 
Bay Region, Alaska, Bull. Geol. Soc. Amer., Vol. 17, 1906, pp. 29-64; 
Geog. Journ., Vol. 28, 1906, pp. 30-43; Changes in Shorelines in 1899, 
Prof. Paper 69, U. S. Geol. Survey, 191 2, pp. 18-32. 

T. W. Vaughan. Geology of the Keys, Carnegie Institution, Year Book 8, 
1909, pp. 140-144; Geologic History of the Floridian Plateau, ibid., 
Publication 133, 1910, pp. 99-185; Physical Conditions under which 
Paleozoic Coral Reefs were Formed, Bull. Geol. Soc. Amer., Vol. 22, 
1911, pp. 238-252. 

T. L. Watson. Evidences of Recent Elevation of the Southern Coast of 
Baffin Land, Journ. Geol., Vol. 5, 1897, pp. 17-33. 

E. Wertn. Fjorde, Fjarde, und Fohrden, Zeitschrift ftir Gletscherkunde, Vol. 
3, 1909, pp. 346-358. 

W. H. Wheeler. The Sea Coast, New York, 1902, 78 pp. 






SHORELINES 



387 



A. W. G. Wilson. Cuspate Forelands along the Bay of Quinte, Journ. Geol., 

Vol. 12, 1904, pp. 106-132 ; Shoreline Studies on Lakes Ontario and Erie, 

Bull. Geol. Soc. Amer., Vol. 19, 1908, pp. 471-500. 
J. E. Woodman. Shore Development in the Bras d'Or Lakes, Amer. Geol., 

Vol. 24, 1899, pp. 329-342. 
J. B. Woodworth. Note on the Changes of Level of the Coast of Southern 

Chile, Bull. Mus. Comp. Zool., Vol. 56, 191 2, pp. 1 16-132. 



Charlestown, R. I. 
Martha's Vineyard, Mass. 



TOPOGRAPHIC MAPS 

Bars shutting in Bays 

Duluth, Minn. 
Boston Bay, Mass. 



Pulaski, N. Y. 
Oswego, N. Y. 



Coos Bay, Ore. 



Beaches and Tied Islands 

Boston Bay, Mass. Cleveland and Vicinity, O. 



Coral Reefs 

U. S. Coast and Geodetic Survey charts, Nos. 15, 170, 1007. 
Cartes des bancs et recifs de coraux, 4 sheets, Paris, 1912. 



See also Joukin's 



Boothbay, Me. 
San Francisco, Cal. 
Charlestown, R. I. 
Martha's Vineyard, Mass. 



Drowned Coast 

Coos Bay, Ore. 
New Haven, Conn. 
New London, Conn. 
Stonington, Conn. 



Seattle, Wash. 
Brooklyn, N. Y. 
Boston Bay, Mass. 
New York City Special 



Leonardtown, Md. 
Barnegat, N. J. 



Drowned Coastal Plains 

Pt. Lookout, Md. 
Sandy Hook, N. J. 



Choptank, Md. 
Norfolk Special. 



Duluth, Minn. 
Oswego, N.Y. 



Drowned Lake Coast 

Pulaski, N. Y. 
Rochester Special 



Plattsburg, N. Y. 



Seattle, Wash. 

New York City Special 



Harbours 

San Francisco, Cal. 
Duluth, Minn. 



New Haven, Conn. 
Norfolk Special 



Lake Shores 

Charts of the Great Lakes, U. S. Engineer's office, Detroit, Mich., or Buffalo, 
N. Y. ; Maps 1, 5, 6, also Lake Ontario, Niagara River, Lake Erie, Lake 
St. Clair, etc. 



Ocean Shores 

U. S. Coast Survey charts, Nos. 6 (General Chart, coast of Maine and Massa- 
chusetts) ; 103, 104, 105, 106 (Maine coast, more detailed); 108 (coast 
from southern Maine to Cape Ann) ; 109 (Boston Bay) ; 8 (approaches 
to New York, Gay Head to Cape Henlopen) ; 113 (Narragansett Bay); 



3«8 



COLLEGE PHYSIOGRAPHY 



52 (Montauk Point to New York, with Long Island Sound) ; 119 (south- 
ern shore of Long Island) ; 121, 122, 123 (New Jersey coast, Sandy Hook 
to Cape May) ; 376 (Delaware and Chesapeake bays) ; n (Cape Hatteras 
to Cape Romain) ; 142 (Cape Hatteras) ; 147 (Cape Lookout) ; 188 
(Mobile Bay) ; 19, 194 (Mississippi delta and vicinity) ; 21 (Galveston to 
the Rio Grande); 212 (bar from Rio Grande northward); 5400, 5500 
(California coast); 3089, 8100 (fiorded Alaskan coast). 



Atlantic City, N. J. 



Of shore Bars, enclosing Lagoons 
Barnegat, N. J. 



Sandy Hook, N. J. 



Brooklyn, N. Y. 
Oceanside, Cal. 
Plymouth, Mass. 



Sand Bars and Hooks 

Atlantic City, N. J. 
Martha's Vineyard, Mass. 
Stonington, Conn. 



New London, Conn. 
Port Orford, Ore. 
Sandy Hook, N. J. 



Wave-cut Cliffs and Islands 
Port Washington, Wis. Wellfleet, Mass. Oswego, N. Y. 



CHAPTER XII 

MOVEMENTS OF THE EARTH'S CRUST, 
OR DIASTROPHISM 

Changes in Level 

Nature of Diastrophic Movements. — The instability of the rela- 
tive level of land and sea has been frequently referred to in the pre- 
ceding pages. In a great many cases this instability is the direct 
result of movements of the level of the sea itself. It is evident that 
a rise in the sea level will produce results similar to those due to a sink- 
ing of the land; and that the effects of lowering the sea level will 
resemble those of an uplift of the land. So close is the resemblance 
that it is not always possible to tell which of these processes has oper- 
ated to produce a given change in relative position of land and sea. 

Isostasy. — The causes for change in the level of the land are not 
well understood, and the consideration of them may, for the present, 
be deferred. It may be pointed out, however, that the crust of the 
earth is easily disturbed (a) by the operation of forces from within the 
earth, (b) by changes in load, a weighing down of the crust by deposit 
causing subsidence, a lightening of the crust by denudation causing a 
rising. At a given moment the earth form is in essential equilibrium, 
or isostatic adjustment, and if this equilibrium is disturbed, subter- 
ranean flow takes place to restore it, as would be the case in a liquid. 
The theory of isostasy, which teaches this mobility, seems now well 
established. Still a third possible cause for crustal movements is 
subterranean flowage to bring about adjustment to changing earth 
figure resulting from rotational variations. 

Causes for Changes in Sea Level. — Changes in the sea level may 
result from (a) deepening of the ocean basins; (b) shallowing of ocean 
basins through deposit ; (c) changes in the volume of water in the 
oceans; (d) variation in density or volume of the bordering crust, 
causing variation in gravitational attraction; (e) rotational variation. 
It is easily understood that a deepening of the ocean basins would 
withdraw water from the continent borders; that a shallowing of 
these basins would cause the sea to encroach on the land ; and that 
an increase or decrease in the volume of the ocean water would bring 
about the same results. The ocean waters are held in place by grav- 
ity ; and if gravitational attraction is locally increased or decreased, 
a local distortion of the sea level occurs. For example, the uplift of 

389 



3QO COLLEGE PHYSIOGRAPHY 

a great mountain chain, like the Andes, might produce a very decided 
distortion of the sea level by gravitational attraction; and the de- 
velopment of a great ice sheet, like those of North America and Europe, 
may affect the sea level not only by the withdrawal of much water, 
but also by exerting a lateral attraction upon the bordering sea. 
Variations in rotation, either in rate or in position of the axis of rota- 
tion, will cause a change in the sea level, since the oblate spheroid 
form will necessarily be adjusted to the changed conditions. 

There are, therefore, a number of causes for change in relation of 
sea and land. Several of these causes have been in operation in bring- 
ing about the many changes that have occurred in the recent past, 
and that are still in progress; and it is not at all improbable that 
more than one process has been in operation in a single locality. The 
causes are so complex, and so little understood, that it is at present 
impossible to speak more definitely. Indeed it is quite common to 
speak of these changes as uplift or subsidence, as though the changes 
in level were all the result of actual crustal movement or diastrophism. 
In the use of these terms, however, it is tacitly understood that they 
do not necessarily mean to assert actual land movement, any more 
than the inherited term sunset asserts actual sun movement. 

The change of level of the land in its relation to sea level may be 
either (a) upward or (b) downward, giving rise to either greater ele- 
vation of the land above the sea or to lowering of the land surface. 
These changes may be either (a) local, affecting only a slight area, or 
(b) general, affecting extensive areas ; they may give rise to differential 
movement or to general change of fairly uniform character ; and they 
may take place rapidly, or they may proceed with great slowness. 

Evidences of Change of Level 

There are many different kinds of evidence of a change in the rela- 
tive level of land and sea, the greatest number and the best being 
those observed along the sea coast, for there even very slight changes 
are registered and easily detected. 

Man's Observation of Emergence. — In some places actual human 
testimony proves uplift of the land, as in Yakutat Bay, Alaska, where 
the coast line was uplifted during the earthquakes of September, 1899 
(Fig. 257). In still other cases human structures, such as piers or 
buildings, have been raised, as in Crete, where old docks now stand 
27 feet above sea level. Upward movements have also been deter- 
mined by actual measurement, as in northern Sweden, where marks 
placed on the coast for the purpose of testing the common belief that 
the level of the land was changing prove an uplift of 7 feet in 154 
years. 

Evidence from Elevated Shorelines. — Equally clear evidence of 
change in the relative level of land and sea is the presence of elevated 
shorelines, with wave-cut cliffs, sea caves, chasms, stacks, beaches (Fig. 



MOVEMENTS OF THE EARTH'S CRUST 391 

258), and marine clays, — all the phenomena of shorelines excepting 
the presence of the ocean water. Such elevated shorelines, as already 
stated, are found back of many coasts, proving conclusively either 
that the sea level has been lowered, or the land level raised. It is a 
noteworthy fact that such shorelines are commonly tilted, and very 
often at such a sharp angle as to make it certain that it was the land 
that was raised, not the sea level that was lowered. Where the tilt- 
ing of the shoreline is more gradual it is possible that the apparent 
uplift is the result of a deformation of the sea level itself, or it may 
equally well be due to a tilting of the earth's crust. 

Evidence from Marine Organisms. — Another evidence of eleva- 
tion of the land relative to the sea is the presence of remains of marine 
organisms in deposits on the land. Thus the firiding of the skeleton 
of a whale in deposits in the Lake Champlain valley is accepted as 
proof that the sea 
once stood there; W^^&^r 
the presence in \, '"^^^i 

the deposits of the 
Texas coastal 

- . P ^wcicucii WJMMfiWM** <JPMS£D BEACH OF ,'8S9 

plain ol marine "WMmm^M* ! 

shells t of species 
now living in the 

(^ l-C Z T\T • ' 'OLD T/?£E-COy£fi£0 

Gulf of Mexico is &ach 

proof that they v ^-~ 

wprp rprpnt i v u Fig. 257. — Beach uplifted m 1899, and older elevated beach, 
were recently be- Yakutat Bay, Alaska. 

neath sea level ; 

and the presence of existing or recent species of marine organisms in 
hundreds of other places testifies to either uplift of the land or to a 
lowering of the sea level. Geological history records a complex suc- 
cession of emergences and submergences of the land in all the con- 
tinents ; and it is as a result of these changes of level that we have 
so great a series of sedimentary rocks which were originally deposited 
in the ocean waters, now forming parts of the continents. Doubtless 
many of these changes are the result of variations in the ocean level ; 
others are without doubt due to crustal movements. 

Evidences of Submergence less Numerous. — As in the case of 
emergence there is human testimony of submergence also. Thus 
parts of the Yakutat Bay coast line sank during the earthquakes of 
September, 1899, while there was uplift in other portions; and in 
Crete, while there was uplift in one part, there has been sinking in 
another part, proved by the submergence of structures built by man. 
Since the sea covers the submerged land, the evidence of shorelines 
and marine fossils cannot be utilized to prove the change in this direc- 
tion. It is, therefore, much less easy to discover evidence of submer- 
gence and very difficult to prove the exact amount. 

Evidence from Stumps and Peat Beds. — Submergence is often 
indicated by the presence of tree trunks or stumps standing in place 




392 



COLLEGE PHYSIOGRAPHY 




Fig. 258. — Beach uplifted over 40 feet during earthquake in 1899 at Yakutat Bay, Alaska. 




Fig. 259. — Barnacles and mussels attached to the rock on elevated shoreline, Yakutat 

Bay, Alaska. 



MOVEMENTS OF THE EARTH'S CRUST 393 

at and below tide level, and by the presence of peat bogs beneath 
the salt water. Since such vegetation can grow only on the land, its 
presence below sea level is proof of a downward change of level of the 
land. The use of such evidence, however, is possible only when it 
can be demonstrated that there has not been local downsliding or 
local change in water level due to change in exposure to waves or 
tides (Fig. 260). 

Evidence from Irregular Coasts. — One of the best evidences of 
land submergence is the drowned land topography of many coast 
lines, such as northeastern North America and northwestern Europe. 
Where the sea enters the land valleys, transforming them to bays, 




Fig. 260. — Forest killed by submergence in Yakutat Bay earthquake of 1899. 

harbours, estuaries, and straits, while the divide areas form peninsulas, 
capes, islands, and shoals, the evidence is fairly clear either that the 
land has subsided or the sea level has risen. The only important ex- 
ceptions are where differential crustal movements have given rise to 
coastal irregularity, and where glaciers have eroded valley bottoms 
below sea level. The former are limited to a few sections, the latter 
to regions of powerful glacial scour where they form fiords. Else- 
where the cause for the irregularity is certainly a downward change 
in relative level of land to sea. Further proof of such a change has 
been revealed by soundings, which have discovered drowned river 
valleys, like the channel of the Hudson River on the sea bottom south- 
east of New York City (Fig. 116), and completely drowned valleys 
off the New England coast, off the mouth of the St. Lawrence, and in 
the North Sea. 



394 



COLLEGE PHYSIOGRAPHY 



Instances op Change of Level 

Thousands of instances of recent or present day changes of level 
of the land are now known, and from among these only a few can be 
selected for specific treatment. 

Yakutat Bay, Alaska. — During a series of several earthquakes in 
September, 1899, the coast line of Yakutat Bay, which pierces the 

St. Elias Range of Alaska, 
was greatly deformed. 
One part of the coast was 
uplifted 47 feet; other 
portions were raised less ; 
there was no uplift along 
some sections of the coast ; 
and in some parts there 
was actual depression 
(Fig. 261). In this case 
the change of level was 
certainly the result of di- 
astrophism ; the move- 
ments were local and 
differential; and t'hey 
were abrupt, occurring 
certainly within a period 
of about three weeks, and 
possibly in a single day. 
When studied in 1905, 
barnacles and mussel 
shells were still clinging 
to the uplifted shore (Fig. 
259); and, in the area 
of depression, dead and 
dying trees were still 
standing in the salt water 
where they were lowered 
during the earthquakes. 
In the zones of uplift 
there were beaches so perfect in form that one could scarcely realize 
that the waves no longer reached them ; but annual plants and young 
shrubs had begun to grow amid the barnacles and on the sand and 
pebble beaches where the waves beat a few years before. There were 
also wave-cut sea cliffs, chasms, sea caves and stacks ; but at the new 
sea level there were no such shoreline features, because the waves had 
not yet had time to develop them. 

Changes Associated with Growing Mountains. — The neighbour- 
ing region furnishes evidence of other earlier changes of level. The 
rocks near Pinnacle Pass, west of Yakutat Bay, at an elevation of 




Fig. 261. — Map showing fault lines and amounts of 
uplift and depression during Yakutat Bay earth- 
quake of i8gg. 



MOVEMENTS OF THE EARTH'S CRUST 395 

5000 feet above sea level, contain willow leaves and mussel shells of 
species still living in the adjacent ocean. This is the common condi- 
tion in regions where mountains are still in process of growth. There 
are evidences of local, differential changes of level of the land along 
the California coast ; for example, local subsidence admitting the 
sea across the Coast Range at San Francisco and giving rise to San 
Francisco Bay, and uplift on Santa Catalina Island, and along the 
coast south of Los Angeles; but the time of occurrence of these 
changes of level is not recorded. Similar differential movements 
have occurred in many parts of the Mediterranean, in New Zealand, 
in the West Indies, in Japan, and in many other parts of the world ; 
and uplift associated with mountain growth has left clear records 
along the western base of the Andes. 

It is certain that areas of subsidence are associated with many, if 
not all, mountain uplifts. The great depth of the sea near the West 
Indies is explicable only on this .theory ; and sinking of the ocean 
bottom off western South America seems necessary to account for 
the great depth of the ocean there. The floor of parts of the Medi- 
terranean is evidently still sinking, for submarine cables are some- 
times rent asunder during periods of slipping. Hidden from view, 
these submarine movements attract less attention than those of the 
land, though there is reason to believe that they are actually more 
important than the changes of level in that part of the crust that is 
exposed to direct observation. 

Pozzuoli, on the Bay of Naples. — One of the most famous in- 
stances of change of level is that recorded by the ruins of the temple 
of Jupiter Serapis near Naples (Fig. 262). This temple was built be- 
fore the Christian era, and then came a series of changes of level, as 
follows : (1) After the temple was built, subsidence occurred, so that a 
new pavement had to be built. (2) Following this subsidence of 5 feet 
came a period of rest and as late as the year 235 a.d. the temple was 
above sea level. (3) Then followed a slow subsidence of 12 feet dur- 
ing which the marble columns were encased in mud as they were 
lowered beneath the sea. (4) A further subsidence of 9 feet occurred 
so rapidly that the columns were not enclosed in sediment, and, there- 
fore, the boring shell lithodomus was able to perforate the upper part 
of the limestone columns. (5) A period of rest followed, during which 
the lithodomus extensively perforated and roughened the limestone 
columns. (6) Then came uplift of 23 feet or more, bringing the col- 
umns above sea level, in which position they were found in 1749. 
(7) Subsequent to this there has been a slight subsidence. There 
is indication that sinking is still in progress, and careful measure- 
ments are now being made to determine the rate. 

In this case there can be no doubt that most, if not all, the move- 
ments are really crustal changes ; and they are probably in some way 
related to volcanic activity, for Pozzuoli lies between the volcanic 
Vesuvius and Ischia, and in the midst of a group of smaller cones. 



396 



COLLEGE PHYSIOGRAPHY 



Instability of the crust is common in volcanic regions, and numerous 
other illustrations could be given. Such changes of level, whether 




up or down, are doubtless related to migrations of molten rock beneath 
the crust. 



MOVEMENTS OF THE EARTH'S CRUST 397 

Scandinavia. — In the time of Linnaeus in the middle of the eigh- 
teenth century it was a matter of common belief that southern Sweden 
was slowly sinking, for rocks and reefs were reported to be gradually 
disappearing beneath the water, and streets, as at Malmo, were sub- 
merged. In the north of Sweden, on the other hand, the evidence 
pointed to uplift. This led Linnaeus to start a series of records ; and, 
by careful study of the evidence, it has been found that while the 
land has been rising north of Stockholm, having risen about 7 feet in 
154 years, it has been sinking in the south. There is, however, evi- 
dence that this subsidence has now ceased. 

Both in Sweden and Norway there have been still earlier changes 
of level. There was, for example, the great subsidence that gives 
the irregular coast line ; then, after the Glacial Period, there has been 
an uplift, and the beaches, wave-cut cliffs, and marine clays are plainly 
to be seen, not at uniform level, but varying from point to point, and 
rising toward the fiord heads. Between these periods was one in 
which the land stood for a long time from 200 to 300 feet higher than 
now, and the wave-planed bench and the sea cliff of this stage are 
prominent features of the Norwegian coast. The bulk of the popu- 
lation of Norway dwells on the bench of this stage, or else on the ma- 
rine clays of the last uplift. 

It is noteworthy that the (1) great depression occurred during 
the stage of glaciation; (2) the great uplift occurred during inter- 
glacial time; (3) the last uplift succeeds the withdrawal of the ice. 
This has naturally led to the theory that the glaciation is responsible 
for these changes of level, (a) partly by actual depression as a result 
of the ice load on the crust, and subsequent rebound, still in progress, 
when the load was removed; (b) by the attraction of the ice mass 
distorting the sea level, which might explain part of the rise in the 
beaches up the fiords in which ice tongues lay as the glacier receded. 

Other Northern Lands. — Subsidence during glaciation and sub- 
sequent uplift are also observed in many other regions of former gla- 
ciation. There are upraised sea beaches and associated sea cliffs 
from 20 to 25 feet above sea level along the western coast of Scotland ; 
there is a series of well-preserved beaches in Spitzbergen, in Baffin 
Land, Labrador, eastern Canada, and Maine. The beaches of 
Spitzbergen, where extensive glaciers still remain, are not nearly 
so high as those of Labrador and Baffin Land, from which the ice has 
largely withdrawn. In Greenland, still the seat of a great ice sheet, 
subsidence is still in progress along some 600 miles of coast line. 

Northeastern North America. — The irregular coast line of north- 
eastern North America clearly proves a great submergence, more in 
the north than in the south ; but in the south the submergence fol- 
lowed an emergence, and put beneath the sea only a portion of the 
previously upraised sea bottom. Whether these changes are due to 
crustal movements or to changes in sea level, or to a combination of 
the two, cannot at present be proved. That the land in the north 



398 COLLEGE PHYSIOGRAPHY 

was higher before the Glacial Period, and that it sank during that 
period, suggests a possible relation to glaciation. One great difficulty, 
however, is the fact that the level of preglacial time has not been 
even approximately restored; for although there has been an uplift 
varying from 5 or 10 feet near Boston to several hundred feet in 
Labrador, the land still lies far below its former level, and it does not 
seem to be still rising. In fact, the latest movement has been one 
of slight subsidence, for submerged peat beds and tree trunks are 
found at various points along the coast of New England and New 
Jersey. In the latter state there is apparently a subsidence at present 
in progress at the rate of about 2 feet per century, although this has 
been disputed. It seems probable, therefore, that, even though 
glaciation may be responsible for some of the changes of level, other 
causes, either for crustal movement or for change in sea level, are 
necessary to explain the phenomena. 

In the Continent Interior. — Away from the sea coast changes of 
level, even of considerable amount, might occur without detection. 
Therefore there is little proof of such change as a result of direct ob- 
servation. There are cases in which points visible from a certain 
locality are reported to have become invisible as a result of change 
of level at one of the points, or at an intervening point. There are 
also cases where lake waters have been tilted so far as to enter into 
and drown stream valleys that were formed before the tilting. This 
is well illustrated along the southern shores of the Great Lakes ; for 
instance, at Chicago, where the lake waters enter and form a small 
harbour in the Chicago River ; along the southern shore of Lake On- 
tario, where many stream valleys are drowned and transformed to 
bays and lagoons ; and at the mouth of Niagara, where the lower 
course of a former period is now wholly beneath lake water. 

A commonly accepted indication of change of level in the interior 
is the fact that the beaches and other shoreline phenomena of formerly 
expanded lakes are tilted. Thus the shorelines of the expanded 
Lake Iroquois, south of Lake Ontario, rise at the rate of about 5 feet 
per mile in a northeasterly direction ; and the shorelines of former 
Lake Agassiz rise at the rate of 1.3 feet per mile. This apparent 
tilting of the land may, in part at least, be due to original tilting of 
the lake waters, attracted toward the ice dam which held them in. 
That it is not wholly due to this cause is indicated by the careful 
measurements made by Gilbert, as a result of which he concludes 
that there is at present in progress an uplift of about 9 inches a cen- 
tury at Toledo, 6 inches at Duluth, and 9 to 10 inches at Chicago, 
giving a tilting toward the northeast of about 5 inches per hundred 
miles in a century. Still earlier, Spencer had reached the conclusion 
that there was present uplift of about if feet per century in the Niag- 
ara River region. 

The shorelines of former Lake Bonneville are also deformed (Figs. 
218, 219, 254), and here the deformation is certainly in large measure 



MOVEMENTS OF THE EARTH'S CRUST 



399 



the result of diastrophic movements, subsequent to the formation of 
the shorelines, for some parts of the shoreline are 350 feet higher than 
other parts (Fig. 263). In this mountain region changes in level are 
undoubtedly still going on in connection with mountain growth ; and 
in other mountain regions similar changes are certainly in progress. 
Instances of this kind are given in the discussion of earthquakes. 



Instability of the Earth's Crust 

The instances of change of level given in the preceding paragraphs 
are only a few of the many that are established by complete evidence. 

There are no coast lines that do 

not furnish evidence of some 
change in the recent past; and 
there are probably few, if any, 
coast lines that are at present in 
a state of actual stability. Sub- 
sidence, elevation, or warping are 
common phenomena of coasts ; 
and there is no reason for doubt- 
ing that, if we had equally clear 
means of detecting changes away 
from the coast, the same state- 
ment could be made with regard 
to the interior of the continents 
and the ocean floor. 

Some of these changes of level 
are rapid enough to be called 
paroxysmal ; but the great ma- 
jority are slow movements of 
the earth's crust, or of the 
sea level, or of both combined. 
These movements have been in 
progress throughout past time, 
and prodigious changes have 
taken place as a result of their 
continued operation. Thus sedi- 
mentary strata with marine fos- 
sils are found on plateaus thou- 
sands of feet above sea level, and 
among lofty mountains, 5000, 

10,000, and even 15,000 feet above the sea. The movements of the 
past are continuing in the present ; and there is every reason for be- 
lieving that they will operate in the future. 

The land is being attacked by denudation, and the fragments are 
being borne into the sea now, as throughout geological time. Were 
it not for the effect of diastrophism, by which elevation above sea 




Fig. 263. — Map to show deformation of the 
shorelines of Lake Bonneville. Contours of 
goo to 1 200 feet pass through points of equal 
warping. (Gilbert.) 



4 oo COLLEGE PHYSIOGRAPHY 

level is being renewed here and there, the lands would long since have 
been lowered to a surface of low relief, standing but little above the 
level of the sea. There has, without doubt, been actual upward move- 
ment of the crust in places, downward movement in others, and warp- 
ing elsewhere ; and there has been distortion of the sea level, and rising 
and sinking of the surface of the ocean. As a result of these complex 
movements, changes in the relation of sea and land have been fre- 
quent and great in extent. It is not possible to assign exact value to 
each of the types of movement, nor, in some cases of change, to state 
the exact nature of the cause ; but, speaking generally, the evidence 
indicates that the change in level through diastrophic movements 
of the crust is the most common, widespread, and effective. 

Disturbance of the Strata 

Structures Produced by Earth Movements. — One of the most 
striking proofs of crustal deformation is the condition of the strata 
themselves. Not only are beds that were deposited in the sea now 
found in all the continents and even in the loftiest mountains and 
plateaus ; but these strata, originally deposited in horizontal, or 
nearly horizontal, position are now found tilted at all angles. This 
tilting has been brought about (i) by folding, (2) by breaking or fault- 
ing, along certain planes. A third result of the diastrophic move- 
ments has been the development of a complex series of cracks known 
as joint planes. 

Nature of Folding. — Subjected to the slow stresses which give 
rise to crustal deformation, even the brittle rocks yield by bending 
when weighted down by superincumbent layers. In many cases 
this is the result of mechanical gliding of grain on grain ; but in rocks 
under heavy pressure there is actual flowage. 

Anticlines and Synclines. — The simplest form of folding is that 
in which the strata are thrown into a somewhat symmetrical series of 
upfolds and downfolds, wave-like in their form (Fig. 264). The upfold, 
or arch, is an anticline, in which the strata incline, or dip, away from the 
central axis ; the downfold, or trough, is a syncline, and here the layers 
dip toward the axis of the fold. Anticlines and synclines on the lands 
rarely stand out as topographic forms in their perfected state; for, 
like all land surfaces, they are subjected to denudation during and 
after formation. The inclined, strata of which they are composed 
do, however, give rise to very striking topographic features, as is 
shown in the discussion of the denudation of mountains. 

Forms of Folds. — Both anticlines and synclines may be either 
symmetrical, or unsymmetrical, in the latter case with one side steeper 
than the other. Very often one limb of an anticline is pushed over 
past the perpendicular, and it is then said to be overturned, or, if pushed 
over to a nearly horizontal position, recumbent. There is great com- 
plexity of folding among mountains, the strata sometimes being 



MOVEMENTS OF THE EARTH'S CRUST 



401 



thrown into a series of folds, in which the layers, though greatly 
folded, are all inclined in a single direction, a condition known as 
isoclinal folding. There 
is also complex contor- 
tion and crumpling , as 
one might crumple 
sheets of paper (Fig. 
265). When exposed to 
denudation, all these 
varieties of rock posi- 
tion give rise to ap- 
propriate influence on 
topographic form. 

In a region of moun- 
tain folding the rocks are 
thus thrown into a com- 
plex system of folds, 
each individual fold 
having a linear exten- 
sion along an axis, but 
dying out in both di- 
rections. The strata, 
therefore, not only dip 
on either side of the 
axis, but they have an 
inclination along the 
axis, known as the pitch 
of the fold. The direc- 
tion of the axis of a fold 
is the strike. Anticlines 
and synclines may be 
long and narrow, or 
short and broad, and 
the pitch may be either 
steep or gentle. The 
folded layers of a sym- 
metrical syncline of me- 
dium length, breadth, 
and pitch often has the 
form of a canoe ; and 
a similar anticline • the 
form of an inverted 
canoe. 

Geosynclines and 
Geanticlines. — Some 
areas of the earth's crust subside for a long period of time, as was the 
case in the western Appalachian Mountains before these mountains 

2D 




Fig. 264. — Anticline, above; Syncline, middle; and 
Monocline, below. The first two greatly modified by 
erosion. (After Willis.) 



4 02 COLLEGE PHYSIOGRAPHY 

were uplifted, and for such areas Dana proposed the term geosyncline 
or earth syncline. The Appalachian geosyncline was a trough of de- 
pression through several geological ages before uplift took place, and 
during this subsidence over 25,000 feet of strata were laid down, later 
to rise in a series of mountain folds. The opposite condition of long- 
continued rising is a geanticline. 

In a much folded series of strata there are groups of folds, roughly 
parallel. If such a group is in general anticlinal, though including 
both anticlines and synclines, it is called an anticlinorium ; if syn- 
clinal, it is called a synclinorium. 

Domes and Monoclines. — In some parts of the earth the strata 
are raised in domes, as where lava has been thrust into the crust, lift- 




Fig. 265. — Anticline, syncline, crumpled or contorted strata, faults, and an igneous dike 
in southwestern Alaska. (Stanton, U. S. Geol. Survey.) 

ing the rocks above. In such cases the strata dip in all directions 
from the centre of the dome. Another type of fold is the monocline, 
in which there is a single sharp bend (Fig. 264), as is common in the 
plateau country of southwestern United States, where the horizontal 
strata are interrupted by an abrupt dip, then the horizontal position 
is resumed. Monocline folds are commonly associated with faulting, 
and often merge into faults. 

Relation of Folds to Topography. — All these types of folding are 
definite proof of diastrophism, for the rocks of the crust have clearly 
been deformed as a result of stresses applied in crustal movements. 
They cause notable effects on the topography, first by the general 
upward or downward movements, as a result of which elevations and 
depressions of portions of the surface are caused, and, secondly, by 
inclining at various angles rock strata differing in character and in 
degree of resistance to the agents of denudation which are engaged in 
the task of reducing the elevations. 




MOVEMENTS OF THE EARTH'S CRUST 403 

Nature of Faulting. — When the strain in diastrophic movements 
is applied too rapidly, or in too brittle rocks, or in strata not heavily 
enough weighted by superincumbent load, breaking, instead of fold- 
ing, will result. This breaking is naturally more apt to be common 
at or near the surface than at considerable depths below the surface ; 
and it is very probable that superficial breaking commonly grades 
downward into folding. In a given rock a strain slowly applied may 
cause folding, while the same strain rapidly applied causes breaking ; 
a given strain, applied at a given rate, may cause folding in one stra- 
tum and breaking in another ; and a given strain, in a given rock 
applied at the same rate, may cause breaking under the atmospheric 
pressure, and folding under the pressure of a thousand feet of strata. 

Fault Planes. — The breaks produced by stresses during crustal 
deformation are known as faults, and the plane along which the slip- 
ping occurs is the fault plane (Fig. 266). 
The fault plane may lie vertically, or 
at any angle from this to horizontal, 
and the movement along the fault plane 
may be vertical, or horizontal, or di- 
agonal. There may be movement on 
both sides of the fault plane, or on only 
one side. Ordinarily there is a vertical 
element in the movement along a fault Fig. 266. — a fault plane with the 
plane so that one side is. left higher ^UelT (Poweuf 1 * * ****" 
than the other. The higher side is 

called the upthrown side, the lower side the downthrown ; but this 
does not mean that one side has been thrown up and the other down, 
for either a downward movement on one side of the fault plane, or 
an upward movement on the other side, will give the same result. 
The fault plane may be a single break, though more commonly there 
are numerous parallel breaks close together, and the rock along the 
fault plane is often crushed and broken, giving rise to a fault breccia. 
As the moving rock grinds together, the walls are commonly polished 
and grooved, giving the appearance called slickensides . 

Step Faults and Graben. — There is great variety in faulting, ac- 
cording to the angle of inclination, or hade, of the fault plane, and to 
the position of the strata crossed by the fault plane. There are 
parallel faults, or step faults (Fig. 267), giving rise to steps in the 
strata that are raised or lowered by faulting ; there are parallel faults 
between which a block of the earth's crust has settled, forming a 
graben or trough fault (Fig. 268) ; and there are single faults. The 
latter may cross either horizontal or inclined strata, and in the latter 
case they may be parallel to the dip or may cross it at various angles. 

Normal Faults. — A large number of faults, called normal, or gravity 
faults (Fig. 269), have the fault plane inclined, or hading, toward the 
downthrown side. In such a fault the dislocated strata are separated 
by movement along the fault plane, so that there is not only a vertical 






404 



COLLEGE PHYSIOGRAPHY 




Fig. 267. — Step faults near Nunatak Glacier, made during Yakutat Bay earthquake of 

1899. 




ig. 268. Rift or graben fault mar Nunatak Glacier, Alaska. Photographed 14 years 
after the faulting took place. (Hatch.) 



MOVEMENTS OF THE EARTH'S CRUST 



405 



displacement or throw, but also a horizontal displacement, or heave, 
leaving the ends of the dislocated strata apart. In a normal fault a 
vertical line dropped from the end of a layer on the upthrown side will 
be separated from the end of the same stratum on the downthrown 
side by a certain horizontal distance. 

Overthrust Faults. — The opposite type of fault is the reversed 
fault, in which layers are thrust over one another. In this case a verti- 
cal line dropped from the end of a layer on the upthrown side will pass 
through the same layer on the downthrown side. In reversed faults 
the angle of hade is often so great that the fault plane approaches the 
horizontal, and the layers are thrust over one another along these 




Fig. 269. — Normal fault with throw of over 2,2 feet, made during Yakutat Bay earthquake 
of 1899. Photographed 6 years after faulting took place. After 8 years more this 
fault was much more obscured by talus. 

planes of low inclination. Such a fault is an overthrust fault, common 
among many mountain regions, where older strata are sometimes 
thrust forward over younger for a mile or more. In an extreme case, 
as in the Rocky Mountains of Glacier National Park, the overthrust 
may be 7 to 12 miles. Such thrust faulting is now known to be a 
normal feature of many mountains, like the Alps, the Scottish High- 
lands, and Scandinavia (Chap. XV). It proves great crustal move- 
ments accompanying mountain deformation and great horizontal 
transfer of upper layers of the crust. 

Horizontal Movements along Faults. — Movements along verti- 
cal and highly inclined fault planes may also involve horizontal trans- 
fer of portions of the crust, as was the case during the California 
earthquake of 1906, when the surface on one side of the fault plane 
300 miles in length was shifted horizontally from 8 to 20 feet (Fig. 270). 



406 



COLLEGE PHYSIOGRAPHY 



More commonly the surface is raised or lowered by either upward 
or downward movement on one side of the fault plane. Such move- 
ments are actually observed during the earthquakes ; and, after the 
movement is over, the surface is left permanently higher on one side 
of the fault plane than on the other. In the Japanese earthquake 
of 1 89 1 both movements were observed in connection with a fault 
plane 40 miles long, on one side of which the surface sank from 2 to 
20 feet, while there was a lateral shift of 13 feet in places. (Fig. 282.) 




Fig. 270. — Horizontal movement along fault line in California. Before the earthquake 
of 1906 the two parts of the fence in the foreground were continuous and in the same 
straight line. (Gilbert, U. S. Geol. Survey.) 



Great Faulting accomplished Slowly. — Evidence of movements 
accompanying faulting is furnished by geological study of strata in 
all parts of the lands, but especially in mountain regions, the throw in 
some cases amounting to thousands of feet. It is not to be inferred 
that such movements occurred in a brief period of time, but rather 
that, as the stresses were applied, successive slippings occurred until 
in the course of long periods of time a great total throw was attained. 
Doubtless the faults in which present-day movements are observed 
give a clear indication of the nature of the movements by which the 
great faults of past ages were formed. 

Fault Scarps, Rift Valleys, and Horsts. — Faulting produces a 
direct effect on topography by forming a cliff, or fault scarp, on one 
side of the fault plane. Such fault scarps are often developed during 



MOVEMENTS OF THE EARTH'S CRUST 



407 



earthquakes, as in Japan in 1891 and in Alaska in 1899. If the move- 
ment continues, the scarp may rise higher and higher, giving rise to 
a pronounced cliff, of which there are many instances in mountains 
and plateaus of recent uplift, as in the Basin Ranges of the Great 
Basin and in the Colorado Plateau. Davis has emphasized the con- 
trast between fault scarps, due to 
diastrophic movement by faulting, 
and fault-line scarps., due to erosion 
of a faulted structure. Fault scarps 
may even be completely obliter- 
ated by erosion, whose continued 
action later produces a fault-line 
scarp facing in the opposite direc- 
tion (Fig. 271). 

Linear depressions known as rift or graben valleys may also be 
formed by diastrophism as in the Coast Ranges of California, the 
Dead Sea valley and elsewhere. The sinking of great blocks of the 
crust between fault planes gives rise to a large graben, like the Low- 
lands of Scotland, and the broad valley of the upper Rhine. It has 
been shown that the Lake Superior basin is a graben (Fig. 272), at 
least its western portion, due either to recent faulting, or, more probably, 




Fig. 271. — Fault scarp BC, obliterated by 
erosion F, fault-line scarp G. (Davis.) 




K EWE ETNA WAN 



EWEENAWAN 



Fig. 272. — The rift valley or graben of western Lake Superior. 



to ancient faults whose escarpments have been buried beneath sedi- 
mentary strata and resurrected by later denudation. Even the Medi- 
terranean depression is explained as the result of the subsidence of a 
portion of the crust between a series of fault planes; and similar 
subsidences are doubtless also in progress in parts of the ocean bot- 
tom, in intermontane valleys on the land, and along the fronts of 
growing mountains. Portions of the sea bottom may be raised to form 
peninsulas or islands, and coast lines may be raised above the sea, as oc- 
curred in the Yakutat Bay region 
of Alaska in 1899. Up faulted 
blocks are called horsts (Fig. 273). 
They may also be due to down- 
faulting on both sides of a sta- 
tionary block of the earth's crust. 
Relation of Faulting to Topography. — Indirectly faulting is also 
important in modifying topography. In some places the fault plane 
serves as a guide to drainage, especially where a series of parallel or 
branching faults so crush the rock as to render it weak; but the 
guidance of streams by this cause is much less important than was 



l^Kt VATTLRN 




Fig. 273. 



A horst in southern Sweden, 
(de Geer.) 



4o8 



COLLEGE PHYSIOGRAPHY 



at one time thought, for the fault is usually only a narrow break, and 
is a much less efficient cause for influencing stream erosion than other 
causes, such as variation in strata. In fact, streams very often cross 
great fault planes, or flow parallel to them without actually coinciding 
with them. 

One very important influence of faulting is the placing of strata of 
varying degrees of resistance within the reach of the agents of denuda- 
tion. They are then attacked and worn away irregularly. Thus 
a fault scarp wastes away as it rises, and when movement ceases it 
is given over wholly to the attack of the agents of denudation. The 
cliff recedes from the fault plane, and its form varies with the influ- 
ence of the component strata, so that ultimately it may be far removed 




Fig. 274. — Joint planes near Ithaca, N.Y. 



from the fault plane with which it was originally associated. It may, 
indeed, be worn down to such low relief as to lose all resemblance 
to a fault scarp, and the position of the fault plane be recognizable 
only after careful geological study of the strata. Many fault planes 
find no surface expression in the topography ; and others are indicated 
in the topography only by the differences in form of the land, caused 
by the influence of the strata on the two sides of the fault plane as 
etched out by denudation. 

Nature of Joint Planes. — The cooling of lava rocks, causing con- 
traction, gives rise to internal strains in the rocks as a result of which 
breaking occurs along planes called joint planes. Drying of sediments 
also causes contraction and jointing (Fig. 274). But much more im- 
portant than either of these causes for joint planes is the introduction 
of strains, either of tension or compression, as a result of which the 
rock breaks along a series of planes. Such joint planes are developed 



MOVEMENTS OE THE EARTH'S CRUST 409 

in all classes of rock, and they give rise to natural breaks, often of 
great regularity and definiteness of direction, as a result of which the 
rock is traversed by planes which divide it into rectangular or rhombic 
blocks. These planes may be far apart or near together, according 
to the nature of the rock and the intensity of the strain, sometimes 
traversing the rock so close together that it is broken into a multitude 
of small blocks a few inches in area. 

Joint planes do not necessarily signify any visible motion, such as 
occurs along fault planes ; but there is good evidence that there is 
sometimes actual motion along the joint planes. Associated with 
jointing, either during its formation or subsequently, there is often 
a crushing, and in much jointed rocks there are frequently broken, or 
brecciated, areas or zones. 

Relation of Joints to Erosion. — All these forms of breaking are 
highly important in guiding the work of the agents of denudation, as 
we have seen. The joint planes offer paths for the entrance of per- 
colating waters ; they ' are seats of solution, chemical change, and 
frost action; they aid in the plucking action of glacial erosion, in 
the tearing out of blocks by wave attack, and in the erosive work of 
streams. Very often the topographic detail of coast line or gorge 
wall is joint-determined; and everywhere joint planes are guiding 
and aiding in denudation. Joint planes are one of the most impor- 
tant single factors of rock structure in influencing the shaping of the 
earth's surface under the agencies of denudation; and we may be 
certain that, without such influence, the land would waste away much 
less rapidly, and the surface features would be far different. 

Earthquakes 

Nature of Earthquakes. — Delicate instruments, known as seis- 
mographs, reveal the fact that tremors, not detected by the senses, 
are of common occurrence ; and it is a well-known fact that more vigor- 
ous shaking, known as earthquakes, occurs every now and then, at 
times attaining such force as to cause great destruction of both prop- 
erty and life. It is probable that no moment passes without some 
tremor or quake occurring in some part of the earth. They originate 
in all parts of the earth, both on the land and on the sea bottom, but 
they occur far more commonly in some parts of the earth than in 
others ; there are, indeed, well-defined earthquake belts in which by 
far the greatest number of shocks, and practically all the violent ones, 
occur. 

Causes of Earthquakes 

Small Shocks in Relation to Man's Activities. — Any jar that 
arises within the earth or on its surface is a cause for an earthquake, 
using the term in its largest sense and including even the most minute 
tremors. Thus the rumble of a heavily loaded cart over a paved 






410 COLLEGE PHYSIOGRAPHY 

street starts a series of tremors that may be measured by a seismo- 
graph near by, and which may be even detected by the senses. An 
explosion sets in motion more vigorous earth waves, as was the case 
with the great explosion at Hell Gate some years ago, when a small 
earthquake was artificially generated, which was measured on the 
instruments at Cambridge, Massachusetts, nearly 200 miles distant. 

Other Causes of Minor Earthquakes. — The descent of an ava- 
lanche, or the falling of a mass of rock from a sea cliff will also cause 
a small earthquake. For example, the people of Niagara Falls are 
often made aware of the fall of a piece of limestone from the crest of 
the cataract by a trembling of the ground. Another cause for earth- 
quakes is the falling of a portion of a cavern roof, a cause observed 
in England ; and another is the snapping of rocks under strain. The 
latter cause is illustrated in the granite quarries of Monson, Mass., 
where, when the rock is stripped off, and pressure thus removed, 
strain in the layers finds relief by snapping, sending an earthquake 
tremor through the quarry. Doubtless this cause is in common oper- 
ation in regions of denudation where, by the removal of overlying 
load, relief from strain becomes possible by bending and breaking of 
the layers on which the load hitherto rested. Subterranean move- 
ments of imprisoned gases or liquids, and change in temperature of 
the rocks or the ground are other causes for earth tremors. 

Two Classes of Great Earthquakes. — These and other causes are, 
without doubt, operating to cause small earthquakes ; but the great 
majority of earthquakes, and all or nearly all of the really destruc- 
tive ones, are due to causes associated either with diastrophism or 
vulcanism. Those due to diastrophism are called tectonic, those due 
to vulcanism may be called volcanic shocks. 

Tectonic Shocks. — Tectonic shocks are the direct result of move- 
ments associated with crustal deformation. When slipping occurs 
along a fault plane, there is a disturbance of the earth (1) by friction 
along the fault plane, (2) by breaking and crushing of the rocks, and 
(3) by the movement of the upraised or down-sunken layers. There 
is reason for believing that in great movements along fault planes there 
is also transfer of deep-seated rock by a process analogous to rlowage. 
A tectonic shock may be only a minute tremor, generated by a slip- 
ping of minute extent ; or it may be a movement involving many 
cubic miles of crustal layers, and giving rise to such disturbance that 
the earth near by is violently shaken, while waves sweep outward 
and completely encircle the earth so that the occurrence of a violent 
earthquake is recorded on the seismographs of the antipodes. Such 
great tectonic shocks consist of a complex series of waves generated 
by the slipping and breaking along the fault plane, and the bodily 
movement of great masses of rock. The plane along which the slip- 
ping occurs may extend for scores or even hundreds of miles ; and the 
depth of the movement may reach thousands of feet into the earth. 
Tectonic shocks are the greatest of earthquakes, and the largest are 



MOVEMENTS OF THE EARTH'S CRUST 411 

truly world-shaking, though perhaps not noticeable to the senses 
except within a few hundred miles of the centre of origin. Tectonic 
shocks are also the most common of earthquakes, for the strains of 
crustal deformation are widespread, both on the lands and on the sea 
bottoms. They naturally occur most abundantly and with most 
vigour in belts of growing mountains, for there the strains of crustal 
deformation are most concentrated. 

Volcanic Shocks. — Volcanic shocks are also common, and abound 
on and near volcanoes either at present active, dormant, or recently 
extinct. They result (1) from the explosion of an active volcano, (2) 
from the subterranean movements of lava, seeking escape. Vol- 
canoes that have been reduced by denudation often reveal the presence 
of fissures rilled with solidified lava, known as dikes. When these 
fissures opened it is possible that the earth was shaken in the neighbour- 
hood, and the inward rush of the molten lava must also have caused 
a disturbance in the rocks round about. Earthquakes are common 
in the neighbourhood of volcanoes before an outbreak, and these are 
doubtless due to the intrusion of dikes and other movements of molten 
rock. When the final outburst occurs, there may be a great and violent 
earthquake in the region about the volcano, as was the case in the great 
eruption of Krakatoa in the Straits of Sunda in 1883. 

Violent though such earthquakes sometimes are in the immediate 
neighbourhood of the volcano, they are not in the same class with the 
great tectonic shocks, for there is less material involved in the move- 
ment, and the area of disturbance is more limited. The volcanic 
shock is caused by movements within a limited area around and 
beneath the volcanic vent ; the tectonic shocks may involve move- 
ments along a linear belt scores or hundreds of miles in length, 
probably extending as deep, if not deeper, than the volcanic move- 
ments. It is not meant to intimate that great destruction may not 
occur at or near the centre of a violent volcanic earthquake ; but 
merely that the area of violent shaking is more limited, and the world- 
shaking character is less noticeable in the great volcanic than in the 
great tectonic shocks. 

Nature op the Earthquake Shaking 

The Focus and Epicentrum. — The nature of the movements in an 
earthquake shock will most easily be understood if we consider the 
impulse to start from a point, or from a single small area, as is undoubt- 
edly the case in many earthquakes, notably the volcanic and minor 
tremors. A jar applied to such a point generates a series of elastic 
waves, which spread outward in all directions from the centre, or 
focus, as a series of waves will pass through a stone which is struck 
a blow with the hammer. If the medium through which the waves 
pass is assumed to be uniform, these waves will spread with equal 
rapidity in all directions, but will gradually lose in intensity equally 






412 COLLEGE PHYSIOGRAPHY 

in all directions from the centre of disturbance. Therefore the nearer 
the focus the sooner the shock is felt and the greater its violence. 

Passing through the earth, the waves will in time rise to the surface, 
reaching it first and with greatest violence directly above the focus. 
This point is called the epicentrum. The origins of earthquakes are 
often very deep below the surface, the depth of focus of the Cala- 
brian earthquake of 1857 being estimated as 5 miles and others being 
calculated at depths up to 14 miles. 

Coseismals and Isoseismals. — The shock diminishes in violence 
in all directions and the time of appearance of the waves becomes 
later and later with increasing distance from the epicentrum. A series 
of lines connecting places at which the shaking appears at the same 
time are called coseismals, and lines passing through places of equal 
intensity of shaking are called isoseismals. The isoseismal lines are 
often roughly circular and concentric around the epicentrum. 

Complexity of Earthquake Movement. — As a matter of fact the 
phenomena of the transmission of earthquake waves is far less simple 
than this assumed case, especially in the great earthquakes originat- 
ing as tectonic shocks. Instead of a single wave or related series of 
waves generated from a single point, there may be a multitude of waves 
of varying strength, generated from many points and planes within 
the epicentral area, which may extend scores of miles, and reach 
thousands of feet into the earth. These waves, with different ampli- 
tudes, and from different centres, pass through strata of different 
kinds. Thus there is, in reality, a complex of waves. In a great 
earthquake the ground may be shaken for several minutes, with a 
violence varying as the different waves reach it, and at times reaching 
such strength that a person is thrown to the ground, and strong 
buildings are rent asunder. One shock may succeed another at in- 
tervals of several minutes, or hdurs, or days, as further motion takes 
place along the fault plane, or as adjustments occur in the disturbed 
strata. There is perhaps no natural phenomenon to which man is 
subjected that is more terrifying than a violent earthquake, and even 
wild beasts are subdued by terror during such a convulsion of the 
normally stable earth. 

Destructive Effects 

Changes in the Earth's Surface. — During the passage of the waves 
of a violent earthquake the ground is so shaken that unstable objects 
may be overthrown. Thus trees are overturned and avalanches 
are caused, sometimes forming temporary lakes. Loose earth is 
shaken down, and depressions and elevations are introduced in the 
loose soil. Fissures open and close, water is squeezed out of the ground, 
springs have their supply cut off, and even stream flow is interfered 
with, so that brooks dry up and later resume their flow. The water 
in lakes may rise and fall for several hours, even far from the shock. 



MOVEMENTS OF THE EARTH'S CRUST 413 

With the disturbance of underground water, there is often an eruption 
of sand from craterlets or sand vents on the surface. Leading up to 
these are cracks filled with sand, known as sandstone dikes. Along 
the fault plane the surface may be permanently raised or lowered on 
one side ; but this is an accompanying phenomenon, not a result of the 
earthquake ; it is a surface expression of the movement by which the 
earth shaking is generated. 

Destruction of Buildings. — Where the epicentrum or fault line 
passes through a settled country, there is introduced the great danger 
of falling buildings and subsequent fire, as a result of which vast de- 
struction of life and property have been brought about. This danger 
decreases rapidly with distance from the source of the shock ; it also 
varies with the nature of the underlying rock. Made ground and 
loose, unconsolidated strata are far less secure than solid rock, for, 
added to the direct shaking due to the earthquake waves, is the set- 
tling and movement and fissuring of the unstable foundation. Even 
the strongest building may succumb to the combined shaking and 
undermining of an unstable foundation. 

There is much difference in the effect of earthquakes according to 
the construction of the building. Old houses of massive construction, 
with heavy floors and roofs, undoubtedly led to a large part of the 
terrible destruction during the Messina earthquake of 1908 ; and in 
San Francisco there was noticeable difference in destructiveness of the 
1906 earthquake, not only in relation to stability of foundation, but 
also according to the construction (Fig. 275). The Japanese, living in 
an earthquake country, have given careful study to the subject of earth- 
quake-proof buildings, and their light, low, bamboo houses are able 
to resist all but the most severe shocks. In Italy, too, thought is 
now given to construction of earthquake-resisting structures in the 
Calabrian region of the southern part of the peninsula, which has 
been visited by a succession of earthquakes of great destructive- 
ness. 

Perilous Location of Towns. — Attention ought also to be paid 
to the question of location of towns in earthquake countries. There 
are towns and even cities built on or close by fault lines, along which 
movements are known to have occurred, and where it is practically 
certain that other movements will take place. With the present 
knowledge of earthquake cause it is certainly folly to tempt fate by 
rebuilding on a fault line a town that has been destroyed by an earth- 
quake generated through movement along such a fault. In Italy 
some village sites have been abandoned by government order since 
the Messina earthquake of 1908. 

Seismographic Records 

Movement of Earth Waves. — The establishment of stations in 
which seismographs are kept in various parts of the earth is adding 



414 



COLLEGE PHYSIOGRAPHY 



greatly to our knowledge, not only of the distribution of earthquakes, 
but of the condition of the earth's interior. The seismograph is so 
made as to magnify and automatically record any vibration that 
passes through the earth on which it rests. It is found that the waves 
of a great earthquake (Fig. 276) pass around the earth, in both direc- 




Fig. 275. — Dwellings displaced during the San Francisco earthquake of 1906. Not being 
adapted to standing the shock the three-story house lurched off its foundation, while 
those to the right were unharmed. (Gilbert, U. S. Geol. Survey.) 



tions, and may even make a second circuit, travelling with a velocity 
of a little over two miles a second. Other waves travel through the 
earth at the rate of 6i miles per second, so that they are able to pass 
through the earth along one of its diameters in about 20 minutes. 
From their rate of travel and the uniformity of the velocity it is in- 
ferred that the earth's interior is of somewhat uniform composition 
and is one and a half times as rigid as steel. 



MOVEMENTS OF THE EARTH'S CRUST 415 

The upward motion of a particle in an earthquake wave may be 
as little as 5 or 6 millimeters, or less, though with even this slight 
motion chimneys will be thrown down. 

Location of Epicentra. — The seismograph records many earth- 
quakes that would otherwise be unknown, such as those occurring 
in unsettled regions or on the sea floor. It records the intensity and 
duration, and from the records of three or more stations it is possible 
to determine both the position of the earthquake and the time of its 
occurrence. Thus it is now frequently stated that a vigorous earth- 



.vuy-i^i— 



Sei&moaroms of the great Yakutat Bay, earthquake, Sept. A 
as recorded ar Cafan/a, ffa/y. 



ww#j(!W^^ ||| i W 



t W*^vf^Wyw~v^' — » 



Sei'ssnooroms of fhe Son franc/SCO, Co//r"omja, eortfiquoke Apr /3 /906 
as recorded or Co/on/o, fto/y. 



WWW 1 ^v-A^~rfr~~ w-jw: 



Fig. 276. — Seismograph records from an instrument at Catania, Italy, showing the records 
of vibrations of the earth's crust after the Yakutat Bay earthquake of 1899 and the 
California earthquake of 1906. 

quake occurred at a certain time and place, perhaps even before the 
world has been notified of its occurrence by telegraph. 

Distribution of Earthquakes 

Scores of Earthquakes Daily. — It is estimated that there are 
30,000 earthquakes every year that are recognizable by the senses. 
Most of these are very light, and only a few are of the first order; 
but every year there are some shocks of great violence, and now and 
then £>ne of these occurs in a settled region where sufficient destruc- 
tion is accomplished to attract world-wide attention. It does not 
follow, however, that such shocks are the most violent ; for their 
notoriety may be due rather to the accident of location than to ex- 
ceptional vigour. Every year great earthquakes pass unheeded by all 
but seismologists because they happen to occur where no human life 
could be lost. 



416 



COLLEGE PHYSIOGRAPHY 





Fig. 277. Zones of most frequent earthquakes, in black, (rie Montessus de Ballore.) 



MOVEMENTS OF THE EARTH'S CRUST 417 

The Two Belts of Earthquakes. — While earthquakes of minor in- 
tensity may occur in any place, the great majority of recorded shocks 
occur in two well-defined belts, or great circle zones. A few areas 
of frequent earthquakes lie outside these belts, and occasional great 
earthquakes have occurred in these outside areas. These facts have 
long been recognized in a general way, but it remained for Count de 
Montessus de Ballore to give it numerical proof on the basis of the 
great series of earthquake records that have been accumulated. He 
has studied and tabulated the records of no less than 170,000 earth- 
quake shocks; and on the. basis of these studies has put out the two 
maps reproduced here as Fig. 277. From these maps it is clear that 
there is one belt of abundant earthquakes encircling the Pacific, and 
another in a great circle approximately east and west around the 
earth, through the Mediterranean, southern Asia, the East Indies, 
and the West Indies. 

Most Shocks within these Belts. — De Montessus finds that 41.05 
per cent of all recorded shocks occur in the circum-Pacific belt, while 
53.54 per cent occur in the other belt. Thus 94.59 per cent of 170,000 
earthquakes studied occurred in these two belts, forming but a small 
part of the earth's surface; and 
only 5.41 per cent occurred in all 
the rest of the earth. Doubtless 
future study will cause some mod- 
ification of this conclusion, now. 
that seismographic records reveal 
to us the location of submarine 
earthquakes and tell us of earth- ^cvy^ «« 4 «»< 

quakes in uninhabited parts of «**- i^-— ■••- '*'." 
the lands. For example, it is cer- 
tain that a future map will assign Fl ? hr ^ h - la T c ^ ^gJiM&ETS 
greater seismicity to Alaska than the years listed. (Suess.) 
de Montessus has given it on the 

basis of existing records ; and seismic regions will without doubt be 
added in the oceanic areas. But it is equally certain that future maps 
will bring out with similar clearness the two great earthquake belts 
which he has demonstrated ; and that they will show the greater part 
of the earth to be relatively immune to earthquakes of vigorous 
character. 

Relation to Mountain-making and Vulcanism. — The reason for 
these two belts is not difficult to see. These are belts in which moun- 
tains are now in most active process of growth, and in which, there- 
fore, diastrophism is giving rise to those movements by which tec- 
tonic shocks are generated. It is in these belts, too, that most of 
the active volcanoes of the world lie, and, accordingly, it is here that 
the volcanic earthquakes must develop in greatest numbers. What 
causes the mountain growth and the location of volcanoes in these 
belts is a far more difficult question, and one to which definite answer 

2E 




4 i8 COLLEGE PHYSIOGRAPHY 

cannot be given. It is noteworthy, however, that, in earlier ages, 
growing mountains and volcanic activity were distributed along dif- 
ferent belts ; for instance, when mountains rose and chains of vol- 
canoes existed in eastern United States, in England, central France, 
and northwestern Germany. Then the belts of greatest earthquake 
intensity were doubtless far different from now. 

That some regions are relatively immune from vigorous earth- 
quakes is due to the fact that diastrophism and vulcanism are either 
absent or only moderately active. Minor shocks may rise from local 
causes, like those mentioned in discussing earthquake cause (p. 409) ; 
and if diastrophism is still in progress locally, as is apparently the case, 
even vigorous shocks may occur in regions outside the great earth- 
quake belts. The earthquake belts lie in regions where, for some 
reason as yet unknown, earth movements are concentrated inten- 
sively ; but some movement, less intense and less widespread, is 
occurring in other parts of the crust, and there occasional slipping 
gives rise to earthquakes, either great or small. 

Earthquake Periodicity 

The Desirability of Predicting Earthquakes. — It is a matter of 
high importance to determine whether there is any recognizable 
periodicity of earthquake occurrence, upon the basis of which it may 
be possible to make predictions. Up to the present time it is not 
possible to demonstrate such periodicity ; but there are signs of a 
beginning which holds out promise of important future results, now 
that a large body of students are working upon seismological problems 
in all parts of the world. 

Possible Relation to Atmospheric Pressure and Terrestrial Tides. — 
It has been suggested that there is a relation between variations in 
atmospheric pressure and earth shaking ; and it seems reasonable 
to believe that, if a strain has been applied almost to the slipping 
point, change in the weight of air pressed down on that portion of 
the crust may give the necessary last cause for the movement. Varia- 
tions in attraction upon the earth occur as the moon and sun change 
in relative position, as is well known from the phenomena of oceanic 
tides. Such variations may give the necessary last cause for slipping 
along a fault plane on which strain is already concentrated. It 
cannot be said that either of these causes is at present demonstrated ; 
it will require a careful tabulation of a large mass of data to give such 
demonstration. 

Relation to Shifting of the Poles. — Within recent years it has been 
proved that the pole is steadily changing position along a somewhat 
irregular path. As it changes there is necessarily a constant ten- 
dency for the earth form to change in adjustment to the new axis of 
rotation ; and at certain points in the curve of the polar path there 
is such a change that there may well be sudden applications of press- 



MOVEMENTS OF THE EARTH'S CRUST 419 

ure on parts of the crust. Milne has announced his belief that there 
is a well-defined periodicity of earthquake intensity related to this 
polar movement. A tabulation of the earthquake shocks of Japan 
indicate that periods of great seismic activity recur once in about 
13 years ; and for the city of Kioto, once in about half that time, or 
6i years. 

Theory of Alternation. — The eminent Japanese seismologist, 
Omori, has worked out a law, first applied to Japanese and Formosan 
earthquakes. This is that the stress applied along one of the great 
earthquake belts, on finding relief by an earthquake movement, will 
not for a time affect that neighbourhood ; but when next the stress 
finds relief, it will be at a distant point along the belt. Upon the basis 
of this law he made the prediction shortly after the California earth- 
quake of April 18, 1906, that the next great earthquake in that belt 
would occur in South America south of the equator. Immediately after 
this prediction came the great Chilean earthquake of August 17, 1906. 

Relation to Vulcanism. — There are indications that there is sym- 
pathetic relationship between diastrophism and vulcanism, and that 
the mobile zone of flowage is affected by large, general causes, which 
react upon the rigid zone of fracture. The laws of behaviour of this 
mobile zone are not yet clear, but a beginning has been made, and in 
the study of seismology seems to lie one of the chief hopes for their 
discovery and demonstration. When these laws are understood, it 
may be possible to predict the times and places of occurrence of earth- 
quakes and thus lead to great saving of life. Already the great zones 
of seismic intensity are marked out ; and the location of many of 
the earthquake " rifts," or fault planes, is known. If the time when 
movements are likely to take place is known, and if the premonitory 
signs are understood and recorded, there need be no such terrible 
disaster as the world is made familiar with every now and then. 

Elastic Rebound Theory. — Careful surveys of the region near 
San Francisco before and after the 1906 earthquake prove that for 
perhaps a century a slow northward movement had been taking place 
under the Pacific Ocean and in a strip along the west coast. This 
set up a shearing strain which finally became so great that faulting 
was renewed along an old line of fracture. The two sides then sprang 
back into positions of equilibrium, the rebound being distinguishable 
only within about 6 miles of the fault. Upon this is based Reid's 
elastic rebound theory of earthquakes. It may be stated as follows : 

" 1. The fracture of the rock, which causes a tectonic earthquake, 
is the result of elastic strains, greater than the strength of the rock 
can withstand, produced by the relative displacements of neighboring 
portions of the earth's crust. 2. These relative displacements are 
not produced suddenly at the time of the fracture, but attain their 
maximum amounts gradually during a more or less long period of 
time. 3. The only mass movements that occur at the time of the 
earthquake are the sudden elastic rebounds of the sides of the fracture 



420 COLLEGE PHYSIOGRAPHY 

towards positions of no-elastic strain; and these movements extend 
to distances of only a few miles from the fracture. 4. The earth- 
quake vibrations originate in the surface of fracture ; the surface from 
which they start has at first a very small area, which may quickly 
become very large, but at a rate not greater than the velocity of com- 
pressional elastic waves in the rock. 5. The energy liberated at the 
time of an earthquake was, immediately before the rupture, in the 
form of energy of elastic strain of the rock. 

" These statements, which may be called the elastic rebound theory 
of tectonic earthquakes, do not broach the original cause of earth- 
quakes, which lies in the source of the slow movements accumulating 
the elastic energy, but merely give the modus operandi of the accu- 
mulation and liberation of this energy." This theory offers promise of 
earthquake prediction. 

Specific Instances 

The Lisbon Earthquake. — One of the most terrible of recorded 
earthquakes occurred on November 1, 1755, at Lisbon, Portugal. 
A noise like thunder was first heard, then came a violent shock which 
threw down a large part of the city. The sea drew away from the 
land, then rolled in, rising 50 feet or more above the normal level. 
In less than six minutes 60,000 people perished. A large number 
of people gathered on a pier, or quay, to escape the danger from fall- 
ing buildings ; but a fissure opened beneath it, and it is said that it 
sank with all the people, and a number of vessels were drawn into the 
whirlpool and sank out of sight. The depth of water where the quay 
stood is said to have been 600 feet. The great water wave swept 
over the neighbouring coast ; huge avalanches descended from the 
mountains ; and the shock was felt as far away as Sweden, North 
Africa, and the West Indies. Previous earthquakes occurred in 
Lisbon in 1309 and 1531. 

Southern Italy. — Earthquakes occur throughout Italy, some tec- 
tonic in character, others volcanic. Of the latter may be mentioned 
the earthquake which destroyed the town of Casamicciola on the 
volcanic island of Ischia in 1883. Although completely destroying 
the town, and much life, no damage was done in the city of Naples 
only 22 miles distant — showing clearly how limited is the area of 
destruction of volcanic shocks. Earthquakes are common before and 
during eruptions of Vesuvius and Etna. 

Far more widespread and destructive are the Calabrian earthquakes 
south of Naples, where great tectonic shocks have occurred on nu- 
merous occasions. The earthquake of 1688 destroyed 20,000 lives; 
that of 1693, 43,000 ; that of 1783, 32,000, and there have been a series 
of shocks in the region down to that of 1905 in which 800 lives were 
lost, and the Messina earthquake of 1908 in which 100,000 lives are 
said to have been lost. In each of these great earthquakes the dis- 



MOVEMENTS OF THE EARTH'S CRUST 421 

tribution of destructiveness has been along lines or narrow belts, 
one of which is the Strait of Messina, located on a fault line. It is 
not surprising, therefore, that Messina has been visited by a series 
of violent earthquakes, the last of which so completely devastated 
the city. It is a region in which crustal movements are actively in 
progress, and between two volcanic areas. There is indication that 
there is relationship between volcanic activity and diastrophic move- 
ments in this locality. 

The Calabrian earthquake of 1783, which received careful study, 
presented some interesting phenomena. The ground cracked open 
and closed, the surface heaved in great undulations, and people were 
nauseated by the motion. Large trees swayed so that their tops 
touched the ground, monuments were twisted by vorticose motion, 
thousands of fissures and circular pits were formed on the surface, 
and water was forced out of the ground. 

Other parts of Europe have earthquakes frequently, including 
Spain, England, Germany, and the Austrian Alps (Fig. 278). There 
are also frequent severe earthquakes in Asia Minor. 

Indian Earthquakes. — Throughout the eastern Mediterranean 
and western Asia earthquakes are abundant and often of great vio- 
lence ; and it is well known that the Asiatic region was visited by 
destructive earthquakes in the days recorded in the Bible. The earth- 
quake belt also extends through northern India. There was a great 
and destructive earthquake in the Indus valley in 181 9, the shocks 
recurring through a period of 4 days. A great tract of land sank 
and another portion rose. In a few hours a tract of 2000 square miles 
was transformed to an inland sea, and an area 50 "miles long, and in 
some parts 16 miles wide, was raised, to a maximum of 10 feet. A fort 
standing on the submerged area sank partly beneath the water. 

A great earthquake, known as the Assam earthquake, occurred in 
India in June, 1897. There was a violent initial shock, and in 15 
seconds practically all the destruction was accomplished, while the 
heavy shock had all passed in two minutes and a half. An area of 
150,000 square miles was laid in ruins. The ground was fissured, 
and movements occurred along fault lines, in one place with a throw 
of 35 feet. One fault line extended parallel to a winding stream course. 
Where it crossed the stream it formed small ponds in some places 
where the upthrown side of the fault caused a dam ; elsewhere where 
the stream fell from the upthrow to the downthrow side a waterfall 
resulted (Figs. 279, 280). The ground was heaved and moved, and 
railway tracks were twisted in a remarkable manner. 

Another earthquake, known as the Kangra earthquake, devastated 
a great extent of country in northern India, on April 4, 1905, de- 
stroying 20,000 lives. It spread from two well-defined centres, and 
was felt over an area of 1,625,000 square miles. In this case there 
was no visible faulting at the surface, though there was a slight up- 
ward bulging of the surface in one of the centres. 



422 



COLLEGE PHYSIOGRAPHY 



Japanese Earthquakes. — Between India and Japan earthquakes 
are frequent and often very destructive in the East Indies and in the 
Philippine Islands. Japan is a centre of great seismic activity, and, 
owing to the destructive effects (Fig. 281), the study of seismology has 
received great attention there. On the average there has been one de- 
structive shock in every two and a half years since the beginning of the 
seventeenth century, and there is record of 223 destructive shocks in the 




Sea/e. - » 

Fig. 279. — Ponds along fault scarp in India. 

(Oldham.) 



Figures show amounts of uplift in feet. 



last 1500 years. In addition, there are vast numbers of lighter earth- 
quakes, many detected only by the seismograph. Since 1885 there 
has been an average of 1400 shocks a year, or at the rate of about 4 
a day. 

Among these earthquakes that of October 28, 1891, known as the 
Mino-Owari earthquake, is notable as being one of the most destruc- 
tive, and the first large earthquake to receive careful study by a trained 

seismologist. It shook an area 
of 243,000 square miles, or more 
than three-fifths of the entire area 
of Japan, but the area of great 
destruction was far smaller and 
was confined to a plain in a basin 
among the mountains in which lie 
the provinces of Mino and Owari, 
a densely settled plain with nearly 
a thousand people per square mile 
and a multitude of village^. The 
earthquake came without warn- 
ing, and in a single minute 20,000 
buildings were thrown down, 7000 
people were killed, and 1 7 ,000 injured. Fire followed, as is often the case 
after earthquakes, and added to the destruction of life and property. 
As in the case of many earthquakes, there was a succession of after- 
shocks, 102 on October 28, 318 on October 29, and a decreasing num- 
ber for several months afterwards. Over 2500 shocks were recorded 
at the city of Gifu within the period of five months succeeding the 
great earthquake. 

The ground was cracked and fissured, mud volcanoes and sand 
craters were developed, and subterranean drainage was interfered 




#<- 



Fig. 280. — Waterfall over fault scarp, formed 
during Assam earthquake of 1897. (Old- 
ham.) 



MOVEMENTS OF THE EARTH'S CRUST 



423 



with. Even the light bamboo houses were thrown down, railroad 
bridges were broken and caused to collapse, and a railway track was 
bent into a sinuous, serpentine course. A long fault line, extending 
in a northwest-southeast course, was traced a distance of 40 miles by 
the disturbance of the surface. On one side of the fault the surface 
sank from 2 to 20 feet (Fig. 282), the fault line being marked sometimes 
by a cliff, sometimes by a cracked and fissured dome. There was also 
lateral shifting along the fault line, movement occurring in a northerly 
direction on the average from 3 to 6 feet, and in one place 12 feet. 




Fig. 281. — Building destroyed by one of the Japanese earthquakes. 

Alaskan Earthquakes. — The earthquake belt of Japan swings 
along the Aleutian chain to the Alaskan coast region, and throughout 
the Aleutian Islands and the coast of Alaska earthquakes are abundant, 
and often of great violence. Owing to the sparsely settled character 
of this 'region, little attention has been paid to these earthquakes. 
The single exception is the series of shocks that affected the St. Elias 
region in September, 1899, and that, originating in the Yakutat Bay 
region, are known as the Yakutat Bay earthquakes. Although no 
destruction of life occurred, and almost no damage was done to prop- 
erty, these are to be reckoned among the most notable of modern 
earthquakes. They were of such strength that they attracted at- 
tention in Europe from the seismographic records alone, and the place 
and time of their occurrence were determined even before anything 
was known about the remarkable phenomena associated with them. 

During these earthquakes, which occurred in a period of 27 days, 
from September 3 to 29, there were four or five that were world shak- 



424 



COLLEGE PHYSIOGRAPHY 



ing, and hundreds of minor shocks. There were especially violent 
shocks on September 3, 10, and 23, and strong earthquakes also on 
the 15th, 17th, 26th, and 29th. Two world-shaking earthquakes 
occurred on the 10th. The violent shocks were recorded on seismo- 




— Fault made during the Japanese earthquake of 1891. (Milne and Burton.) 



graphs all over the. earth, and an area of at least 400,000 or 500,000 
square miles was sensibly shaken, and perhaps 3 times that area. 

As already stated (p. 394), the neighbouring shorelines were notably 
deformed (Fig. 283), in one place being uplifted 47 feet, in other places 
being depressed. The crust was broken and moved along a series of fault 
planes and the mountain blocks tilted, a part of the movement of the 

growing St. Elias Range. Be- 
sides major fault lines there 
was a series of smaller fis- 
sures and faults, some with a 
throw of over 3 feet. Vast 
quantities of snow, ice, and 
rock were avalanched from 
the mountains, and, as a re- 
sult of this abrupt accession 
of supply to the reservoirs of 
the glaciers, a wave of ad- 
vance was started which dur- 
ing succeeding years swept 
down the glaciers and caused 
notable change and advance 
in the glacier ends. 
At least one great water wave swept through Yakutat Bay and tore 
up the forest to an elevation of 50 feet in some places (Fig. 284). Only 
a small group of prospectors were in the bay at the time, and their 




Fig. 283. — Elevated sea cliff and rock bench which 
was raised over 17 feet in Yakutat Bay earth- 
quake of 1899. 



MOVEMENTS OF THE EARTH'S CRUST 



425 



escape from death was marvellous. From them and from the inhab- 
itants of the native village at the mouth of the bay we have the only- 
direct knowledge of these earthquakes in this central area; but in- 
formation concerning the shaking in other sections has been obtained 
from a number of people. Both in Yakutat Bay and at Muir Glacier 
the vigorous shaking dislodged huge masses from the tidal glaciers. 

South American Earthquakes. — The western coast of South 
America is another noted seismic area, and has been the seat of repeated 




Fig. 284. — Forest destroyed by earthquake water wave, or tsunami; and scars of 
avalanches on mountain side, Yakutat Bay, Alaska. 



shocks, some of great violence and destructiveness. In Chile, for in- 
stance, a great earthquake occurred on May 24, 1751, and the coast 
was devastated by an earthquake water wave that rolled in upon it. 
A violent earthquake occurred in Peru on October 28, 1746, during 
which Lima was destroyed. Nineteen ships were sunk in the harbour 
of Callao, and most of the 4000 inhabitants of the city were destroyed. 
In 1687, 59 years earlier, Callao was also overwhelmed by an earth- 
quake and accompanying water wave. Chile was visited by other 
destructive shocks in 1822, 1835, and 1837. During the earth- 
quake of February 20, 1835, there was also a destructive water wave; 
and the island of Juan Fernandez 865 miles from Chile was also vio- 
lently shaken, while a submarine volcano broke forth about a mile 
from the shore. An earthquake in Peru and Ecuador in 1868 affected 
a strip of country 2000 miles long. Since then there have been other 
earthquakes on the western coast of South America, the last being 
the Valparaiso earthquake of 1906. 



426 COLLEGE PHYSIOGRAPHY 

The occurrence of the water waves proves that at least a part of 
the movement in the earthquake occurred beneath the sea. It is 
probable that there is subsidence of the sea bottom along this steeply 
rising coast ; and uplifted shorelines prove that there has also been 
recent rising of the land. It is a noteworthy fact that many of the 
belts of greatest seismic activity are on or in close association with 
steep slopes, along which subsidence is apparently in progress on one 
side and uplift on the other. 

West Indian Region. — Central America, northern South America, 
and the West Indies form another region of frequent earthquakes, 
some on the land, some on the sea floor. There have been numerous 
severe earthquakes in this region, some of them evidently volcanic, as 
in Guatemala, where the site of the capital, near the base of a volcano, 
had to be changed because of the frequency of destructive earth- 
quakes. Others are of tectonic origin. Such was probably the case 
in the earthquake which destroyed Caracas in 1812, and killed 10,000 
people in about half a minute. 

Earthquakes have occurred in islands and the surrounding waters. 
Jamaica has suffered especially in this respect. Up to 1692 the capital 
of the island, Port Royal, stood at the entrance to Jamaica harbour, 
partly on a low rocky point, partly on a sand bar connecting the rock 
with the main island. In 1692 a violent earthquake visited the is- 
land, causing the land to rise and fall " like a rolling sea," landslides 
fell from the neighbouring mountains, houses were thrown down, and 
fissures opened and closed, in some of which people were caught and 
entombed. The part of Port Royal that stood on the sand bar slid 
into the water, something like 1000 acres of land being thus engulfed, 
and for years afterward it is said that the chimney tops of the houses 
could be seen in the/waters of the harbour. 

The new capital, Kingston, was built at the head of the bay, and 
it was visited by an earthquake of great vigour on January 14, 1907. 
First there came slight preliminary tremors, then in about ten seconds 
a shock so violent that persons were thrown to the ground. In thirty- 
five seconds all damage by the shaking had been accomplished, but 
then fires broke out and destroyed a large part of the city. On and 
near the site of the former Port Royal there was extensive subsidence, 
palm trees being lowered beneath the water. The depth of the har- 
bour was also increased, in one place being 27 feet deeper than before 
the earthquake. 

Earthquakes in Eastern United States. — The greater part of the 
United States is apparently free from the danger of violent earth- 
quakes. Earth tremors and minor shocks are common, and in some 
places are especially frequent, as in the neighbourhood of East Haddam, 
Connecticut (Fig. 285). There have also been some shocks of con- 
siderable violence. The earliest of these of which we have record 
occurred on February 5, 1663, and, although central in the St. Lawrence 
valley, was felt also in New England. It seems to have been a shock 



MOVEMENTS OF THE EARTH'S CRUST 



427 




NEWBURYPORT 
BOSTON 



A^HADDAM 



YORK 
k DELPHIA 



Fig. 285. — Seismicity of New England. Fre- 
quency of earthquakes indicated by sizes of 
black dots, (de Montessus de Ballore.) 



of catastrophic violence, judg- 
ing from the accounts of it kept 
by the Jesuit priests. 

Three shocks of considerable 
violence occurred in New Eng- 
land in 1685, 1727, and 1755, 
respectively. The second af- 
fected the region nearNewbury- 
port, Mass., and is reported as 
having consisted of a series of 
successive shocks, through a 
period of four years, some hun- 
dreds in all, and some of them 
quite violent. Contemporary 
accounts refer to accompany- 
ing strange noises described by 
Shaler as follows : " There came 
from the earth a wonderful 
thundering, or bellowing noise, 
loud enough to startle people 
from sleep, even when they had 
long been used to it. Many believed that it was the Evil One him- 
self, raving in his empire be- 
neath the earth, and threatening 
to burst it asunder in his rage." 
Sounds accompanying earth- 
quakes, and coming from under- 
ground sources, are reported as 
occurring in association with 
many earthquakes. 

The shock of 1755 was most 
violent near Boston, and is 
reported to have been strong 
enough to throw people from 
their feet, but little destruction 
was accomplished by it. 

In 1811-1812 there occurred 
a series of violent shocks in the 
lower Mississippi valley, com- 
monly referred to as the New 
Madrid earthquake. The re- 
gion (Fig. 286) now embracing 
parts of northern Arkansas, 
southern Missouri, and western 
Kentucky and Tennessee, was 
sparsely settled, and, therefore, 
little destruction was accom- 




1886 
Sonora 

1887 
la °Yakutat Bay- 
San Francisco 
1906 



120° 110° 



LtlAMS ENGRAVING CO., 



Fig. 286. — Map showing as much as is known 
of the area affected by the New Madrid earth- 
quakes in 1 8 1 1- 1 8 1 2, and comparison with four 
other great seismic disturbances in North 
America. 



428 



COLLEGE PHYSIOGRAPHY 



plished, though some of these shocks were evidently of the world-shak- 
ing order. The first shock occurred on the night of December 16, 
i8n,and from that time till March 16, 1812, there were 1874 recorded 
shocks, of which eight were of the first order of violence, the most vio- 
lent being February 7. These earthquakes were so violent that they 
were felt throughout eastern United States. The ground opened in 
long fissures, and water spouted out of them to heights as great as 
40 feet. A large area of country was depressed, and is still called 
the " sunk country," while lakes still exist which were formed at the 
time (p. 316), with trees still standing in them. 




Fig. 287. — Destruction of buildings during the Charleston earthquake in 1886. 
(Hillers, U. S. Geol. Survey.) 

Lesser shocks have occurred in this region since, and it is evident 
that this is a region of seismic activity in the midst of a country in 
the main exempt from the danger of violent earthquakes. 

The last notable earthquake to affect eastern United States was 
central near Charleston, S.C., August 31, 1886, and is commonly 
known as the Charleston earthquake. Strange noises were heard 
and slight tremors were felt before the earthquake, notably on August 
27 and 28. Just before ten o'clock at night on the 31st a rumbling 
sound was heard, increasing to a great roar, and the shaking became 
violent. There was a second violent shock a few minutes afterwards,, 
and a number of after shocks of lesser violence. 

The earthquake spread at the rate of about 1 50 miles per minute, 



MOVEMENTS OF THE EARTH'S CRUST 



429 



from two epicentra a few miles 
westward from Charleston, and the 
shaking was felt over much of east- 
ern United States, the total shaken 
area being between two and three 
million square miles. The usual 
phenomena of fissures and crater- 





pacific 

OCEAN 



Fig. 2 8g. — Map of fault scarps at Owens 
Valley, California. Figures give height of 
scarps. (W. H. Hobbs, after W. D. John- 
son.) 



Fig. 288. — Diagram showing by size of the 
black dots the frequency of earthquakes in 
California, (de Montessus de Ballore.) 

lets were developed in the area of 
vigorous shaking, and railway 
tracks were bent and buckled, giving 
evidence of lateral shifting. 

Although a violent earthquake, 
and one that affected a wide area 
and therefore received careful study, 
the Charleston shock probably does 
not rank among the most violent. 
Some damage was done to most of 
the large buildings in the city, but 
few were destroyed (Fig. 287), and 
only 27 lives were lost. One of the 
chief kinds of damage was the de- 
struction of chimneys, there being 
about 14,000 of these thrown down. 



43° 



COLLEGE PHYSIOGRAPHY 



Earthquakes in Western United States. — The country from the 
Rocky Mountains westward to the Pacific is a seismic region, though 
throughout most of it no really destructive earthquake has occurred 
during the period of settlement. There are, however, fault scarps 
which indicate recent movement, and earthquakes in almost any part 
of the region need not be unexpected. 

One of the greatest earthquakes of the West occurred March 26, 
1872, in Owens Valley, California. Fault scarps were developed here 
(Fig. 289), and the disturbance of the surface during this earthquake is 
still plainly to be seen. It was a sparsely settled region and little 
destruction was therefore accomplished, though it doubtless ranks 

among the great earthquakes of 
recent times. The Sonora earth- 
quake of 1887 was also very 
severe, being felt over an area 
of 500,000 square miles (Fig. 
286) in Mexico and southwest- 
ern United States. 

The Coast Ranges of the Pa- 
cific coast have been the seat 
of a number of vigorous earth- 
quake shocks since the region 
was settled, and there are well 
recognizable rift valleys and 
fault scarps along which move- 
ment has recently taken place. 
In this region the section of 
central western California, in 
and near San Francisco, is a 
centre of special frequency of 
earthquake shaking. Scores of 
earthquakes have occurred in that centre, and several of them have 
been of destructive violence, the last one being that of April 18, 1906 
(Figs. 288, 290). 

During this shock there was horizontal shifting of a large mass of 
country on the southwest side of a fault plane, the movement being 
generally in a northeastward direction, and varying from 3 to 20 feet. 
At one point there was movement in the opposite direction, and lo- 
cally there was uplift on one side, but nowhere more than 4 feet. 
The fault line, or rift, or " earthquake crack," or fault trace, was 
followed across country by the furrowing of the surface, the dislocation 
of the roads, breaking of water pipes, separating of fences, and even 
the splitting of trees beneath which it passed (Figs. 270, 292). 

This great rift, traced for about 400 miles, has been the seat of earlier 
movements, at least as far back as the Glacial Period, and its course 
is marked by a succession of linear valleys, small lakes and pools, 
fault scarps, and narrow bays. Doubtless there have been many 




Map of California. The heavy lines 



the principal faults. 



Fig. 290. — Fault lines in California 

son.) 



(Law- 



MOVEMENTS OF THE EARTH'S CRUST 



43i 



earlier earthquakes as a result of movements along this line ; and it is 
probably a safe prophecy to state that there will be others in the future. 
The rift runs just west of San Francisco, and, therefore, the move- 
ment along it caused severe shaking in the city, and much destruction 




Fig. 291. — The Agassiz statue at Leland Stanford Junior University after the California 
earthquake of 1906. (Davey.) 



there. The greatest damage was, however, accomplished by the 
fire that followed the earthquake, for, as is so commonly the case, 
fire broke out at several points in the damaged city, and spread with 



43 2 



COLLEGE PHYSIOGRAPHY 



great rapidity and destructiveness, increased by the fact that the city 
water supply was cut off by the breaking of the mains by the earth 
movement. The city was laid mainly in ruin ; it would have been 
well if the new city had been built up with the possibility of a future 




'IG. 292. — Horizontal shifting of a road in California. Before the 1906 earthquake this 
road was straight. (Sinclair.) 



recurrence of a similar disaster in mind. The destruction of San 
Francisco by earthquake and resulting fire stands out as one of the 
great human disasters, and the greatest to which the United States 
has been subjected as a result of the terrible natural phenomenon of 
earthquake shaking (Figs. 275, 293). Fortunately there was no 
earthquake water wave in connection with this seismic disturbance. 



MOVEMENTS OF THE EARTH'S CRUST 433 

Earthquake Water Waves 

Earthquakes below Sea Level. — It is a well-known fact that earth- 
quakes originate on the ocean floor as well as on the land. The occur- 
rence of such shocks is sometimes observed on shipboard ; telegraph 
cables are sometimes snapped apart by submarine movements ; and 
the seismographic records of the present day have located many such 
shocks. Another proof of such shocks is the development of earth- 
quake water waves, or tsunami, sometimes spoken of as " tidal waves." 
Such submarine earthquakes are most common in the two great earth- 
quake belts, and especially in places where there are abrupt changes 




Fig. 293. — Train overturned during the California earthquake of 1906. (Gilbert, U. S. 

Geol. Survey.) 

in the slope of the ocean floor, along which fault movements are evi- 
dently taking place. In some sections of the sea bottom there are 
very abrupt slopes, and even great precipices, as in the neighbourhood 
of Zante in the Mediterranean, where there are submarine cliffs from 
3000 to 5000 feet high. The cable between Zante and Crete has 
several times been broken by movements along submarine fault 
planes ; and in some parts of the sea bottom the floor is so uneven that 
it is not feasible to lay cables on it. 

Nature of the Water Wave. — When such a movement takes place 
beneath the sea, the entire body of water above is lifted or lowered 
with the moving crust. Thus a broad, low swell is formed, affecting 
the ocean from top to bottom. It is so low that its passage would not 
be noticed, unless concentrated by movement into shallow water. 
If originating in the open sea, since it spreads outward in all directions 
from the centre, it may be dissipated before travelling a great dis- 
tance ; but if it starts near the coast, it may rise in height on passing 
into the shoaling water near shore, and rush upon the land as a great 
and destructive surge. 



434 COLLEGE PHYSIOGRAPHY 

Large Areas Affected. — Although such destructive inundations 
are possible only on coasts near the centre of disturbance, the great 
wave may sweep completely across the oceans, and cause recog- 
nizable fluctuations in the tide gauges on the opposite shores. The 
wave generated during the explosion of the volcano Krakatoa in 1883, 
for example, was measured on all the tide gauges of the Pacific and 
Indian oceans; and the water wave generated during the Lisbon 
earthquake of 1755 is said to have swept all the coasts of the civilized 
world. Their great extent, and their destructiveness on near-by 
coasts, are due to the fact that they differ from ordinary waves in 




Fig. 294. — The Water ee washed ashore during the Chilean earthquake of 1868. The 
surf line is an eighth of a mile beyond the farther ship. 

being a motion of the whole body of the ocean water from top to bot- 
tom, not of the upper layers alone. At sea they will pass unnoticed 
because so low, but on the shallowing coasts the great body of water 
involved causes a piling up of the water as the more slowly moving 
tide does at regular intervals. 

Damage to Life and Property. — Instances of such waves have 
been mentioned in the preceding pages; for instance, the earthquake 
water wave that devastated Lisbon, the similar waves on the coast 
of South America, and the water wave that swept through Yakutat 
Bay. Such waves have also swept portions of the coast of Japan, 
and other portions of the Asiatic coast. During the inundation of a 
tsunami on the coast of Japan in' 1896, the earthquake water wave 
10 to 50 feet high devastated 175 miles of coast, wrecked 9300 houses, 
stranded 300 large crafts, and crushed or carried away 10,000 fishing 
boats, and killed 27,000 people. A water wave during the South 



MOVEMENTS OF THE EARTH'S CRUST 435 

American earthquake of 1868 carried a United States warship inland 
half a mile, leaving it stranded (Fig. 294). By such waves trees and 
buildings are torn loose and floated about, and complete devastation 
follows in the wake of the rushing waters, which may rise 50 to 100 
feet above normal tide level. 



References to Literature 

J. G. Aguilera. Sonoran Earthquake, De los Fenomenos Seismicos del 3 de 
Mayo de 1887, Anales del Ministerio de Fomento, Vol. 10, Mexico, 1888, 
PP- 5-56. 

M. Baratta. La Catastrope Sismico Calabro Messine, Rome, 1910. 

W. M. Davis. Nomenclature of Surface Forms on Faulted Structures, Bull. 
Geol. Soc. Amer., Vol. 24, 1913, pp. 187-216. 

C. Davison. A Study of Recent Earthquakes, London, 1905, 355 pp. 

C. E. Dutton. The Charleston Earthquake of August 31, 1886, 9th Ann. Rept., 
U. S. Geol. Survey, 1889, pp. 203-528; Earthquakes, in the Light of the 
New Seismology, New York, 1904, 314 pp. 

M. L. Fuller. Our Greatest Earthquakes, Pop. Sci. Monthly, Vol. 69, 1906, 
pp. 76-86 ; Notes on the Jamaica Earthquake, Journ. Geol., Vol. 15, 1907, 
pp. 696-721 ; The New Madrid Earthquake, Bull. 494, U. S. Geol. Survey, 
191 2, 119 pp. 

J. Geikie. Structural and Field Geology, New York, 1905, 435 pp. 

G. K. Gilbert. Modification of the Great Lakes by Earth Movement, Nat. 
Geog. Mag., Vol. 8, 1897, pp. 233-247; Recent Earth Movement in the 
Great Lakes Region, 18th Ann. Rept., U. S. Geol. Survey, Part 2, 1897, 
pp. 595-647 ; A Theory of Earthquakes of the Great Basin, Amer. Journ. 
Sci., Vol. 27, 1884, pp. 49-53; Earthquake Forecasts, Science, Vol. 29, 
1909, pp. 121-138. 

G. K. Gilbert and Others. The San Francisco Earthquake and Fire of April 
18, 1906, Bull. 324, U. S. Geol. Survey, 1907, 161 pp. 

J. W. Gregory. The Great Rift Valley, London, 1896, 422 pp. 

J. F. Hayford. The Earth Movements in the California Earthquake of 1906, 
U. S. Coast and Geodetic Survey, Rept. for 1907, Appendix 3, 1908, pp. 
67-104. 

W. H. Hobbs. Earthquakes, New York, 1907, 336 pp.; Some Principles of 
Seismic Geology, Beitrage zur Geophysik, Vol. 8, 1907, pp. 219-362; A 
Study of the Damage to Bridges during Earthquakes, Journ. Geol., Vol. 
16, 1908, pp. 636-653 ; The Evolution and the Outlook of Seismic Geology, 
Proc. Amer. Phil. Soc, Vol. 48, 1909, 44 pp.; The Earthquake of 1872 in 
the Owens Valley, California, Beitrage zur Geophysik, Vol. 10, 1910, pp. 

352-385- 
R. Hoernes. Erdbebenkunde, Leipzig, 1893, 452 pp. 
E. Huntington and J. W. Goldthwait. The Hurricane Fault in Southwestern 

Utah, Journ. Geol., Vol. n, 1903, pp. 46-63; ibid., Bull. Mus. Comp. 

Zool., Vol. 42, 1904, pp. 199-259. 
C. G. Knott. The Physics of Earthquake Phenomena, Oxford, 1908,^ 283 pp. 
K. Kobayashi. Report on Earthquake Observations in Japan, Tokio, 1892, 

43 PP- 
B. Koto. On the Cause of the Great Earthquake in Central Japan, 1891, 

Journ. College of Science, Imperial University, Japan, Vol. 5, Part 4, 

1893, pp. 295-353. m 

A. C. Lawson. Post-glacial Faults near Banning, Ontario, Bull. Seism. Soc. 

Amer., Vol. 1, 191 1, pp. 159-166; Recent Fault Scarps at Genoa, Nevada, 

ibid., Vol. 2, 191 2, pp. 193-200. 



436 COLLEGE PHYSIOGRAPHY 

A. C. Lawson and Others. The California Earthquake of April 18, 1906, 2 vols, 
and atlas, Publication 87, Carnegie Institution, 1908, 1910, 451, 192 pp. 

C. K. Leith. Structural Geology, New York, 1913, 161 pp. 

Sir Charles Lyell. Lisbon Earthquake, Principles of Geology, nth edition, 
Vol. 2, 1874, pp. 147-154; Temple of Jupiter Serapis at Pozzuoli, ibid., 
Vol. 2, pp. 164-178. 

Lawrence Martin. Alaskan Earthquakes of 1899, Bull. Geol. Soc. Amer., Vol. 
21, 1910, pp. 339-406; Possible Oblique Minor Faulting in Alaska, 
Economic Geology, Vol. 2, 1907, pp. 576-579. 

C. S. Middlemiss. The Kangra Earthquake of April 4, 1905, Memoir 38, 
Geol. Survey of India, Calcutta, 1910, 409 pp. 

J. Milne. Earthquakes, New York, 1886, 363 pp.; Seismology, London, 
1898, 320 pp.; Recent Advances in Seismology, Proc. Royal Soc. A, Vol. 
77, 1906, pp. 370-373; Movements of the Earth's Crust, Geog. Journ., 
Vol. 7, 1896, pp. 229-250; Recent Earthquakes, Nature, Vol. 77, 1908, 
pp. 59 2 -597- 

J. Milne and W. K. Burton. The Great Earthquake in Japan, 1891, 2d 
edition, Yokohama, 30 plates, 70 pp. 

F. de Montessus de Ballore. Les Tremblementes de Terre, Paris, 1906, 475 
pp.; La Science Seismologique, Paris, 1907, 579 pp. 

R. D. Oldham. Report on the Great Earthquake of 12th June, 1897, Memoir 
19, Geol. Survey of India, Calcutta, 1899, 379 pp. ; The Geological Inter- 
pretation of the Earth Movements Associated with the Californian Earth- 
quake of April 18th, 1906, Quart. Journ. Geol. Soc, Vol. 65, 1909, pp. 
1-20. 

F. Omori. Bulletins of the Imperial Earthquake Investigation Committee, 
Tokio, 1907 to date; also Publications of the Earthquake Investigation 
Committee in Foreign Languages. 

H. F. Reid. The Mechanics of the (California) Earthquake, Carnegie Institu- 
tion, Publication 87, Vol. 2, 1910, 192 pp. ; The Elastic Rebound Theory of 
Earthquakes, Bull. 6, Dept. Geol. Univ. Cal., 191 1, pp. 413-444. 

E. R. Scidmore. The Recent Earthquake Wave on the Coast of Japan, Nat. 
Geog. Mag., Vol. 7, i8q6, pp. 285-289. 

N. S. Shaler. The Stability of the Earth, Aspects of the Earth, New York, 
1904, pp. 1-45; First Book in Geology, Boston, 1885, pp. 130-140. 

A. Sieberg. Ffandbuch der Erdbebenkunde, Brunswick, 1904, 362 pp. 

R. S. Tarr. Pacific Coast Earthquakes, The Independent, Vol. 60, "1906, pp. 

954-962; The World's Earthquake Belts and Causes of Seismic Shocks, 

Leslie's Weekly, Vol. 103, 1906, pp. 422-423. 
R. S. Tarr and Lawrence Martin. The Earthquakes at Yakutat Bay, x\laska, 

in September, 1899, Prof. Paper 69, U. S. Geol. Survey, Washington, 191 2, 

135 PP- 
C. R. Van Hise. Earth Movements, Trans. Wis. Acad., Vol. n, 1898, pp. 465- 
516. 

B. Willis. Mechanics of Appalachian Structure, 13th Ann. Rept., U. S. Geol. 

Survey, Part 2, 1892, pp. 211-281. 
J. B. Woodworth. Postglacial Faults of Eastern New York, Bull. 107, N. Y. 
State Museum, 1907, pp. 5-28. 

C. W. Wright. The World's Most Cruel Earthquake (Messina), Nat. Geog. 

Mag., Vol. 20, 1909, pp. 373-39 6 - 

PERIODICALS 

Bulletin Seismological Society of America, Palo Alto, Cal. 

Gerland's Beitr&ge zur Gcophysik, Leipzig. 

Transactions Scismological Society of Japan; Seismological Journal of Japan. 

Bolletfino delta Societd Sismologica Italiana, Rome. 

Die Erdbcbemvarle, Vienna. 



MOVEMENTS OF THE EARTH'S CRUST 437 

TOPOGRAPHIC MAPS 

For topographic maps showing the fault trace of the California earthquake 
of 1906 on U. S. Geol. Survey quadrangles and on special contour maps by 
F. E. Matthes, see atlas accompanying report on California earthquake of 
1906, Publication 87, Carnegie Institution. For hachure maps of fault scarps 
in Owens Valley by W. D. Johnson, see Hobbs' Earthquakes, and his paper 
on Owens Valley earthquake. For maps showing relations of topography to 
various folded and faulted structures, see the folios of the U. S. Geol. Survey. 
For topographic maps of fault scarps and fault block mountains, see the end of 
Chapter XV. 



CHAPTER XIII 
VULCANISM 

Nature of Vulcanism 

Lava Blown into the Air. — In some parts of the earth molten 
rock or -lava rises to the surface and spreads out over the surrounding 
country. It is always accompanied by steam (Fig. 295) and other 
gases, the expansion of which is commonly responsible for the 




Fig. 295. — Katmai volcano, Alaska, in eruption in June, 1913. (M. Horner.) 

expulsion of the liquid rock. In some cases the expansion of the 
associated gases blows the lava into bits which settle around the 
orifice as volcanic ash, pumice, and lava blocks of various sizes 
and shapes. These are usually very porous because of the expansion 
of the included gases. 

Lava Flowing out of Openings. — In other cases the liquid rock 
flows out more quietly as a lava flow, from which great quantities of 
steam rise, and in the upper portion of which a porous condition is 
also caused by the expanding of the gases as the lava cools. 

438 



VULCANISM 439 

In recent geological periods lava rose to the surface through cracks, 
or fissures, and spread over the surrounding region in great floods ; 
but at present this condition is practically unknown, though some of 
the modern eruptions of Iceland have come from fissures, or from 
numerous vents along fissures. 

Lava Building up Cones. — The surface expressions of vulcanism 
to-day are mainly those of eruption from restricted vents, which we 
call volcanoes. These volcanoes are, however, often along lines, as if 
associated with fissures, from only portions of which is there emission 
of lava at present. Very often in a chain of volcanoes only one or two 
are now active, and in some cases activity is apparently at an end in 
all the volcanoes of a chain. In some cases, at least, it seems as if a 
fissure had opened, from many parts of which lava outflowed ; then, 
with diminution of the expelling force, eruption was confined to a few 
points along the fissures ; and, finally, one after the other of the vents 
became closed. A final stage in volcanicity is the escape of steam, 
sulphurous and other gases, and hot water. 

Lava Underground. — Besides the form of vulcanism which is 
expressed in surface outflow, there are underground manifestations 
of importance, the nature of which will not be considered for the 
present. 

The Volcanic Products 

The two .products of volcanic eruption are molten rock and asso- 
ciated gases, each appearing in different forms according to conditions. 

The Lava Flow. — Where the lava rises in sufficiently liquid condi- 
tion, it flows away from the vent, as molten iron would flow. At first, 
while hot, it cools to a dull red glow, then to a rock either black or 
other colour, passing from the liquid to the pasty and then to the solid 
state, when it may be crystalline or glassy. On issuing from the 
vent, the temperature may be 2000 F. or more ; but by radiation and 
conduction it rapidly cools as it flows away from the vent and spreads 
out ; but it may be months and even years before a lava flow becomes 
completely cold. The lava may be solid enough to walk upon, while 
glowing hot within. The lava crust is such a poor conductor that it 
requires a very long period for it to completely cool. For example, 
the lava from the eruption of Vesuvius in 1787 was still hot and steam- 
ing seven years later ; steam still issued from the flow of 1858 when ob- 
served by Geikie in 1870 ; and it is said that 21 years after a lava flow 
issued from the volcano Jorullo in Mexico, in 1759, a cigar could still 
be lighted at its fissures, and it was still steaming 44 years after the 
eruption. 

Gases from Lava Flows. — From the moment the lava reaches to the 
surface until after it has become completely solidified, steam and other 
gases rise from it ; and at first so much steam may rise that the lava 
flow is almost completely covered by an overhanging cloud. While 
the lava is liquid, these gases may escape without any effect on the lava, 



44o COLLEGE PHYSIOGRAPHY 

though there may be minor eruptions from the surface as the highly 
heated gases rapidly escape, or where the lava flows over snow, or 
springs, or other bodies of water. When it becomes pasty, the es- 
caping gases may form cavities which do not close, giving rise to po- 
rous, slaggy, or cinder-like texture. When solid, the gases cease to 
escape, excepting from the crevices and fissures, some of which open 
as the solidified lava cools and contracts. 

Pahoehoe and Aa. — If a part of the liquid lava cools without 
further movement during the pasty or solidified state, it will assume 
a smooth, or fairly regularly rounded surface, called, in the Hawaiian 
Islands, pahoehoe ; but usually there is subsequent flow, and the sur- 
face, therefore, becomes much rougher. Flow during the pasty state 
draws the partially solidified lava out in the form which is well de- 
scribed by the term ropy structure, the surface resembling the braided 
form of a coarse rope or coil of rope, due to the stretching of the nearly 
solid rock. A solid crust may form while there is still liquid lava below ; 
and then, if motion continues, the crust may be fissured, broken, and 
splintered, giving rise to a field of clinker-like fragments. Such a 
rough lava surface is called aa in the Hawaiian Islands. During such 
a stage in lava flow motion, one can hear the blocks break and grind 
together. 

Variations in Lavas. — There are many differences in the appearance 
of a recently formed lava flow, partly for the reasons stated, partly 
because the lavas vary in composition, in temperature, and in the slope 
over which they flow. Some lavas are quite viscous, even at the point 
of emission, as in the case of Vesuvius ; and such lavas are normally 
rough and clinkery. Others are more liquid, like the Hawaiian lavas, 
and in these the smoother form is more common, while the lavas 
spread out in a thinner sheet. The degree of liquidity may be due 
either to difference in temperature or to difference in composition. 
The basic lavas, or basalts, of the Hawaiian Islands, for example, 
melt at a lower temperature than the more acid lavas. There is a 
difference in rate of flow also according to the slope, as in the case of 
any liquid body. 

Rapidity of Flow. — A lava flow may escape from the crest of a 
volcano, but much more commonly it issues from one or more fissures 
on the slopes of the cone. It may well out with moderate volume, or 
it may spout out fountain-like, especially where it issues from the 
lower flanks of a volcano and is, therefore, under hydrostatic pressure 
from the column standing in the volcanic vent. For instance, in the 
eruption of Mauna Loa in 1852, a fountain of lava 1000 feet broad rose 
to a height of 200 to 700 feet. At the same time vast volumes of 
steam escape and condense in a heavy cloud over the white-hot lava. 
At first the lava flows down the slopes rapidly, spreading as it goes, 
naturally seeking the lowest points, and, therefore, entering any 
valleys that may lie in its course. It is reported that it may flow as 
fast as a mile a minute, though the rate is ordinarily less rapid, moving 



VULCANISM 44i 

near the point of outflow from ten to fifteen miles an hour. A lava 
flow from Etna in September, 191 1, moved as a stream 1500 to 1800 
feet wide, 35 to 45 feet high at the front, advancing \ of a mile an hour. 
Still slower rates are the rule, the lava at Teneriffe in the Canary Islands 
in 1909 flowing only 50 or 60 feet an hour on a io° slope. 

Cooling of Lava Flows. — As a crust forms on the lava, its rate of 
flow diminishes, and finally the forward motion of the end may be 
almost imperceptible. The advancing end is apparently a broken 
mass of lava blocks, slowly pushing forward with an accompanying 
sound of rupturing and grinding of the solid mass urged forward by the 
underflow of the liquid lava. If it comes to the edge of a steep slope, 
or if for any other reason the front is ruptured, the liquid lava may rush 
forth with rapid flow from beneath the pile of broken fragments, 
overwhelming all in its path. This outrush of lava sometimes leaves 
caverns beneath a solidified roof, and in the Hawaiian volcanoes they 
are at times ornamented with lava stalactites and stalagmites. 

Size of Flows. — Lava flows commonly extend only part way down 
the slopes of volcanoes, though now and then they flow out to the sur- 
rounding land. Some of the large lava flows of the Hawaiian Islands 
are 30 or 40 miles long, and two or three miles broad. Dana estimates 
that the flow of 1852 from Mauna Loa may have contained as much 
as 10,560,000,000 cubic feet of lava, assuming its average width to 
be 6000 feet and its average depth 20 feet. Daly states that Mauna 
Loa emitted 455 million cubic metres in 1855 ; that the lava from 
Skaptar Jokull in Iceland in 1783 amounted to 12,360,000,000 cubic 
metres; and that Etna sent out 980 million cubic metres in 1669, 
or 34,608,160,000 cubic feet. 

Effects of Flows. — The lava flow overwhelms everything in its 
path, and leaves a train of destruction, blotting out not only plants 
and human structures, but even completely changing the topography. 
There are few more desolate scenes than that of a recently formed lava 
flow ; and it is many years before a new soil can form on its surface 
and vegetation once more occupy it. This varies with the lava, some 
of the streams from Vesuvius being occupied in less than a century ; 
others, as in Sicily, remain barren for centuries. Where the flow ends, 
however, trees may still stand, for the heated lava is buried beneath 
the solidified crust, and the vegetation is not injured unless the end 
of the flow overturns and overrides it. Even islands of trees and vine- 
yards may stand in the midst of a lava flow that has surrounded 
them. Lavas have even flowed over snow fields and ice, without 
melting them. On the slopes of Etna, for example, there is a mass of 
ice, originally a snow bank, which was buried by a lava flow over a 
century ago. At other times the lava melts the snow, causing great 
floods ; and, by the steam thus caused, eruption occurs within the 
flow itself. Small cones are thus sometimes formed on the surface 
of lava flows where the liquid rock flowed over and evaporated snow 
or water. In a few minutes a lava flow may blot out a valley, and bury 



442 



COLLEGE PHYSIOGRAPHY 



it beneath hundreds of feet of rock. By entering the sea, the lava may 
notably extend the land area, as in 1868, when a half mile was added 
to a portion of the island of Hawaii, and in 1906 when a lava flow from 
the island of Savaii poured into the sea for several weeks, extending 
the coast (Fig. 296). The lava streams often form dams across valleys 
in which lakes gather; and they at times force streams to outflow 




Fig. 296. — Lava flow entering the sea in Savaii. (After Sapper.) 



across low portions of their valley wall, thus dividing their courses, or 
even inverting their flow by turning them so that they outflow across 
former divides. 

Fragmental Products. — Even as lava is flowing, the explosive 
action of included water frequently throws fragments into the air 
and even builds small cones of such ejecta on the surface of the lava 
flow. Similarly, though on a far grander scale, lava fragments are 
hurled from the volcanic vents, and as they rise the expansion of the 
included water renders the lava porous, forming volcanic ash. Al- 
though the name ash is used, it is not to be implied that combustion 
has taken place, as in the ash from coal. It is often as porous as a 
sponge, and so light that it will float, as the pumice does. 

These fragments are of all sizes, from bits the size of dust to huge 
stones, tons in weight. They may rise only a few feet or scores of 
feet, and fall back into the vent ; or they may rise thousands of feet 
into the air, the largest falling near the vent, while smaller fragments 



VULCANISM 443 

may be drifted in the air currents for scores or even hundreds of miles 
before settling to the earth. 

Bombs and Ash in the Air. — The explosive action of the included 
gases in molten lavas must be very great, for the temperature is far 
above 773 F., the critical point of water, that is, above the temperature 
at which water is always a gas, no matter what the pressure may be. 
It is sufficient to carry ash to a height of two or three miles in the air, 
and to hurl huge stones several miles. It is reported, for example, 
that a block weighing 200 tons was hurled a distance of 9 miles from 
the vent during an eruption of the volcano Cotopaxi in Ecuador. 
As the gases expand, while the fragments cool, they are not merely 
rendered porous, but are blown into bits. Fragments of volcanic dust 
that were collected 65 miles from Cotopaxi, whence they came, were 
found to be so small that from 4000 to 25,000 were required to weigh 
a grain. Such dust will float for a long time in the air, and it enters 
almost any cavity, no matter how small, sifting under windows, even 
entering into the interior of watches. 

So great a quantity of ash and dust rises from a violent volcanic 
eruption that it completely obscures the sun for miles around. During 
the eruption of Coseguina in Nicaragua, in 1835, for example, darkness 
prevailed throughout a radius of 35 miles from the vent. Near the 
volcano there was a fall of ash which covered the ground to a depth 
of 10 feet, while volcanic dust fell four days later in Jamaica 700 
miles distant. How great a quantity is expelled during a violent erup- 
tion may be inferred from the following estimates : the ash erupted 
in 1880 from Cotopaxi is estimated to have been fully 2,000,000 tons ; 
nearly 5 cubic miles of ash fell during the eruption at Katmai, Alaska, 
in 191 2 ; between 28 and 50 cubic miles of volcanic material is esti- 
mated to have come from the volcano Tomboro on the island of 
Sumbawa near Java in 1815, or an amount equal to one hundred and 
eighty-five mountains the size of Vesuvius. 

Variations in Fragmental Material. — There is much difference 
in the matter of expulsion of volcanic fragments, according to the as- 
sociated conditions. In a very liquid lava the gases rise and escape 
with little commotion, though now and then great bubbles may rise 
and throw up fragments of the lava. This result is quite certain to 
follow when the surface of the lava column cools to a pasty state, 
or when it becomes frozen over with a solid crust. In the Hawaiian 
volcanoes, for instance, clots of lava are thrown out, and, falling back 
around the vent, build small, steep-sided cones, to the sides of which 
the still plastic lava lumps cling as they fall. 

Lapilli and Bombs. — Even in normally viscous lavas there is 
boiling and escape of steam bubbles from the surface of the lava 
column, but the explosive force of the included gases is much stronger 
than in the liquid lavas. Consequently, the lava is tossed higher 
in the air, and the fragments are more porous. In Vesuvius, for ex- 
ample, pieces of slag, lapilli, and volcanic bombs are thrown up, even 



444 COLLEGE PHYSIOGRAPHY 

during stages of comparative quiet ; and they often fall to one side of 
the vent so that one does not venture near it. The lapilli are small 
fragments, as large as a pea or a nut, often rounded, but sometimes 
angular and usually porous. The term slag applies to porous frag- 
ments of various sizes and shapes, resembling furnace slag or cinders. 
Volcanic bombs are rounded, elliptical, or pear-shaped masses, varying 
in size from a few inches in diameter to several feet, and usually some- 
what cellular inside. They were evidently hurled out while still in 
an unsolidified state, and gained their rounded form while whirling 
through the air and forming their solid crust. Sometimes they are 
flattened on one side, evidently by impact when they struck the 
ground before being quite solid ; at other times they have been broken 
by falling. Very often they are cracked by fissures and planes de- 
veloped by contraction during cooling. 

Volcanic Ash and Tuff. — If a crust forms on the upper portion of 
a lava column, the rise of the included gases may be so checked that 
a great strain is applied, which ultimately may blow out the consoli- 
dated lava and even blow away a part of the cone. It is at such times 
that the most violent eruptions take place, and the imprisoned gases, 
rapidly expanding, throw the lava high in the air. As already stated, 
these gases disrupt the lava into bits of ash, sand, and dust, the 
coarsest of which fall back near the vent, while the finer particles 
drift far and wide. 

The volcanic fragments that settle on and near volcanoes make 
deposits of various kinds from the coarse-textured lapilli, bombs, 
and slag to deposits of volcanic sand, ash, and dust. The latter some- 
times forms extensive beds near volcanoes, known as volcanic tuff. 
Scattered through the deposits near the vent are oftentimes found 
fragments of non- volcanic rocks, such as limestone, schist, etc., 
evidently torn off by the ascending lava in its passage through the 
strata underlying the volcano in the early stages of its formation. 

Volcanic Gases. — The vast quantities of steam which rise from vol- 
canoes, as well as other phenomena associated with eruption, proved 
conclusively that there are great quantities of included gases, evidently 
dissolved in the molten magma, and consequently having the same 
temperature as the lava. Among these gases are the elements of 
water, — hydrogen and oxygen, — at a temperature far above the 
critical point of water (773 F.). Just what happens in the process 
of escape is unknown ; but water vapour rises from lava flows, from 
the lava column in the vent, from the ash eruptions, and from cracks 
and crevices in the volcanic cone. During great eruptions prodig- 
ious quantities of steam rush out and form a great cloud, thousands 
of feet high, above the vent. It is estimated that during an erup- 
tion at Etna enough vapour escaped in a period of 100 days to form 
462,000,000 gallons of water. 

Rain following Eruptions. — Condensing as it rises, the vapour forms 
clouds and rain, and, therefore, much water falls back upon the vol- 



VULCANISM 445 

cano. There are copious rains, and thunder and lightning develop 
in the steam cloud, so that great volumes of water rush down the 
slopes, often causing great destruction in their path. These floods 
are sometimes augmented by the melting of snows or by the emptying 
of lakes. 

Poisonous Gases. — Many other gases arise from volcanoes, in- 
cluding hydrochloric acid vapour, sulphurous acid, chlorine, oxygen, hy- 
drogen, and carbon dioxide. It is highly probable that the oxygen and 
hydrogen exist uncombined in the lava, and that they unite on escape, 
giving rise to the explosions and to the great volume of steam that rises 
from the vents. Long after volcanic activity ceases, steam continues 
to rise from and near the vents ; and carbon dioxide issues even after 
the steam ceases. Thus in the volcanic Eifel district of western Ger- 
many, long since extinct, carbon dioxide issues from a multitude of 
points, and many of the numerous acid springs there are due to this 
gas. Many medicinal and hot springs are due to the volcanic condi- 
tions, and may be interpreted as the last stages of expiring volcanic 
activity. Geysers are evidently the product of one stage in the dying 
out of vulcanism. 

In some places so much carbon dioxide escapes from the earth that 
the air is locally charged with it, and animals may be suffocated by it. 
It is said that in former days birds flying over Lake Avernus, in a small 
crater on the Bay of Naples, were often suffocated by the noxious 
vapours ; but this is not true to-day. In a small valley in the Yellow- 
stone Park, however, bears are sometimes killed by the carbon 
dioxide that issues from the ground ; tigers and deer are killed in the 
" Valley of Death " in Java, a deep hollow from which great quantities 
of carbon dioxide escape ; and insects, birds, and mice are sometimes 
killed near the orifice whence carbon dioxide escapes along the shores 
of Laachen See, a lake in one of the extinct craters of the Eifel. 

Mud Flows. — The rains that fall upon the volcanoes, finding loose 
ash freshly fallen on the steep slopes, wash it down in such quantities 
as often to form great flows of liquid mud, called mud flows or mud lavas. 
These are masses of pasty mud, sufficiently liquid to flow, yet not stiff 
enough to stand upon. They move with a velocity varying with the 
liquidity and the slope, and may cause even more destruction than 
lava itself. Everything in the path of such a mud flow is enveloped 
and overwhelmed, as was the case during the eruption of Vesuvius 
in the year 79 when a mud flow swept over Herculaneum, quickly 
covering and entering the houses, and sealing them effectively in a 
mass of mud which has since solidified. Similar mud flows descended 
the flanks of La Soufriere in the island of St. Vincent during the 
eruptions of 1902, and they were steaming hot, probably partly 
because of the heated rains, partly because of the hot ashes upon 
which they fell. From their surface jets of steam arose, and even 
minor eruptions occurred, erecting small cones on the surface of the 
mud flows. 



446 COLLEGE PHYSIOGRAPHY 



The Volcanic Cone 

The Building of Volcanic Cones. — A single eruption from an 
orifice builds up a mound-shaped deposit, or a cone, around the vent ; 
and successive eruptions may so add to the cone as to make it a moun- 
tain of large size. Thus Vesuvius is 3880 feet high (PL VII) ; 
Etna is 10,870 feet; and Mauna Loa is 13,675, or, reckoned from its 
base beneath the sea, fully 30,500 feet high. So far as is known these 
volcanoes are composed entirely of lava or fragmental products, 
poured forth from within the earth during successive eruptions that 
have occurred during an unknown number of preceding centuries. 
There is, therefore, a vast amount of molten rock extruded from 
within the earth, even in a single volcano ; and since great quantities 
have drifted away in the form of ash and dust, there is even more than 
the volcanic cone alone would indicate. 

Lava Cones and Ash Cones. — The volcanic cone varies greatly in 
shape as well as in size, and one of the main causes for variation is 
the condition in which the molten rock is expelled. If the lava flows 
forth in streams, practically all that comes out remains near the vent, 
and, therefore, contributes toward the growth of the cone ; but, if 



m 



Fig. 297. — Contrasted profiles of a broad lava cone like one of the Hawaiian volcanoes and 
a steep ash cone like Vesuvius. 

it is blown out in fragments, much may settle at a distance from the 
vent. A lava cone is, therefore, apt to be larger than an ash cone 
having an equal number of eruptions. The lava cone will, however, 
be less steep than the ash cone (Fig. 297) ; for while the ash settles 
and tends to assume the angle of slope of loose materials in the air, 
the lava tends to flow away from the vent. Lava will congeal on 
any slope, up to the vertical ; but it is able to flow over the most 
gentle of slopes. Some of the liquid lavas of the Hawaiian Islands, 
for example, have flowed down slopes of less than i°. 

Ash cones often have a slope of from 30 to 40 , but of course denuda- 
tion is always at work removing fragments from higher to lower points, 
and thus tending to flatten the slopes. Lava cones have a much more 
gentle slope; that of Mauna Loa, for example, having an average 
slope of only 6°. It is, therefore, a much less striking mountain than 
many a smaller, steeper cone. An additional reason for the broad 
and gently sloping lava cones is the fact that the eruptions commonly 
come from the flanks of the cone, and, therefore, can flow farther out 
than if they came from the summit of the cone. Most active volcanoes 
erupt both lava and ash, sometimes both together, sometimes one or 
the other. Such cones have slopes intermediate between the lava 
and ash types. 



VULCANISM 447 

The Crater. — The crest of the volcanic cone is usually truncated, 
and in it is sunk a pit, or crater, whose diameter varies from a few hun- 
dred yards to a mile or more. Some very large craters, like that of 
Mauna Loa, which is about 8000 feet in diameter, are called calderas. 
Both the form and depth of the craters vary greatly according to the 
state of eruption ; that of Vesuvius, for example, was several times 
larger after the eruption of 1906 than before it. The inner slope is 
commonly precipitous; and from its porous and rifted walls steam 
and other gases issue, while still greater volumes issue from the crater 
bottom. 

Conditions within Craters. — During stages of inactivity, or after 
eruption is at an end, the crater bottom is covered over with solidified 
lava, fallen blocks, and scoriae, through which, here and there, small 
fumaroles of steam arose. This was the condition of Vesuvius when 
the author saw it in March, 1910 ; but during a visit seven years 
earlier the crater bottom was in a far different state. Vast quantities 
of steam, sulphurous and other gases were pouring out, a fiery glow 
was visible through the steam cloud, and, now and then, it was lighted 
by the bursting forth of the imprisoned gases. Immediately followed 
detonations which shook the crater edge, the volume of steam in- 
creased, and masses of lava, some of them of large size and still glowing, 
were hurled above the crater edge. 

Several descents have been made into the crater of Vesuvius 
(PL VII), notably by Cappello in 191 1, by Malladra in 191 2, and by 
Storz in 1913. Times of inactivity were chosen, but the dangers 
seem to have been less from poisonous gases than the falling of 
stones from the cooling and rapidly disintegrating crater walls. 
Temperatures of 200 F. were recorded in the air at the bottom of 
the crater, which was 984 feet deep in 191 2. A thermometer 
lowered 200 feet into an opening in the bottom of the crater recorded 
a temperature of ii7o°F. in December, 1913. 

Volcanic Eruptions. — This circular pit is the opening of a vent, 
filled with molten rock forced upward from a reservoir at an unknown 
distance beneath the surface. Now and then the volcanic forces 
gather energy enough to propel the lava high in the air, and a destruc- 
tive rain of ash, scoriae, slag, and bombs falls upon the slopes of the 
cone ; at the same time the tremendous strain of the rising lava and 
its included gases rends open the side of the cone, and lava flows out 
through one or more fissures. The volcano is in eruption. 

In a few days the activity may cease, or eruption after eruption 
may occur through a period of weeks or months before there is suffi- 
cient relief to permit a period of quiet. After each eruption of this 
kind the mountain slopes are coated with freshly fallen fragments, 
as snow covers a land surface, and soon stream work begins its 
task of removing it, gullying the surface and bearing the loose 
fragments to lower levels. To prevent this, and to protect vine- 
yards on the lower levels of Vesuvius from inundation of debris thus 



448 COLLEGE PHYSIOGRAPHY 

washed down, extensive retaining walls have been built since the 
1906 eruption. 

Parasitic Cones. — Where the lava outflowed, the side of the cone 
is scarred by a great black track of utter desolation, and at the point 
of outflow there is, perhaps, a small cone built up. Such secondary 
cones on the flanks of a volcano are called parasitic cones, of which 
there are 200 on the slopes of Etna, some of them 700 feet high. 

Slopes of Volcanoes. — Besides these features there are on the 
slopes of volcanoes, as on alt land forms, the excavations made by 
running water. In an active volcano, however, this denudation is 
frequently interfered with, and the valleys often completely blotted 
out by the falling ash, or by mud flows, or by lava flows, which nor- 
mally enter the valleys as they pour down the slope. 

Destruction of Volcanic Cones from Within. — It is by such pro- 
cesses that the volcanic cone usually grows ; but it is sometimes sub- 
jected to conditions which interfere with the development of the normal, 
symmetrical cone. In some cases, the withdrawal of the lava from 
beneath the cone permits collapse, and a truncated cone is produced 
with a huge caldera, like that of Crater Lake in Oregon. In other 
cases, after a period of inactivity, a great explosion occurs, which 
blows away the top of the cone, also causing a huge caldera, 
or, as in the case of Krakatoa in the Straits of Sunda, blowing 
away one side of the cone and leaving the other part standing. 
Subsequent eruptions may build a new cone in the caldera, or on the 
site of the wrecked cone, and partially repair the damage of an earlier 
explosion. This has been the case in Vesuvius, where Monte Somma, 
a part of the old crater rim that stood before the eruption of 79, still 
remains on one side of the present cone. Double vents and overlap- 
ping craters are not uncommon, as on Mount Shasta," but one is always 
younger than the other and continues activity longer. 

Active, Dormant, and Extinct Volcanoes. — It is very common to 
class volcanoes as active, dormant, and extinct, though the three 
kinds grade so into one another that there can be no hard-and-fast 
line drawn between them. A truly active volcano cannot be mistaken, 
for, even though it may be temporarily quiescent, there is clear evi- 
dence of recent activity; and the presence of slumbering energy is 
plainly shown by the steam that issues from its crater or slopes. On 
the other extreme, a volcano may be so certainly extinct that one 
cannot fail to recognize the fact. Even though the cone, with its 
crater, may still remain, it no longer emits steam, though carbon 
dioxide may still rise from it. Such is the condition of the volcanoes 
of the Eifel district of western Germany ; of the Auvergne in central 
France ; and of a multitude of volcanoes in western United States. 
"Between these two extremes, however, is a condition of quiescence 
which may be either temporary or permanent, and it is quite im- 
possible to state which. Doubtless Vesuvius before its eruption of 79 
was in a condition which would warrant the assumption that it was 



VULCANISM 449 

extinct, but we know that, after centuries of quiet, it broke forth in 
the most terrible eruption that it has experienced within historic times. 
Ever since, it has been intermittently active, though at one time, 
from 1500 to 1 63 1, it was dormant for a period of 131 years. Even 
centuries of inactivity do not necessarily prove extinction. 

Some volcanoes are in a state of almost incessant activity, but most 
of them are intermittent, the periods of inactivity varying in length, 
and the intervals usually being irregular. When the period of quiet 
has been long and the dormant volcano breaks into activity, the erup- 
tion is apt to be of exceptional violence, for during the period of quiet, 
the lava in the vent has solidified and given rise to an obstacle which 
only a great explosion can remove. 

Specific Instances of Volcanic Eruptions 

It is clear that while there are certain features in common among 
volcanoes, there are also marked differences. A fuller understanding 
of these differences may best be gained from a brief description of a 
few typical volcanoes and their eruptive activity. 

Italian Volcanoes 

Present and Former Volcanoes in Italy. — There is a chain of 
volcanoes extending almost the entire length of the Italian peninsula, 
from near the base of the Alps to Vesuvius, then continuing southward 
through Lipari Islands to Sicily, and to the Mediterranean south of 
that island. North of Vesuvius the volcanoes are extinct, though 
some of them have been active in recent geological times ; their cones 
are still quite perfect, and their craters are unbreached, and occupied 
by lakes, like Lake Nemi near Rome. On the Bay of Naples there is 
a group of volcanoes that have been active during historic times, in- 
cluding Vesuvius ; south of it is another group in the Lipari Islands ; 
on Sicily is Etna; and south of it there has also been eruptive 
activity. 

Lipari Islands. — This small group of islands, north of Etna and 
between it and Vesuvius, consists of a series of seven large and a 
number of small volcanic islands. Several of these are apparently 
extinct, though rising vapours, hot springs, and the cones and craters 
testify to recent activity. Vulcano has a crater over 500 yards across, 
from which steam constantly rises and explosive eruptions occasionally 
take place. At the north end of the island is the small Vulcanello, 
which was upheaved from beneath the sea in the year 200 B.C. and 
is now connected with the main island. It has three overlapping 
craters, each evidently due to an eruption from a slightly different 
vent, caused by the closing of the earlier vent between eruptions. 

The northern island, Stromboli, sometimes called " the lighthouse of 
the Mediterranean/' is remarkable for the fact that it is in a constant 



45o COLLEGE PHYSIOGRAPHY 

state of activity, usually of a very moderate character. The cone 
itself rises about 3000 feet above the sea, but including the portion 
beneath the sea it is about a mile high. On the northern side of the 
summit, and about 1000 feet below it, is the active crater, from which 
steam, often called " smoke," is constantly rising. In the bottom of 
the crater, lava may be seen, and at intervals of from 3 or 4 to 10 
minutes it swells up in a blister and is exploded by the rising gases, 
much like boiling oatmeal. With the explosion, which may generally 
be safely watched from the crater edge, a shower of hot stones is 
thrown up to a height of several hundred feet. Stromboli has been 
in similar constant activity throughout historic times. There has, 
however, been variation in intensity of eruption, and there is a belief 
among the fishermen that the amount of steam and the force of erup- 
tive activity vary with the weather, so that it can be used as a kind of 
barometer. This is not at all improbable, since the eruptive activity 
is apparently in such delicate balance with atmospheric pressure that 
a diminution in pressure, such as accompanies a storm, may easily 
induce a more copious discharge of the included gases. 

Graham Island. — Between Sicily and Africa lies the extinct 
volcano of Pantellaria. About halfway between it and the coast of 
Sicily, and 30 miles distant from Sicily, a submarine eruption occurred 
in 1 83 1, preceded by earthquake shocks felt at sea and on the coast 
of Sicily. About July 10 a column of water was seen to rise 60 feet 
in the air, followed by a column of dense steam rising 1800 feet. On 
July 18 there was a small island 12 feet high with a crater in its centre, 
from which volcanic ejecta were being thrown, accompanied by great 
volumes of steam. By August 4 the new island had a circumference 
of three miles and rose to a height of 200 feet at a point where, before, 
there had been 600 feet of water. Before the end of the year the island 
had disappeared, doubtless by wave attack upon the loose ash. 

The Graham Island eruption illustrates a very common type* of 
volcanic action. From a vent, usually not far from a centre of vol- 
canic activity, enough material is erupted in a brief interval of time 
to build a cone of considerable size ; then follows inactivity, and the 
cone is left to the attacks of denudation. Such cones abound on the 
slopes of large volcanoes, as on Etna, and near the base of such cones 
as Vesuvius. Many of them have been seen to form on the land, and 
tens of thousands of others have been formed without human witness 
or record. Doubtless they are common in volcanic regions in the 
ocean, for already a number have been seen to form, as Graham Island 
was. 

In the ocean such small cones of loose ash, if they rise to the surface, 
fall ready prey to the wave attack, as we have seen ; their sites are 
marked only by shoals. On the land they are far more permanent, 
for on such small cones large streams cannot develop, and, in such 
porous deposits, there is so much percolation that the erosive action 
of running water is greatly reduced. Accordingly such cones long 



VULCANISM 451 

retain their perfect>form, giving rise to a deceptive appearance of ex- 
treme recency of origin. 

Etna. — This beautiful, symmetrical cone, rising from the sea level 
to a height of 10,870 feet, is the loftiest volcano in Europe. Its base 
is almost circular, and is about 87 miles in circumference. Etna pre- 
sents a complete contrast to Graham Island, for here eruptive activity 
has persisted through a long series of centuries, and a huge mountain 
has been built up by the ash and lava erupted from within. The first 
eruption of which we know occurred in the year 476 B.C., and there is 
record of at least 80 vigorous eruptions since that time. During the 
nineteenth century there have been about 20 eruptions, or an average 
of one every four or five- years. These eruptions vary greatly in 
violence, in the interval separating them, and in the length of time 
they last. During some of the eruptions the volcano is active for 
two months or more, with repeated explosion and lava flows ; and in 
the interval between eruptions a column of steam ordinarily rises 
from the summit crater. 

The violent eruptions are commonly preceded and accompanied 
by earthquakes, some of which have been very destructive. For 
example, in connection with the eruption of 1693 between 60,000 and 
100,000 lives were destroyed by earthquakes. During the eruption 
successive explosions send vast columns of steam and ash into the air 
from the summit crater, while one or more fissures open on the flanks 
of the cone, from which floods of lava escape, flowing down the slopes 
at first rapidly, then, as it solidifies, with increasing slowness. These 
lava flows escape from all sides of the cone, and at various levels, 
most of them ceasing to flow before reaching the mountain base, 
though some spread out at its base, and even enter the sea. 

Over the fissure from which the lava issues, parasitic cones may be 
reared. This habit of the breaking out of lava from the flanks of 
volcanoes is a common phenomenon of volcanic eruption. It is 
evidently due to the great strain exerted upon the flanks of the cone 
by the column of imprisoned lava. The very weight of the column 
exerts a pressure of from 70 to 80 tons per square foot for every 
thousand feet of lava in the column. Added to this is the great 
pressure of the included gases, whose explosive force is clearly 
indicated by the prodigious height to which it expels rock frag- 
ments from the crater. Doubtless also the sides of the cone are 
weakened by the jarring and shaking accompanying eruption; and 
possibly there is also an influence from the melting of the rocks along 
the conduit. 

It seems evident that a volcanic cone can be built higher than the 
lava column can rise in its conduit. The upper part of the cone, 
being made largely of fragmental ejecta, cannot resist the pressure 
upon it, and therefore the lava drains out through fissures, not by over- 
flow of the crater rim. There may be other factors involved, such as 
the failure of the force that is pushing the lava upward to raise it to 



452 



COLLEGE PHYSIOGRAPHY 



the crater rim, but here and elsewhere the weakness of the upper cone 
is evidently the main cause for the lateral eruptions. 

When eruption through the fissures ceases, they are filled with 
molten rock, which, on cooling, forms vertical or highly inclined seams 
or dikes of rock more solid than the fragmental layers. These tend 
to rivet the layers together, and thus strengthen the cone; it is con- 
ceivable that they might finally so strengthen the walls as to check 
further fissuring. Where volcanic cones have been disrupted by 
explosion, and where extinct cones have been dissected by de- 
nudation, the dikes in former fissures are clearly exhibited, extend- 
ing in various directions. 

One of the greatest eruptions of Etna was that of 1669, which was 
preceded by a violent earthquake. Six parallel fissures opened, one 
after the other, one of them 12 miles long and extending to within a 
mile of the summit of the cone. Near Nicolosi, which had been de- 
stroyed by the earthquake, a double cone was formed of scoriae, rising 
to a height of about 450 feet. The lava that issued from the mountain 
side flowed 13 miles the first twenty days, or at the average rate of 162 
feet per hour; but twenty-three days were required for it to flow 
the last two miles, or at a rate of but 22 feet per hour. It entered the 
sea with a current 600 yards broad and 20 feet deep, pushing the coast 
outward. In its course this lava flow overwhelmed fourteen towns 
and villages, and finally reached the walls of Catania on the coast. 
It slowly rose and, falling over a wall 60 feet high, covered part of 
the city; but it stopped its forward movement there. This lava 
flow still forms a terrace on one side of Catania, and houses and streets 
are built upon it. 

Etna had severe eruptions in 19 10 and 191 1, in the latter year 
pouring forth more lava in 5 days than in the 26 days of the 1910 
eruption. Scores of vents opened on the sides of the mQuntain and 
the main crater ejected ash which buried the adjacent country to a 
depth of several inches. 

The lava flows of recent date are readily distinguished on the flanks 
of Etna, and in general they are identified by the different degrees of 
disintegration and vegetation cover. The most recent are great, black 
bands of rough-surfaced lava, wholly uninhabited and utterly desolate 
in appearance ; but the more ancient ones have a soil on which the 
vine and other cultivated plants are raised, in some cases merely in 
pockets where rough fragments have accumulated, or to which soil 
has been carried by the peasants. Some of the lavas of the past cen- 
tury are now in part covered by vegetation ; but on the other hand a 
period of over 500 years has not sufficed to clothe the flows of 1381 
with vegetation. 

Etna is a perfect type of a symmetrical volcanic cone erupting both 
ash and lava. The symmetry of its form is interrupted by the parasitic 
cones already mentioned ; but, though these are conspicuous features 
in the detail of the landscape, they are lost in a general view of the 



VULCANISM 453 

cone as a whole. The symmetry of the cone is also somewhat modified 
by the presence of a great depression, or valley, known as the Val del 
Bove, evidently the product of some catastrophic, prehistoric eruption. 
With the copious outflow of lava from the flanks of the cone, it is being 
broadened at the base. Like all volcanoes it is gullied by erosion, 
but the successive lava flows tend to seal up these valleys before time 
enough has elapsed for them to progress far in the cycle of denudation. 

The Bay of Naples. — The northern side of the Bay of Naples 
is volcanic in character, with Vesuvius on one end, the volcanic 
island of Ischia on the other, and a group of small cones between. 
Some of these are small islands, others are on the land, in what 
is known as the Phlegraean Fields. This region was the seat of 
the earliest Greek civilization in Italy ; it was later of importance in 
the days of the Roman Empire ; and the shores of the Bay of Naples 
are to-day densely settled. 

Ischia and the Phlegraean Fields. — Ischia, an island 19 miles in 
circumference, is entirely volcanic, and there are a number of small 
cones and craters upon it in addition to the main cone, Mount Epomeo, 
2782 feet high. It is generally believed to have been quiet for 17 
centuries ; but before the Christian era this volcano was active, the 
island being deserted by the Greek inhabitants because of the great 
eruptions of 474 B.C. Another eruption occurred in the year 92 B.C., 
and there were other eruptions before and after this. The last erup- 
tion, however, was in 1302, and since then the volcano seems to have 
become extinct, though there are occasional earthquakes, like that 
of 1883, which destroyed Casamicciola. This may indicate that the 
volcanic forces are merely slumbering. 

In the Phlegraean Fields the numerous low cones and craters are 
evidently all of recent origin. Among them are two known to have 
originated within historic times. The first of these is Solfatara, which 
erupted in 1198, but is known to have existed before. Steam still 
rises from the floor of its crater in great jets, and sulphurous gases 
are depositing sulphur crystals around the orifices of the multitude of 
smaller steam jets. 

Monte Nuovo. — Near by is Monte Nuovo, a circular cone 8000 
feet in circumference and about 440 feet high, rising from the shores 
of the bay. It is truncated on the top and contains a perfect crater. 
Before 1538 the site of this cone is said to have been a lake and for 
two years the region was visited by frequent earthquakes. On the 
28th of September, 1538, there were twenty earthquake shocks, and 
on the 29th, the ground was rent and showers of hot stones and ashes 
were thrown into the air, and the cone was speedily built before the 
eyes of the horrified observers. Other accounts of this eruption differ 
from this one in minor details. To-day the slopes of the cone are 
terraced and occupied by vineyards. 

Vesuvius. — It is Vesuvius, however, that forms the central point of 
interest in this volcanic district (PI. VII) . Up to the year 79 a.d. it had 



454 



COLLEGE PHYSIOGRAPHY 



been in a long period of repose. There is every reason to believe that, 
during the centuries of Greek and Roman occupation of the region, it 
had not been active. It was a low, broad cone, with a great crater, or 
caldera, at its summit, some three miles in diameter. Woods covered 
the rugged slopes and crater walls, cultivated farms dotted the lower 
mountain side, while villages and cities skirted its base then as to-day. 
Among these were the populous cities of Pompeii and Herculaneum. 

In the year 63 there was a destructive earthquake, which did damage 
in Pompeii that had not been fully repaired when that city was de- 




Fig. 298. — A street in Pompeii. 



stroyed in 79. Other shocks followed, becoming more and more 
numerous and violent in August of the year 79 ; then came the most 
violent eruption that Vesuvius has experienced in historic time. One 
side of the crater wall was blown away, and a cone was started on its 
site, from which eruptions have occurred at intervals ever since; 
the other part of the crater wall, called Monte Somma, still rises on 
the north side of the cone. It is made of volcanic ejecta and is riven 
by numerous dikes, and is separated from the present day cone by a 
crescentic valley which neither ash nor lava have as yet succeeded 
in completely filling. 

Ashes fell upon the surrounding country, a huge column of steam 
and ash darkened the sky, and great torrents of water fell upon the 
flanks of the mountains. Pompeii was buried beneath a cover of ash 
and dust, which penetrated every crevice and so sealed the objects in 



Plate VII 



>T49' 



3 




SCALE OF FEET 



500 1000 1500 2000 2500 



WILLIAMS ENGRAVING CO., 



VESUVIUS 



The upper part of the cone of the active volcano, Vesuvius, with lava flows of various periods dated. 
Old wall of Monte Sonnsa on the north. Combined contour and hachure map. Elevations in 
feet. (After Friedlaender in Petermanns Mitteilunger, 191 2.) 



VULCANISM 455 

a compact cover. In the excavations which have been made during 
the last century, objects of even a perishable nature have been re- 
covered. From them we are able to tell far more about the life and 
habits of the people of that day than history alone tells. It is a wonder- 
ful experience to walk through the deserted streets of this ancient 
city of 20,000 inhabitants (Fig. 298), to realize under what terrible con- 
ditions the people were driven out or overwhelmed in their efforts to 
escape. It is even more wonderful to examine in the museums the 
perfectly preserved pictures, utensils, and other objects hurriedly 
left behind by a terrified people fleeing before one of the most frightful 
catastrophes of history. A contemporary account of this eruption, 
from the letters of the younger Pliny to Tacitus, has been translated 
by Shaler. 

Herculaneum was overwhelmed by a great mud flow, and this has 
since been covered by a lava flow, on which a village now stands above 
buried Herculaneum. Part of it is now exposed to the air, but it 
has been more difficult to excavate this city, and it has been only 
partly explored by subterranean excavations, whereas Pompeii is 
largely opened to air by the removal of the cover of from 10 to 
30 feet of loose volcanic ash, lapilli, sand, and dust. Doubtless 
other houses and villages, destroyed during that great eruption, lie 
buried beneath the accumulation of ash, lava, and mud flows that 
were thrown out in 79 and subsequent eruptions. 

The record of the activity of Vesuvius after the eruption of 79 is in- 
complete, but there is record of eight eruptions before 1138, after 
which there was quiet for 168 years; but during this interval there 
was an eruption of Solfatara in 1198 and of Ischia in 1302. Vesuvius 
erupted again in 1306, and in 1500, after which the volcano remained 
dormant until 1631, though in the interval Monte Nuovo was formed. 
Between 1 138 and 1631, an interval of 493 years, there is no record of 
vigorous eruption of Vesuvius. This period of tranquillity was in- 
terrupted by the second most violent eruption of Vesuvius in 1631, 
during which vast quantities of ash were expelled, while seven lava 
streams poured down the slope, one of which overflowed the site of 
Herculaneum, destroying a village built there, while others over- 
whelmed other villages at the mountain base. Since that time Vesu- 
vius has been frequently active, and ten years has rarely elapsed with- 
out an eruption, while in the interval the crater has been steaming, 
and usually scoriae have been emitted as described on p. 443. 

The last two violent eruptions occurred in 1872 and in 1906. The 
former began in January, 1871, with ejections from the crater and 
small lava streams from the sides of the cone. This period of activity 
culminated in the great eruption of April 24 to 30, 1872, during which 
vast quantities of ash were thrown high in the air, and numerous lava 
streams issued from fissures in the mountain side (Fig. 299). 

The eruption of 1906 was similar in character, commencing nearly a 
year earlier and culminating in a period of grand eruptions between 



456 COLLEGE PHYSIOGRAPHY 

the 4th and 7th of April, 1906. It is estimated that the volcanic 
dust and steam were shot up to a height of 4 miles. It settled on 
the surrounding country, even in Naples, 10 miles distant, in sufficient 
quantities to cause the roofs to collapse. The mountain side was 
covered with ash, as with freshly fallen snow, and vineyards and or- 
chards were badly damaged, while the roofs of houses collapsed 
under the unaccustomed load. In places four or five feet of ash fell 
on the surface. Four years later, when the author visited the volcano, 
the coat of ash was still notable, though it was evidently being rapidly 
removed by running water. 

Lava issued from several fissures on the slopes of the volcano, 
coming apparently from near the site of the ancient crater rim of 




Fig. 299. — Vesuvius in eruption in 1872. 

Monte Somma, blown away in the eruption of 79. It flowed down the 
slope of the cone in narrow streams, one of which invaded the village 
of Boscotrecase, overwhelming a part of it, but stopping short of com- 
plete destruction. 

Thus Vesuvius resembles Etna in the nature of its eruptions ; but 
the cone is far smaller than that of Etna, and a less volume of lava is 
erupted. There are a few small parasitic cones on Vesuvius, called 
bocas. They form no such conspicuous feature as do those on Etna. 
The most noteworthy feature of the Vesuvian history is the long in- 
tervals of quiet, during which the volcano might be thought to be 
extinct, then the sudden awakening with an explosion of terrific 
violence, removing the obstruction caused by the solidification of the 



VULCANISM 



457 




458 



COLLEGE PHYSIOGRAPHY 



lava in the vent, and with it a part of the former crater. Even the 
eruption of 1906 completely altered the upper part of Vesuvius, 
lowering its summit by some 500 feet, truncating its top, and forming 
a far larger crater than existed before. In the intervals of quiet other 
neighbouring vents were active ; but in the past three centuries of prac- 
tically incessant activity of Vesuvius there have been no eruptions 
from other neighbouring vents. 

Other Volcanoes in Europe 

Volcanoes of the Eifel. — In Germany there are no active volcanoes 
to-day; yet in former days there has been much such activity, 
notably in the district of the middle Rhine and west of this in 




Fig. 301. — Volcanic necks in the Auvergne district of France. 

the volcanic Eifel. Some of the rocks exposed in the gorge of the 
Rhine are lavas, and there have been successive periods of activity, 
during which different types of lava were erupted. 

The latest phase of vulcanicity in this region occurred in the Eifel ; 
and the perfection of the cones and craters, their relation to the topog- 
raphy of the country, and the emanations of carbon dioxide and other 
gases, prove conclusively that the period of activity was very recent. 
But, so far as can be told, the period of vulcanism is, for the present 
at least, at an end in this district. 

Many of the Eifel volcanoes were the result of a single explosion, 
while others had successive explosions, and from some lava streamed 
forth. Some of the cones are very small and none of them are of that 
large size which results from centuries of activity. In some cases 
no noticeable cone was formed — only a crater-like cavity now occu- 



VULCANISM 459 

pied by one of the circular lakes or maare (Fig. 300). Similar lakes 
also lie in the craters of some of the cones. While most of the erup- 
tions consisted of ash, there were some which threw out no ash or 
lapilli, but only fragments of the country rock. These were merely 
steam eruptions, without the accompaniment either of lava or vol- 
canic fragments. Throughout the region blocks and pieces of 
country rock occur in the ash deposits, indicating that the rising mass 
drilled its way through the upper crust and carried the fragments 
with it. Among these blocks are pieces of granite and other rocks 
not found in place in the region, but evidently lying at a consider- 
able depth below the surface, in the portion of the earth's crust 
through which the lava was forced. 

Auvergne and Other Regions. — Another region of recently extinct 
volcanoes in Europe is in the Auvergne region of the highlands of 
central France (Fig. 301). Earlier eruptions occurred also in Scot- 
land, Ireland, England, and many other parts of Europe not now 
volcanic. 



Pele in the Island of Martinique 

The Lesser Antilles, which border the Caribbean Sea on the east, 
are a chain of volcanic cones, rising from a submarine mountain 
ridge, which sweeps down to the South American coast. One of these 
islands, Martinique, contains the cone of Mont Pele, in which, prior 
to 1902, there was a crater some 2000 feet deep and half a mile in 
diameter. The volcano was breached by a deep gash on the south- 
west side, opening toward St. Pierre, the capital of the island, a city 
of about 26,000 inhabitants (Fig. 302). 

There had been no eruption of this volcano since 1851, when there 
was an outbreak that did little damage; but in April, 1902, signs of 
activity appeared, vents opening in the crater bottom, and steam 
and ashes being thrown out of the crater. Sulphurous vapours poured 
out of the mountain, ash fell in St. Pierre, and frequent earthquakes 
occurred, among other things breaking the cables offshore. In the 
early days of May there was considerable activity, but on the Sth a 
terrific eruption occurred, and the steam and ash rose high in the air ; 
but a portion was propelled through the gash in the crater rim, and 
rushed down upon the city of St. Pierre with terrific force, going the 
distance of three miles, it is estimated, in about two minutes. The 
hurricane of superheated steam and hot ash overthrew buildings, 
hurled an iron statue from its pedestal, and overturned cannons. At 
the same moment the city caught fire, either from the hot gases or 
from the red-hot ash (Fig. 303). With a single exception the entire 
population of the city was instantly killed, for the cloud is estimated 
to have had a temperature of 1400 or 1500 F. and to have con- 
sisted of steam, sulphurous and other gases, and hot dust and other 
volcanic fragments. 






460 



COLLEGE PHYSIOGRAPHY 



Grande Riviere 



Other eruptions took place during the succeeding months, some of 
them extending the destruction, and during their greatest activity 
it is estimated that the column of steam and ashes rose to heights as 
great as seven miles. By these eruptions a cone of volcanic fragments 
has been built up in the old crater, even rising above its former 
walls. 

In the late stages of the eruption a peculiar phenomenon appeared 
in the form of a " spine " that slowly rose out of the crater. As it 
rose it crumbled away on the face, but more followed, though ulti- 
mately it collapsed. This spine consisted of hot, porous lava, ap- 
parently pushed up 
from the vent by the 
expansive force be- 
low. Its rise has been 
compared to the move- 
ment of a cork forced 
out of a bottle by 
the gases within (Fig. 

304). 

During the eruption 
of Pele the volcano 
Soufriere in the ad- 
jacent island of St. 
Vincent also broke 
forth, as if in sym- 
pathetic activity ; but 
with far less dramatic 
results. Ash from this 
eruption also spread 
over a wide area, dev- 
astating much of the 
surrounding country 
and settling on the 
sea round about, but 
especially in the direc- 
tion of the prevailing 
winds. Great mud 
flows swept down the valleys, especially in St. Vincent, and from 
their surfaces jets of steam rose as they flowed along. 

Both of these eruptions are of the explosive type, following long 
periods of quiet. They differ from the ordinary eruptions of Etna and 
Vesuvius in the absence of associated lava flows. The appalling de- 
struction of life at St. Pierre was due less to the violence of the eruption 
than to the peculiar topographic feature which directed the blast 
upon the fated city. It has been compared to a break in the breach 
of a gun by which a part of the discharge escapes through the break 
instead of through the muzzle. Doubtless the lateral motion through 




Fig. 302. — Zone of destruction (oblique lines) on the 
island of Martinique in 1902. 



VULCANISM 



461 




462 



COLLEGE PHYSIOGRAPHY 



the gash was brought about by the overlying column of steam and 
ash. Such a phenomenon gives basis for understanding the enormous 




Fig. 304. — The spine protruded from Mt. Pele. (Heilprin.) 

lateral pressure to which the walls of the volcanic vent are subjected 
by the vapour-charged lavas within them ; and to account for the 
frequent Assuring of volcanic cones. 



Krakatoa 

This volcano, in the Straits of Sunda between the islands of Java 
and Sumatra, was in 1883 the seat of one of the most violent eruptions 
of which there is record. There was an eruption about a century 
earlier, and in the interval there had been such a solidification of 
lava in the vent, and such a gathering of subterranean energy that, 
when the eruption finally occurred, it took the form of a terrific ex- 
plosion. There had been preliminary earthquakes and minor explo- 



VULCANISM 463 

sions, but, on the 27th of August, two-thirds of the island was blown 
into the air, and, with it and following it, vast volumes of steam and 
volcanic fragments. It is estimated that over a cubic mile of 
rock fragments was hurled upward during this explosion ; and on the 
site of the cone the water was 1000 feet deep after the eruption (Fig. 
305). The steam and volcanic dust is estimated to have been 
thrown 17 miles or more into the air. 

Such a vast explosion naturally set a series of air waves in motion. 
Windows were broken at a distance of 100 miles, loud detonations 
were heard at a distance of 150 miles, and the sound was even heard 
in Australia 2000 miles away. A barometrical disturbance, moving 
at the rate of 700 miles an hour, passed through the atmosphere, and 
was recorded in the self-registering barometers, from the records of 




Fig. 305. — Cross-section of the half of Krakatoa left after the explosive eruption of 1883. 

(Symons.) 

which it is believed that the wave which moved westward made the 
circuit of the earth three and three-quarters times, 82,200 miles, before 
finally becoming imperceptible. 

The sea was also greatly disturbed, and waves rose more than 100 
feet above tide level on neighbouring coasts. These water waves spread 
throughout the Indian and Pacific oceans, having been recorded on 
the tide gauges even as far distant as South Africa, 5450 miles from 
Krakatoa. These waves travelled at the rate of 467 miles an hour. 

The falling ash and pumice covered the neighbouring sea as with ice, 
interfering with navigation, and doubtless it was drifted all over the 
surrounding oceans, slowly becoming water-logged and settling to 
the bottom, where not washed upon the coasts. The great volume of 
dust in the air darkened the sky at a distance of 150 miles ; and some 
of the finely commuted particles evidently remained in suspension 
in the air for months, and drifted to various parts of Asia, Europe, 
and America. This conclusion is based upon the fact that a series 
of such unusually brilliant sunsets appeared progressively as to 
attract attention of observers in many places in each of these conti- 
nents. A study of the records of the phenomena of the sunsets has led 
to the conclusion that they were due to the abundance of dust from 
the Krakatoa eruption. 

On the remnant of Krakatoa every vestige of life was destroyed 



464 



COLLEGE PHYSIOGRAPHY 



z 1 ;? 6 /,, p ;>,t Longitude West 155130' from Greenwich 155° 

f^/X, 



( l„,lu 



by the eruption, and had it occurred in a settled region, such as the 
country round about Vesuvius, the destruction of human life would 
have been appalling. As it was, the water waves were the main cause 
for the loss of human life. By them over 36,000 people were killed 
on the neighbouring coasts, and many towns and villlages were de- 
stroyed. 

Hawaiian Volcanoes 

Very different is the volcanic activity of the active cones in the Hawai- 
ian Islands, a chain of volcanic peaks on the crest of a submarine moun- 
tain ridge. There have been no less than fifteen large, active volcanoes 

in this chain, but only 
two are now active, — 
Mauna Loa and Kilauea 
in the largest of the 
islands, Hawaii. This 
large island is a vol- 
canic pile, so far as 
known composed almost 
entirely of volcanic ma- 
terials, mainly lava. It 
rises from the sea bot- 
tom at a depth of about 
16,000 feet and extends 
nearly 14,000 feet above 
sea level, making a great 
volcanic mass 30,000 feet 
or more in height, form- 
ing, so far as known, 
the largest volcanic 
mountain in the world. 
It is not, however, built 
around a single volcanic 
vent, for it is made up 
mainly of three volcanic 
mountains, Mauna Kea, 
Mauna Loa, and Hualalai. The last of these has not been in 
eruption since 1801. Kea is now extinct, but Loa is in frequent 
eruption; and on its slopes, 20 miles from the summit and nearly 
10,000 feet below it, is Kilauea, which projects only about 300 feet 
above the surrounding surface and hardly interrupts the long, gentle 
slope of Mauna Loa (Figs. 306, 307). 

In each of these volcanoes there is a large crater, or caldera, that of 
Mauna Loa being over three miles long and nearly two miles broad, 
while the Kilauea crater is about two miles long and one mile broad. 
Lava rises in these craters, not to be expelled by violent explosion, 
nor usually to flow out of the crater, but ordinarily to find escape 




19^30' TOr- - H >A ,:AV> Ay 
' Y^* .Milium U.W Y = V .' 




Fig. 306. 



Map of the island of Hawaii, with dates of 
some of the lava flows. 



VULCANISM 



465 



through fissures on the mountain side, some from near the summit, 
others from far down the slopes. When the fissures open, there are 
sometimes earthquakes, though not always, and the liquid lava spouts 
out fountain-like, rising several hundred feet in the air, and flowing 
down the mountain side. Some of these flows are from 20 to 40 



MAUNA LOA 



Se<* IcreJ 



I 






Fig. 307. — Relationship of Kilauea to Mauna Loa. (Daly.) 

miles long, and some of them end only when they reach the sea and 
build the coast outward into the ocean. 

There is no regular periodicity of eruption, but on the average 
there is eruption once in eight or nine years, this being apparently 
the time required for the lava column to rise in the vent and exert 




Fig. 308. — View within the crater of Kilauea, Hawaii (Pavlow). 

the necessary pressure to burst through the side of the volcano. It 
is a remarkable fact that Kilauea shows no sympathetic response to 
conditions in Loa, and that lava stands in the crater of Kilauea, 
although it is several thousand feet lower than the lava column of 
the neighbouring volcano. 



466 



COLLEGE PHYSIOGRAPHY 



The crater of Kilauea presents a remarkable spectacle. It is a 
broad, deep pit bordered by black, terraced, lava walls. In the 
bottom is a rough plain of lava, crusted over for the most part, but 
with one or more lakes of liquid lava called Lakes of Fire, whose 
boundaries and position shift from time to time. In these lava lakes 
the molten rock is in a state of ebullition, and it boils and surges 
against the enclosing walls (Fig. 309), and sometimes overflows a 
part of the enclosing rim. From the surface fountain-like jets of 
molten rock rise two or three hundred feet in the air, and some- 



Lava poured into 
subterranean cavern 
with great velocity 




A,B, C,D, path of moving 
founlain which always term- 
inated at D, ihe most violent point. 
A - "Old raithful ' which erupted 
at intervals liberating large 
volumes of gases. 

X bubbled rarely and then 
always accompanying A. 



Lava constantly disappeared 
here 



Fig. 309. 



Sketch map of Halemaumau, in the crater of Kilauea, Hawaii, on Feb. 15, iqoq. 
(C. H. Hitchcock.) . 



times the wind spins from it hair-like threads of natural glass 
known as Pele's Hair. 

The Hawaiian volcanoes differ greatly from any of those previously 
described. They are, in the first place, flat in slope, rarely sloping 
more than 6° or 8° ; they are very broad, thus including a vast amount 
of volcanic material. While there is some ash and there are some 
lapilli, bombs, and other fragmental ejecta, the volcanoes are made 
mainly of successive lava flows. The lava outflows in ^1 very liquid 
state, and with a tranquillity quite unusual in volcanic eruptions. 
Finally, one may watch the eruption, even from near at hand, with 
comparative safety ; and one may go, not only to the crater's edge, 
but even down in it and to the very margin of the lava lakes (Fig. 
308). It is a remarkable phase of volcanic activity. 



VULCANISM 467 

Icelandic Volcanoes 

Iceland is a volcanic island, consisting of an extensive lava plateau 
built during the preceding geological period from fissure eruptions. 
On this island is the large volcanic cone called Mt. Hekla. Iceland 
is of special interest because of the fact that there has been a contin- 
uation of fissure eruption into the historic period, the only known 
case of a phase of eruption once common. The great Icelandic erup- 
tion of 1783 was preceded by a submarine eruption 200 miles away 
some four months earlier. The lava came from a great fissure 12 
miles long. It flowed out in both directions, extending 45 or 50 miles 
on each side with an average depth of 100 feet. River gorges were 
filled, and alluvial plains were flooded by lava lakes from 12 to 15 miles 
wide. There was, in fact, a literal deluge of lava, in which more 
molten rock flowed forth than in any case on record, a bulk estimated 
to exceed that of Mont Blanc. 

Ash was thrown into the air from vents, and it fell not only on the 
island, but on the surrounding sea. Vessels between the Orkney and 
Shetland islands were obliged to shovel it from their decks ; and so 
much fell in Caithness, in northern Scotland, 600 miles distant, that 
crops were destroyed. In Iceland, with a scattered population of but 
50,000, fully 9000 people perished from inundations of water, caused 
by floods where streams were dammed and diverted ; from the advance 
of lava ; from poisonous gases ; and from showers of ashes. The 
latter cause brought about famine by killing the cattle, by destroying 
crops and pasturage, and by the effect of the glassy volcanic dust, 
which clung to the grass that the cattle ate. 

Other Volcanoes 

Besides these there are a multitude of other active volcanoes which 
illustrate similar eruptive phenomena. In the Atlantic Ocean there 
is a chain of volcanoes forming the Azores, including some good-sized 
cones now extinct, and a multitude of small cones. That the volcanic 
activity has not quite died out here is indicated by the fact there have 
been several small eruptions during the past century. The Madeira, 
Canary, and Cape Verde islands are also volcanic, as are Ascension, 
St. Helena, and other islands farther south. The multitude of small 
islands and groups of islands in the Indian and Pacific oceans are also 
volcanic, or else coral islands built on volcanic cones, as are the Ber- 
muda Islands in the Atlantic. Some of these are active, but most are 
now extinct. 

There are volcanoes in the eastern Mediterranean, and in western 
Asia, including Mount Ararat, which was in eruption in 1840. The 
East Indies include a multitude of volcanoes, some of them, notably 
those of Java, having had violent eruptions. The same is true of the 
Philippine Islands, where the Taal volcano had a destructive eruption 



4 68 



COLLEGE PHYSIOGRAPHY 



in 191 1, and Japan, where Fujiyama is well known for its sym- 
metrical cone (Fig. 310). The Sakurajima volcano had two great lava 
flows and ash showers, and destroyed 24 lives in January, 1914. 

In the New World there is a nearly continuous chain of volcanoes 
from the Aleutian Islands to Chile. Many of these are extinct, espe- 
cially those of western United States, but in the Andes and in central 
America and southern Mexico there are many active volcanoes. 

In Alaska there are said to be no less than 57 active volcanoes 
in the Aleutian Islands, which extend westward about 1600 miles. 
Mount Wrangell on the mainland is the easternmost active volcano of 




Fig. 310. — The symmetrical cone of Fujiyama in Japan. 



this chain. Mount Edgecumbe at Sitka is probably only dormant, 
and lava flows near the Blue River in southeastern Alaska are surely 
postglacial. One is less than 50 years old, for there are charred and 
blackened tree trunks near its terminus. 

Little is known about the volcanic history of the Alaskan volcanoes, 
though one, Bogoslof, in Bering Sea, has attracted special attention 
because of its unusual history. There had been here a volcanic rock 
rising from a shoal, but in 1796 a submarine eruption occurred and a 
new volcanic cone appeared, which in four years had grown to a 
height of several hundred feet above sea level. It stands in water 
Oooo feet deep, and Bogoslof is, therefore, only the top of a large 
volcano. The sea normally occupies its crater, and hence we have the 



VULCANISM 



469 



explosions, by which its low, subaerial cones are periodically destroyed. 
Since 1796 there have been frequent eruptions and changes in the form 
and size of this new volcano, as the accompanying maps show 

(Fig. 3"). 

Katmai volcano (Fig. 295), on Alaska Peninsula, had a severe erup- 
tion in June, 191 2. Complete darkness lasted for 60 hours at Kodiak, 
100 miles distant. Dust fell at points 600 to 900 miles away, and 
fumes were reported at Vancouver Island, 1500 miles distant. The 
fall of from less than an inch to over 50 inches of ash near the volcano 




KRUSENSTERN 1826 




CANTTOIL 1084 




DALL 1895 




ew Bogo3lof (Growlngk) 




50 -foot contours 




Llefucm ft., 1851 m. ) 
CAMDEN Oot.1907 



Fig. 311. — Maps of a few of the known stages in the recent history of Bogoslof, where a 
low cone is built up and then destroyed by an explosion. This has happened several 
times more since 1907. (After Jaggar.) 

is shown in Fig. 312. The vegetation was buried, and natives were 
forced to move to new homes outside the afflicted district. Immense 
fields of pumice floated on the sea. The dust in the air was observed 
in distant parts of America and Europe, and may even have affected 
climate during the following year. 



Volcanoes of Western United States 

Former Activity. — So far as can be told, there are no active vol- 
canoes in western United States. There is a partially authenticated 
record that Mount St. Helens was in eruption about 1841 ; and from 



47o 



COLLEGE PHYSIOGRAPHY 



Mounts Baker, Rainier, and Hood sulphurous vapours and steam still 
rise. One of the fumeroles of the latter is said to have melted the 
glacier ice considerably in 1907. Carbon dioxide and other gases 
escape in association with many volcanoes of this region. Many of 
the cones are perfect in form, and the craters are not breached, proving 
that they have not long been inactive. There are also cinder cones of 
great freshness, and lava flows that cannot long have been exposed to 
the air. All these volcanoes, therefore, seem to be dormant. A short 




t— * 8 1 



• Wlfrl'V 



"\ » 1 I I I ' 



One-i ^ 



SCALE OF MILES 



20 30 40 50 



Fig. 312. — Map showing distribution of ash during the eruption of the volcano near 
Katmai, Alaska, in 1912. (G. C. Martin.) 



distance north of Mount Baker, near Vancouver, B.C., there are volca- 
noes which are said to have been active since the Glacial Period. 

In Arizona is Coon Butte, or Meteor Crater, thought by some 
observers to be an impact crater formed by a falling meteorite and by 
others to be related to explosion or subsidence in connection with 
deep-seated volcanic activity. Its walls are not igneous but sedimen- 
tary rock. On the other hand, borings in the centre have failed to 
reveal any meteorite (Fig. 313). 

Volcanic activity in western United States has been present at vari- 



VULCANISM 



47i 



ous earlier periods, as is proved by abundant volcanic deposits and 
associated features. In the last period, which seems now to be nearly 
if not quite at an end, there was activity over a broad area, between 
the Rocky Mountains and the Pacific, and from Mexico to Canada. 
The activity was also prolonged, for there were great fissure eruptions, 
giving rise to lava floods ; and lofty cones were built around some of 
the vents, such as San Francisco Mountain in Arizona, Mount Shasta 
in California, and Mounts Hood, St. Helens, Adams, Rainier, and 




Fig. 313. — Topographic map of Coon Butte (scale 1 : 30,000), showing by black dots the 
positions of the holes drilled in the bottom of the crater in search for a meteorite. 
(Baker.) 



Baker farther north. Some of them rival Etna in size and in grandeur 
— one is, for example, reminded of Etna by Shasta ; and few moun- 
tains in the world have a grander symmetry than Mount Rainier as 
seen from Tacoma, or Mount Hood from Portland. Some of these 
cones may yet awake into activity, though upon this point prophecy is 
not safe (Figs. 157, 314). 

Mount Shasta. — As an instance of a volcanic region in western 
United States we will take the case of Mount Shasta and vicinity. 
Shasta is a very symmetrical cone 14,380 feet high, seventeen miles in 
circumference at its base, having a volume of about 84 cubic miles. 



472 



COLLEGE PHYSIOGRAPHY 



It has been so long extinct that the summit crater is gone, but 2000 feet 
below the summit is a younger well-developed cone, known as Shastina, 
with a crater in its top. Definite lava flows are recognizable on the 
sides of the main cone, and there are numerous parasitic cones on the 




Fig. 314. — Topographic maps of four volcanoes of western United States. (After U. S. 

Geol. Survey.) 



lower slopes and around the mountain base. It has apparently been 
extinct for many centuries. 

Recent Eruption of Cinder Cone. — Eighty miles or more to the 
south of Shasta is Lassen Peak, one of a series of cones in a belt extend- 
ing northwest and southeast. There are several large cones in this 
belt, Lassen Peak rising 10,437 feet, and there are large numbers of 
smaller cones. One of these, known as Cinder Cone, lies about ten 
miles from Lassen Peak. As the name indicates, it is made of volcanic 
ejecta, but a small lava flow extends out from its base. The cone 
rises 640 feet, is 2000 feet in diameter at the base, and has a perfect 
crater in its truncated top. 



VULCANISM 



473 




SECTION FROM A TO B 

Fig. 315. — Map and cross-section of Crater Lake. (After U. S. Geol. Survey.) 



474 



COLLEGE PHYSIOGRAPHY 



It has been shown that there were at least two eruptions, separated 
by an interval of a century or more, the first being mainly ash, the 
second lava. The most interesting feature connected with these 
eruptions is the evidence of their recency ; for trees killed by the ash 
still stand, and there are also standing trees in Snag Lake, which was 
formed by the lava dam, and by whose rise the trees were killed. It 
surely cannot have been a long time since these events happened, else 
the trees would have fallen. Apparently the last eruption occurred 




Fig. 316. 



The inner walls of the caldera of Mt. Mazama, with Crater Lake and Wizard 
Island. (Russell, U. S. Geol. Survey.) 



sometime in the first half of the last century, while the first eruption 
occurred a century or more earlier. 

Since there are similar evidences of recent eruption in other parts of 
western United States, we may be certain that there has been volcanic 
activity there since the discovery of America, and probably even since 
the close of the War for Independence. That there should have been 
no great eruption since the settlement of the west by white men is 
certainly remarkable ; but it does not prove that there may not yet 
be eruptions. Lassen Peak had a series of eruptions of moderate 
activity during the summer of 19 14. 

Crater Lake, Oregon. — This lake lies in a roughly circular crater, or 
caldera, five or six miles in diameter and about 4000 feet deep. The 
lake itself is 1996 feet deep and surrounded by nearly vertical walls 



VULCANISM 475 

from 900 to 2200 feet high. It is evidently in a truncated volcano, as 
are numerous other caldera lakes in the Eifel, in Italy, and in other 
regions. In this case the truncation is ascribed to subsidence, rather 
than explosion, it being believed that the volcano has been engulfed, 
perhaps because of the withdrawal of lava from beneath it (Fig. 315). 

That a volcano, now called extinct Mount Mazama, once existed 
here is proved by the presence of valleys on the outer slopes, which ex- 
tend up to the crater edge and are there truncated. Evidently there 
was once drainage from above, and the valleys have been beheaded. 
Another proof of the same conclusion is the presence of glacial scratches 
on the outer slopes, made by valley glaciers descending from some 
higher region, now gone. That is, there was evidently a mountain 
here, comparable to Mount Rainier, down which glaciers and streams 
flowed, and all the upper part of the mountain has disappeared. 

The most natural explanation of such a phenomenon is that the top 
has been blown away by an explosion, as Vesuvius was in 79 and 
Krakatoa in 1883. But this is negatived by the absence of fragments 
round about. The conclusion is, therefore, forced that the mountain 
top has been lost by subsidence. Since this catastrophe a small cone 
has been built up in the lake, forming Wizard Island (Fig. 316). It is 
possible there are other calderas of the same origin. 

Life History of a Volcanic Cone 

The Young Volcanoes. — Many phases in the life history of a vol- 
canic cone have been presented in the preceding statements with 



Fig. 317. — Mt. Rainier, Washington, a young volcano. (A. H. Barnes.) • 

regard to individual volcanoes. It remains now to state these in 
resume and carry the development further. 

A volcano may start by explosion, great or small, pushing forth the 



476 



COLLEGE PHYSIOGRAPHY 



fragments torn from the vent, together with ash or lava (Fig. 317). 
The site of the volcano may be on the sea floor or on the land ; and 
the eruptive activity may cease with a single outburst, or it may con- 
tinue for centuries. It is quite probable that other volcanoes may be 
built around open vents along a fissure from which lava floods at first 
poured, but which is no longer kept open except at one or more orifices. 
Conflict of Forces. — As soon as the volcano is reared into the air, 
it begins to suffer from the attacks of subaerial denudation, and, if in 
the sea, from the attacks of oceanic agencies also. Thus all the time 
during its activity two opposing tendencies are at work, one tending 




Fig. 318. — Canyons cut 2000 feet or more into the extinct cone of one of the westernmost 
of the Hawaiian Islands. (From map of Island of Kauai, U. S. Geol. Survey.) 



to build it up, the other to remove it ; but under normal conditions 
the work of upbuilding maintains mastery, while denudation serves 
merely to deface it and to render its growth less rapid. As the cone 
grows, its form will depend upon whether it is ash, or lava, or both 
combined, being flattest in the last case and steepest in the first. If 
activity continues, the sides are fissured and crossed by intruded dikes, 
while parasitic cones develop on the flanks and around the base. The 
period and phase of volcanic activity will vary, as we have seen in the 
preceding descriptions, and it may happen that the cone will be partly 
wrecked by violent explosion or by collapse. Each normal eruption 
will add to the cone, tending toward symmetry of form, and both ash 
and lava will serve in part to smooth over the irregularities caused by 



VULCANISM 



477 



denudation. But an explosion of abnormal violence, or a collapse, 
may completely destroy the symmetry. 

Extinct Volcanoes Rapidly Denuded. — When eruptive activity 
ceases, the symmetrical cone will be gullied by radial streams (Fig. 318), 
the crater, in which a circular lake may stand, will be slowly rilled, and 
the crater wall worn away and breached and finally destroyed, as in the 
case of the summit of Mount Shasta. As the cone is slowly worn away, 
the dikes of resistant rock which traverse the weaker inclined beds of 





Fig. 319. — Block diagrams of a youthful volcano and its lava flows (upper), and an old 
dissected volcano (lower) with lava flows represented only by lava-capped mesas. 



the cone may be etched into that wall-like relief from which the word 
dike is derived. Waves also destroy volcanoes in the sea (Fig. 327). 
Old Volcanoes. — The central part of the cone will tend to remain 
the highest (1) because there is more material, (2) because this is the 
centre of the radial drainage, (3) because the rock here is most resist- 
ant. This is due to the fact that when lava solidifies below the sur- 
face, it is more dense and less porous than when ejected into the air. 
The vent therefore becomes filled with a solidified plug of resistant 
rock, known as the volcanic neck, or volcanic plug, and this, because of 



478 



COLLEGE PHYSIOGRAPHY 



its superior durability, will remain highest. In a region of extinct 
volcanoes, all stages in these processes of destruction may be seen, 
even the volcanic necks rising steeply above the surrounding surface, 
from which nearly, if not quite all, of the ejected material has been 




Fig. 320. — Volcanic cap on hilltop where ancient lava flow in a valley covered auriferous 
river gravels (circles). Subsequently erosion by streams on either side has converted 
the former valley bottom into a hilltop. (California State Mineralogist's Report.) 

removed by denudation (Figs. 301, 319, 320). This is the last stage in 
the destruction of a volcano ; and since the plug extends far down into 
the earth, it may remain a witness of eruptive activity of long-past 
ages. Such evidence is one of the proofs of ancient volcanic activity 
in Great Britain. 

Distribution of Volcanoes 

The Two Volcanic Belts of To-day. — The great belt of active and 
recently active volcanoes is around the Pacific, and it has for that 




Fig. 321. — Distribution of active and recently extinct volcanoes. 

reason been called " the ring of fire " (Fig. 321) ; this belt includes by 
far the greatest number of active cones. A second belt is traceable 



VULCANISM 479 

through the Mediterranean, western Asia, the East Indies, Central 
America, the Lesser Antilles, and the Azores. These two belts are in 
general the same as the two earthquake belts already mentioned 
(p. 417) ; they are also belts of present day and recent mountain 
growth. There are, however, many volcanoes outside these two belts, 
such as those of Mexico, Iceland, New Zealand, Mounts Erebus and 
Terror in the Antarctic, and a large number of oceanic volcanoes. 
In detail, groups of volcanoes are usually in linear belts, and along 
one or more lines, usually curved, as if on fissures. 

Relationships of Location. — Most volcanoes are either in or near 
the sea, and a great number of them are on islands ; indeed, most 
oceanic islands are either volcanoes or volcanic reefs. However, there 
are active volcanoes, and still more extinct ones hundreds of miles 
from the sea. The association of volcanoes with the ocean seems to be 
either (1) on swells or ridges rising from the sea floor, or (2) on or near 
the edge of the continent, where it slopes abruptly into the deep oceanic 
basins. In each case, and probably in all volcanoes, there is associa- 
tion with zones of crustal movement. 

The Number of Volcanoes. — It is not possible to state, even approx- 
imately, the number of active volcanoes in the world, one difficulty 
being to determine which are active, for even a century or two of inac- 
tivity may not mean more than that the volcano is dormant. An 
estimate of 400 or 500 active volcanoes would probably not be too 
great, and the number may easily be 'twice that. Schneider states 
that 367 volcanoes are known to have been active in historical times. 
Recently extinct volcanoes are numbered by the thousands. 

The Former Distribution of Volcanoes. — : Many of the volcanic 
belts of to-day are along the line of, or close to, areas of former volcanic 
activity, proving that vulcanism can recur in the same region even 
after long intervals of quiet, as in the Auvergne region of central France 
and in the volcanic complex of the Yellowstone National Park. 
Other belts have apparently not had preceding volcanic activity. 
Again, belts in which there has been recurrent vulcanism have not 
witnessed present day or recent activity, as in the British Isles, and in 
northeastern United States. In both of these cases there has been 
profound volcanic activity in earlier ages, and the activity has come 
again and again ; but for a long period there has been exemptiQn from 
vulcanism. In northeastern United States, for instance, several 
geological periods have elapsed since the last phase of vulcanism in the 
Mesozoic age, a time period probably of millions of years. 

Each age seems to have its own belts of volcanic activity, and while 
these often coincide with earlier or later belts, they may extend quite 
independently of any previous or subsequent vulcanism. The asso- 
ciation in each case seems to be, as at present, with belts of crustal 
deformation, wherever these may lie. It is for this reason, too, that 
certain areas seem to have been exempt from volcanic activity through- 
out geological time. The plains of the Mississippi valley, for example, 



480 COLLEGE PHYSIOGRAPHY 

throughout most of their area have witnessed no vulcanism since the 
earliest geological ages. Their movements up and down have been 
unaccompanied by notable crustal deformation. Yet on the borders, 
as in central New York, dikes in places pierce the horizontal strata, 
though there is no reason for believing that the lava that was forced 
into them ever reached the surface. Lofty, broken plateaus, like the 
Colorado Plateau, share with mountains in volcanic outburst, but 
broad plains apparently do not, unless there is near-by crustal de- 
formation. 

Of these earlier eruptions there is little evidence left at the present 
day, excepting sheets of lava and ash, dikes, and volcanic necks or 
plugs. They are, therefore, of more interest to the geologist than to 
the physiographer. They influence the topography of to-day only 
as other strata of different origins might, if in similar position and with 
similar degree of resistance. They form no dominant topographic 
features, like volcanic cones, and they come in no direct, intimate 
relation to life as active volcanoes do. 

Decline of Volcanic Activity 

Mention has already been made of the former importance of lava 
eruptions from fissures, a phenomenon not now observed excepting in 
Iceland, and, on a small scale, on the flanks of active volcanoes. In 
the Tertiary time, however, lava issued from fissures in great volume, 
and spread over wide areas of country, as it had also done in 
earlier geological periods. There seem to have been periods of great 
volcanic activity, during which fissure eruptions occur, and these are 
followed by a period of declining vulcanism. The present appears to 
be such a period, following one of great activity ; but whether in 
geological time there have ever been times of freedom from volcanic 
activity cannot be said. There is no proof that there ever were such 
periods, though it may also be said that there is no proof to the con- 
trary. 

During the decline in volcanic activity, fissure eruptions cease, 
volcanic vents remain open along fissures, and these, one by one, are 
closed. But even after a vent ceases to pour forth lava or ash, steam 
rises fro/n the crater and from solfataras on and near the cone. This 
stage is followed by the escape of hot water and vapours of various 
kinds, and finally, before complete extinction, by warm springs, mineral 
springs, and carbon dioxide. Mount Hood has possibly reached the 
solfatara stage ; the Yellowstone Park region and New Zealand is in 
the hot spring stage ; and the Eif el is in the final stage when carbon 
dioxide alone issues. 

Fissure Eruptions 

Columbia Lava Plateau. — Although great fissure eruptions no 
longer occur, excepting possibly in Iceland, the effect of such eruptions 



VULCANISM 481 

in a recent geological period is stamped on the topography of parts of 
the earth, notably the Columbia and Snake River valleys in Washing- 
ton and neighbouring parts of Idaho and Oregon (Fig. 322). Here, 
over an area of more than 200,000 square miles, the surface is underlaid 
by sheets of basaltic lava, one on the other, overlapping one another, 
and including between them soil beds and lake deposits, showing that 
sheet after sheet poured out with intervals between. Deposits of lapilli 
and other fragments prove that there was some explosive activity as 
well as the outflow of liquid lava, though such phenomena were excep- 
tional. In places the lava flood of successive layers has covered the 
earlier surface of the land to a depth of 
over 4000 feet. 

The lava flood rises on the bordering 
mountain sides, as the waters of a lake 
would; and they surround as islands 
(steptoes) the higher points which they 
did not completely overspread. Other 

peaks are completely buried by the - t^£._l 7 ]! !£.^!5?^3 

horizontal layers of the encroaching ^ n*i^M\ ..^...„. ■ irrAU *— 

l n J A 1 r 4- v.- 1. • 0-( %JlPi NEVADA I UTAH 

lava flood. One peak 2500 feet nigh is 



buried by i^oo feet of lava. These £■„ ™ , , . ,., 

. nil ,i i n i -i riG. 322. — The lava plateau of the 

lava floods have thus levelled Up the Columbia and Snake rivers. 

surface and formed a broad lava pla- 
teau, not level but undulating, and crossed by deep canyons of 
rivers. These valleys are still in the stage of youth, and there are 
waterfalls and rapids, like the falls in the Spokane River at Spokane, 
and the Shoshone Falls of the Snake River in Idaho. 

In some places there are deep fissures, broadened to valleys or can- 
yons and locally known as coulees. Portions of the plateau have been 
disturbed by orographic movements, and the western part of the lava- 
flooded area has been upturned to form a part of the Cascade Range. 
Most of the surface, however, is little disturbed by subsequent move- 
ment, and over large areas also there has been little denudation. The 
rolling surface apparently represents in large part the original undula- 
tions of the latest flows ; and some of the swells and domes are cracked 
open, revealing the regular forms of the hexagonal basaltic columns. 
At a few points on the surface there are small cinder cones and low 
lava cones, but no large cones, and no visible surface indication of the 
source of the lava ; but Russell discovered dikes in the canyons which 
might perhaps mark the sites of the fissures through which some of 
the overlying Columbia River lava rose. Some of it may have come 
from the low lava cones. As a result of Russell's latest work in the 
Snake River plains, he stated that no facts were known to prove fissure 
eruptions and that there, at least, he thought the lava came from 
numerous vents. 

Other Lava Plateaus. — Similar, though smaller, lava'floods occurred 
in northeastern Ireland and thence in disconnected areas along the 
21 



482 COLLEGE PHYSIOGRAPHY 

west coast of Scotland, through the Hebrides and Faroe islands to 
Iceland, which is in the main a basalt plateau due to fissure eruptions, 
one of which, as already described, occurred in 1783. Similar larger 
basaltic lavas form plateaus in Abyssinia ; and in India, the plateau 
of the Deccan, rivalling in area the Columbia Plateau, is also basaltic 
lava from fissure eruptions ; but this lava is of an earlier age. It is 
also probable that fissure eruptions gave rise to some of the smaller 
lava plateaus of western United States, such as those of the Yellow- 
stone Park. 

Fertility of Lava Plateaus. — The broad, undulating basaltic plateau 
of the Columbia valley is covered with a deep, fertile residual soil 
of very fine grain, caused by the disintegration of the lava. In this 
rich soil lie the wheat fields for which central and western Washington 
and parts of Oregon are famous. Lava soils are often of notable 
fertility, and this fact is well illustrated in the Columbia Plateau. 

Intruded Lavas 

Igneous Rock Cooled below the Surface. — So far we have been 
concerned with the eruption of lava upon the surface of the earth ; 
but there is another important phase of vulcanism, namely, that of 
intrusion of molten rock into the crust. Very probably such intru- 
sions are in progress in parts of the earth to-day, and it is possible that 
some of the earthquakes are a result of the entrance of lava into the 
crust. Probably also some of the changes of level of the land are due 
to subterranean movements of molten rock. Minute changes of this 
sort in Japan have actually been measured. 

When, in the course of long-continued denudation, a land surface is 
worn down to such intruded masses, they exert an influence upon the 
topography. Therefore, a knowledge of some of their characteristics 
is properly within the province of physiography. These intruded 
rocks occur in a variety of forms, among which the most important 
from the standpoint of physiography are volcanic necks, dikes, sheets 
or sills, laccolites, and batholites or bosses. 

Volcanic Necks. — Being formed in the volcanic vent, as already 
stated, these forms of intrusion are roughly circular and of unknown 
depth. They consist of crystalline rock, often so resistant that they 
stand up above the surrounding surface as steep circular hills, some- 
times being surrounded by sedimentary rocks through which the vent 
was drilled. There are numerous such volcanic necks in the Mount 
Taylor region of New Mexico. A form imitating the volcanic neck is a 
plug of lava thrust into the rocks, but not reaching the surface. The 
striking circular hill known as Mato Tepee in Wyoming has been 
interpreted in this way, but others think it is part of an eroded laccolite, 
and still others a denuded volcanic neck. 

Dikes. — The origin of dikes through the solidification of lava in 
fissures in volcanoes and in connection with fissure eruptions has 



VULCANISM 483 

already been mentioned. Lava also rises toward the surface without 
reaching it, and in this way dikes are also formed. When a region of 
former volcanic activity has been worn down by denudation, it is 
often found to be crossed by multitudes of dikes, varying in width 
from a fraction of an inch to several yards. Sometimes the rocks are 
ramified by such dikes extending in all directions, and hundreds of 
them occurring in a single square mile. Some of these may have led to 
volcanic outbursts of long ago, all other signs of which have long 
since been worn away ; others represent merely the ineffectual at- 
tempt of lava to rise to the surface. Such dikes are most common in 
regions of former volcanic activity ; but they are sometimes present, 
as in central New York, where there is no reason to believe lava ever 
flowed out at the surface. 

If dikes are more resistant than the rocks through which they cut, 
denudation etches them into relief. In some places they extend for 
hundreds of yards as steep-sided walls of rock, rising well above the 
surrounding region. If, on the other hand, the dike rock is weaker 
then the enclosing strata, it is worn away more rapidly and a valley or 
chasm is formed. Many chasms along the sea coast are worn out 
along such dikes. 

In some cases rocks are crossed by so many dikes that a large pro- 
portion of their area is made up of these intrusions, and one can read- 
ily see that to find room for these intrusions, there must have been 
important lateral movement. When such an area is exposed to denuda- 
tion, the intruded rock may be even more important in influencing the 
rate and resulting topographic form than is the country rock itself. 
It is said, however, that the Crazy Mountains of Montana and 
Mount Royal at Montreal owe their form primarily to the resistance of 
metamorphosed sediments, altered by contact with a central igneous 
stock and intruded dikes. 

Sheets or Sills. — Lava, rising through fissures, may find a way in 
between the strata and there spread out in sheets between the layers, 
instead of rising and flowing out in surface lava sheets. These are 
known as intruded sheets or sills. They may be very thin, or they may 
have a thickness of scores of feet. It is sometimes difficult to tell the 
difference between an intruded sheet and a surface lava flow that has 
been covered by sedimentary layers. However, the surface of a sill 
is more compact, since the expansion of gases is less easy under the 
load of overlying strata than in a lava flow under only atmospheric, 
pressure. Moreover, by its heat the intruded lava causes changes in 
the overlying rock such as a lava flow could not cause in rocks that 
were laid down after it had cooled. In many cases, too, the sill crosses 
from one layer to another, or small dikes extend from it into the over- 
lying layers. 

Sills are found in Great Britain, in western United States, in New 
Jersey and Connecticut, and in many other places. One of the best 
known instances is that of the Palisades of the Hudson ; but there 



484 COLLEGE PHYSIOGRAPHY 

are others in the trap hills of New Jersey and the Connecticut valley. 
Being more resistant than the enclosing sandstones and shales, these 
intruded lavas have better withstood denudation, and, therefore, 
stand up as hills. Surface lava flows occur in this region also, and they 
produce similar topographic forms, one face being steep because the 
lavas are now tilted. Instances of topographic forms due to such 
lavas are Mount Holyoke and Mount Tom in Massachusetts, East and 
West Rocks at New Haven, Conn., and the trap hills near Orange, 
N.J. 

In western United States lavas, both intruded sheets and surface 
flows, lie in horizontal position, and in the course of denudation they 
have resisted denudation. Therefore many buttes and mesas in this 
region owe their form to the influence of resistant lava. 

Columnar Structure. — Both lava flows and sills often exhibit n 
remarkably perfect system of jointing, which, because best developed 
in the lava known as basalt, is often called basaltic jointing. Such 
jointing is also found in some volcanic rocks. Where most perfectly 
developed, the lava is crossed by a system of planes as a result of which 
the rock breaks away in the form of remarkably perfect hexagonal 
columns. This is true, for instance, at the Giant's Causeway in north- 
ern Ireland, so called because it is like a pavement or causeway 
(Fig. 323), at Fingal's Cave in western Scotland, and along the Rhine 
in Germany. In the latter place the columns have been quarried away 
for centuries, and great pits have been opened by their removal. The 
columns are so perfectly formed that, without further shaping, they 
can be used in building houses, in making dikes in Holland, for corner 
posts, etc. Less perfect jointing is seen in the Palisades of the Hudson, 
the word palisade being applied because of the columnar structure on 
the cliff face. 

These joints are due to the strains set up by contraction of the lava 
during cooling, as a result of which the rock is broken. The hexagonal 
form due to contraction is also developed during the drying of a mud 
flat. It is a normal form resulting from equal contraction in a mass of 
a fair degree of homogeneity, for it is the most economical expenditure 
of the strains set up by the contraction. When a strain reaches the 
breaking point, it finds relief by breaking along three planes meeting 
at an angle of 120 , which is the angle of the hexagon. It is not 
to be assumed, however, that there is mathematical accuracy in the 
result, for variations are introduced due to difference in the rate of 
cooling, in the composition of the rock, and doubtless in other ways. 
While the majority of the columns are more or less perfectly hexagonal, 
there are columns with more than six sides and others with five, four, 
and three sides. 

The hexagonal columns usually develop at right angles to the cool- 
ing surface, and, therefore, extend from the surface toward the base of 
the sheet. The individual columns are commonly broken across by 
gently curved planes, forming a ball-and-socket joint. These are 



VULCANISM 



485 



due to contraction and breaking in the columns themselves. The 
prisms vary in diameter from an inch or two up to a foot and 
a half or more, and they are sometimes over a hundred feet long. 
When most perfectly developed they are so regular as to seem 




Fig. 323. — Basaltic columns at Giant's Causeway in Ireland. 

almost artificial. Because of the remarkable appearance of the 
basaltic columns many people visit Fingal's Cave and the Giant's 
Causeway. 

Laccolites. — Gilbert has called attention to the fact that in the 
Henry Mountains of Utah such large quantities of lava were intruded 
between the layers that the overlying rocks were pushed up in a large 
dome. The lava rising toward the surface 
found it easier to lift the overlying rocks than 
to break through them, and there resulted an 
intrusion of large quantities of lava instead of 
volcanic outflow (Fig. 324). Thus, instead of 
building an ash cone or a lava volcano on the 
surface, a great reservoir of lava was thrust 
into the strata. Such an intruded mass is a 
laccolite, and the dome mountain which is 

raised above the intrusion is a laccolitic mountain. Denudation has 
stripped off much of the overlying cover of stratified rocks in the Henry 
Mountains, and revealed the lava core, now long since cooled and 
solidified (Fig. 343). Other cases of laccolites are now known. 

Batholites or Bosses. — In many cases the wearing down of moun- 
tains has revealed an underlying basement of granitic rock, often many 




Fig. 324. — Cross-section of 
a laccolite. (Gilbert.) 



486 



COLLEGE PHYSIOGRAPHY 



miles in extent, which has risen beneath the mountains, has broken 
through or melted away the strata, and has solidified in complex rela- 
tion to the overlying strata. Such great intruded masses are bosses or 
batholites or batholiths (Figs. 22, 325). It is probable that similar in- 
trusions are at present in prog- 
ress beneath rising mountain 
masses; and they may be the 
reservoirs from which the vol- 
canoes are being supplied. 
Certainly in the past batholitic 
intrusion has been a common 
phenomenon of mountain up- 
lift. 

When by denudation the 
overlying strata are removed, 
and the batholitic basement 
rocks revealed, there is a differ- 
ence in topographic form, as 
is shown in the chapter on mountains (p. 528). The massive, 
crystalline, igneous rocks are very different in character from the 
stratified rocks beneath which the batholites lie. Old mountains, 
long exposed to denudation, are often mainly made up of granites and 
other coarsely crystalline igneous rocks which were raised to their 
present position as batholitic intrusions. 




Fig. 325. — Diagram to show relationships of 
lava cooling underground in large masses. 
(Daly.) 



Imitative Forms 

Geysers, already described, closely imitate volcanoes in important 
respects. They always have a crater ; there is often a cone around 
the crater; and there is intermittent eruption. In geysers, how- 
ever, only steam, hot water, and dissolved substances are commonly 
extruded ; and the cone is made of deposits from solution, not of ash 
or lava. 

Mud volcanoes are formed where steam or other gases rise through 
mud deposits, throwing out the mud, which builds a cone around the 
vent. Small cones of this sort are found in the paint pot areas of the 
Yellowstone Park. Others are formed in Iceland, Sicily, and other 
regions of present or recent volcanic activity. They may erupt con- 
tinuously or intermittently, and, in some places, cones a hundred feet 
or more in height are built. 

Some of the mud volcanoes, also called mud-lumps and mud cones, 
are due to the development of gases underground by some form of 
decomposition or slow combustion. Mud volcanoes in Sicily have 
been explained as a result of the slow combustion of sulphur; 
others are explained by the decay of vegetation, and to other 
forms of chemical change underground. In the lower Indus there 
are great numbers of mud cones over an area of 1000 square miles, 



VULCANISM 



487 



some of them rising 300 or 400 feet and with craters 30 yards across. 
Mud-lumps, from which gases and salt water issue, also occur at 
the mouth of the Mis- 
sissippi (p. 158) ; but 
these are evidently re- 
lated to the river rather 
than to the gases. A 
dome or gas volcano 30 
or 40 feet high and 
3 acres in extent was 
formed in the Carib- 
bean Sea near Trinidad 
in November, 191 1. It 
seems clear that it was 
pushed up by gas, which 
subsequently ignited. 

Subterranean fires 
may also cause imita- 
tion of volcanic phenom- 
ena. An instance of 
this was seen in 1898 
by the author in the 
Bad Lands of North 
Dakota, near Medora, 
where a coal seam had 
taken fire. Gases issued 
from cracks and orifices 
in the ground and there 
were a few small cones, 
while fragments of slaggy 

rock, baked and partly melted, were strewn about, causing a close 
imitation of volcanic phenomena on a small scale. 




Fig. 326. — -Craters on the moon, usually interpreted 
as of volcanic origin. (Nasmyth and Carpenter.) 



Influence of Vulcanism on Man 

Destructive Influences. — The pages of this chapter have recorded 
numerous instances of the destructive work of volcanoes, and this is 
by far the most striking phase of the influence of vulcanism on man. 
Illustrations, similar in kind, could be greatly multiplied, showing the 
destruction of life and property by volcanic eruption. Doubtless 
hundreds of thousands of lives have been lost by this cause, and in the 
sea millions of fish are killed by volcanic eruptions. 

Production of Fertile Soil. — There are, however, other influences 
of importance which may be reckoned as beneficial. One of these is 
the influence upon soil. Many lavas give rise upon their disintegra- 
tion to extremely fertile soils. This is illustrated in the neighbourhood 
of Vesuvius, where the dense agricultural population along the shores 



COLLEGE PHYSIOGRAPHY 



of the Bay of Naples are, over much of the area, tilling fertile volcanic 
soils. The soils of the great lava plateau of Washington and Oregon 
furnish another instance, and there are many others. 

Formation of Ore Deposits. — Vulcanism has also played an impor- 
tant role in the development of many mineral veins. Their heat and 
the gases which they bring have given to water a solvent power of 
great potency in the transfer of mineral from one point to another; 
and the lavas have contributed much of the mineral for solution, 
transportation, and 'deposition in veins. A large part of the mineral 
wealth of the world is accessible to man because of changes 

in which vulcanism 
has played an im- 
portant part, either 
directly or indi- 
rectly. Even min- 
eral deposits are in 
some cases now 
thought to be of 
eruptive origin, as 
in the case of the 
enormous iron de- 
posits at Kiruna in 
Sweden. 

Other Influences. 
— Vulcanism has in- 
troduced rock con- 
ditions which have 
greatly influenced 
topographic form, 
both directly by de- 
posit and indirectly 
by influencing de- 
nudation. It has 
formed lakes, di- 
verted and directed stream courses, and given rise to conditions in 
underground circulation, as a result of which mineral and medicinal 
waters of value have been caused. It is probable also that 
the movements of molten rock beneath the surface are responsible 
for some earthquakes, even some of the greatest, and for changes in 
the level of the land, perhaps even for some of the major topographic 
features of the earth. 

Vast Importance of Vulcanism. — By transferring rock within the 
earth to the surface, vulcanism is responsible for vast and complex 
results through the denudation of the geological ages. It is likewise 
probable that the addition of water vapour, carbon dioxide, and other 
gases is a matter of the highest importance in maintaining that bal- 
ance of conditions upon which life depends ; and it is not at all im- 




FlG. 



327. — Crater harbour of the island of St. Paul, an 
extinct volcano breached by the waves. 



VULCANISM 489 

probable that this balance has been subject to important variations 
with differences in the extent of volcanic activity. This, however, 
is not a point upon which it is as yet possible to speak with definite- 
ness. One can hardly be mistaken in assigning to vulcanism a high 
place in the economy and activities of the earth on which we dwell, — 
a far higher importance than merely that of volcanic outbursts, the 
building of volcanic cones, and the destruction of life and property. 
These are but minor expressions of one of the great phases in earth 
activity, basal in importance in terrestrial development, and perhaps 
even to life on the planet. 

The Cause of Volcanic Action 

Molten Rock and its Extrusion. — The question of the cause of 
volcanic action resolves itself into two parts : (1) the cause of the 
molten rock, (2) the reason why this rock rises into the crust and to the 
surface of the earth. The consideration of the cause of the molten 
rock may for the present be deferred, since it involves the general 
question of the condition of the earth's interior ; and how this con- 
dition affects not only vulcanism, but changes of level of the land, 
earthquakes, mountain formation, and even the formation of con- 
tinents and ocean basins. 

Existence of Molten Rock, or Magma. — In order to explain vul- 
canism alone, we may start with the undoubted fact that there is 
below, the surface, either locally or generally, a supply of heated 
rock, which, under favourable conditions, can be forced into the crust 
and to the surface in liquid form. It is not to be assumed that this 
heated rock is necessarily in the liquid form where it lies ; it may be 
hot enough to melt under atmospheric pressure but prevented from 
expanding to the liquid form by the pressure of the overlying rocks, 
and changed to the liquid state only when that pressure is sufficiently 
relieved. 

Objections to a General Magma. — It is sometimes stated that the 
magma from which the lava is derived cannot be generally distributed 
beneath the crust because (1) vulcanism is present only in limited 
parts of the earth; (2) neighbouring volcanoes may erupt quite differ- 
ent lavas ; (3) neighbouring volcanoes sometimes show no sympathetic 
relation — notably Mauna Loa and Kilauea. These objections are, 
however, not necessarily fatal to the theory of a general magma. 
The areas where volcanoes occur may be lines along which pressure 
is relieved ; one volcano may very well send forth different lava from a 
neighbour if its supply comes from a different level in the magma, or 
from a magma that has been locally differentiated ; and of two vol- 
canoes side by side, but unsympathetic, one may have its supply from 
a local reservoir disconnected from the main magma. 

Facts Indicating a General Magma. — Far more important than the 
three evidences opposed to a general magma is the evidence of quite 



4QO COLLEGE PHYSIOGRAPHY 

the opposite condition. Facts have accumulated to indicate that 
there is a sympathetic response even between distant volcanoes, and 
between volcanic eruption and earthquakes. It can scarcely be an 
accident that Pele and Soufriere, 90 miles apart, were in eruption at 
the same time; that the Icelandic eruption of 1783 was preceded by 
a volcanic outburst many miles away ; that, when Vesuvius is active, 
eruptions occur in other vents of the adjacent volcanic field, to men- 
tion only three of the many cases of apparent sympathy between 
volcanoes. This is a subject upon which it is important to gather 
more facts, but such facts as are known seem to point to the existence 
of widespread magma, locally tapped by fissures or volcanic vents. 

Relation of Deformation to Extrusion. — The parts of the magma 
which are tapped appeared to be mainly, if not entirely, in regions of 
crustal deformation at the present time ; and in earlier geographical 
ages to have been zones of crustal deformation of that day. There 
are three possible ways in which such deformation may induce the 
rise of molten rock from the magma: (1) by relieving the pressure 
locally along lines of upfolding and thus permitting the change to the 
fluid state, (2) by forming fissures and otherwise weakening the crust 
so that the molten rock can rise, (3) by the squeezing up of the lava 
under mountain arches by the downsinking of neighbouring areas. 
All three of these influences may be at work in producing vulcanism. 

Already evidence has been presented in support of the conclusion 
that fissure lines are sought as pathways of escape for the molten 
rock ; but it must be stated that the rising lava itself may be largely 
responsible for the opening of the fissures. This surely must be the 
case at such depths in the earth as lie in the zone of flowage, where 
open cracks cannot exist. That the magma rises under mountain 
arches either by the release of pressure, or by being forced upward by 
downward pressure near by, is already indicated by the great batho- 
litic masses which denudation has revealed in the cores of mountains. 
It is highly probable that these batholites were themselves (1) magma 
that rose up under and into the rising mountains, and (2) the source 
of volcanic supply for ash and lava eruptions that built volcanic cones 
upon the mountains, as similar cones are now building in various parts 
of the earth. 

Relation of Gravity and Included Gases to Extrusion. — The rise 
of the molten lava into the crust and to its surface is evidently due to 
the influence of (1) gravity, (2) included gases. Such a phenomenon 
as the slow ascent of lava in the conduit of Mauna Loa until the press- 
ure becomes great enough to give escape to the lava through the side 
of the cone, and then a recurrence of the ascent of the lava has the 
appearance of hydrostatic adjustment. The ascent of batholitic 
masses beneath mountains seems likewise to be the result of gravitative 
adjustment of a fluid to pressure. The great outflows of basalt from 
fissure eruptions seem explicable only on the theory that they were 
squeezed out by a pressure on the molten rock, and that their rise was, 



VULCANISM 491 

therefore, essentially a result of gravity. It is highly probable that 
the greater amount of the rising of lava into the crust is similarly a 
result of gravitative adjustment. 

But, even granting this, there remain phenomena of vulcanism 
which can be explained only on the basis of the expansive force of the 
included gases. These phenomena may all be essentially surface 
phenomena, expressed only when the liquid rock reaches the upper 
portions of the crust and, therefore, attains points where the pressure 
is so reduced that the expansive force of the gases may express itself 
and expel some of the lava. Certainly the final stage in many volcanic 
eruptions is primarily the result of explosive action of expanding and 
probably combining gases. 

Origin of Gases in Lavas. — There has been speculation as to the 
origin of the included gases. It has been proposed, for example, that 
the water vapour is the result of entrance of sea water into the magma, 
but this seems impossible, first because the sea bottom deposits are so 
compact that percolation must be very slow ; and, secondly, because 
the earth itself is impervious at depths well above the level of the 
magma in the zone of rock flowage, which lies at depths no greater 
than about 12 miles. Some surface water doubtless finds its way to 
volcanic vents, and possibly to reservoirs of lava in the upper crust ; 
but that such vast quantities as are emitted from volcanoes could thus 
find their way to the molten rock is too utterly incredible for belief. 
The amount that thus enters the lava may have an effect in aiding in 
the final eruption. It has been observed that the eruptions of Kilauea 
most frequently occur in the rainy season ; and that Etna and Vesuvius 
have most often erupted in winter and spring, the times when there is 
most rain. These observations are not sufficient to establish the con- 
clusion, but they at least suggest the possibility that when an eruption 
is almost ready to occur, the entrance of surface waters may give the 
last impulse necessary for the outbreak. 

The included gases as a whole, however, seem quite certainly to be a 
component part of the original magma. They may well be a part of 
the original earth material which was never before at the earth's 
surface until expelled during an eruption. If this view is correct, the 
aid of volcanoes in supplying water vapour, carbon dioxide, and other 
gases to the air is a matter of very considerable importance ; for not 
only are huge volumes poured forth during each eruption, but, for 
centuries after volcanic activity ceases, these gases continue to issue 
from the volcanic centres. Even the composition of the atmosphere 
and the volume of the ocean waters may be affected. 

Suggestion of Isolated Subterranean Reservoirs. — On the theory of 
volcanic action as stated above, it is quite possible for some of the 
molten rock to rise into the crust, forming reservoirs more or less 
disconnected from the interior magma, and much nearer the surface. 
Such a condition would account for failure of neighbouring volcanoes 
to erupt sympathetically in some cases, and it might also account in 



492 COLLEGE PHYSIOGRAPHY 

part for differences in composition of lavas. If there are such reser- 
voirs, perhaps batholitic in character, surface waters may descend to 
them, and, in these cases, become a more important factor in the 
extrusion of the lava than is assumed above. Reservoirs of this 
character would gradually fail to supply lava for extrusion, and volcanic 
activity would finally die out. These possibilities are not suggested 
as alternate hypotheses for the theory of volcanic activity stated 
above, but as possible variations of conditions under that theory. 

Cessation of Crustal Movements and of Vulcanism. — On the 
theory here proposed the gradual dying out of volcanic activity in a 
region is assigned to the diminution of those crustal movements as a 
result of which the interior magma is forced upward into the crust ; 
but locally eruption may continue by the accession of underground 
waters even after the crustal movements ceased. If this interpreta- 
tion is correct, the present is a time of relatively slight crustal deforma- 
tion, as it seems to be one of waning volcanic activity. 

Other Hypotheses of Vulcanism. — The explanation of volcanic 
phenomenon here presented is not to be considered a demonstrated 
conclusion. It is an hypothesis framed upon the basis of the known 
data, and it has the merit of satisfactorily accounting for most of the 
facts. There are, however, some difficulties and it does not by any 
means meet with universal acceptance, though to the writer it appeals 
as the best founded of the various hypotheses put forward to explain 
volcanic phenomena. There are numerous such hypotheses, but this 
is not the place for their discussion. It will suffice to state one or two 
of the leading lines of difference between them and the hypothesis here 
presented. 

There are some who assign to percolating waters a far higher value 
in volcanic eruption than is here given, and some students of the sub- 
ject are inclined to assign to downward percolation of water a domi- 
nant part in the expulsion of lava from within the earth. As stated 
above, also, there are students of vulcanism who are not ready to 
admit the existence of a widespread and practically universal magma, 
but who consider vulcanism to be a localized phenomenon because of 
localized cause for heat. Three causes for the development of local 
areas of sufficient heat to melt rocks have been proposed : (i) the influ- 
ence of radium, (2) chemical changes, (3) heat developed by pressure 
and by movement during crustal deformation. That each of these 
is a source of heat is an undoubted fact ; but that either is competent 
to account for the melting of the rocks on the vast scale required by 
present and past vulcanism is by no means demonstrated. 

Perhaps the strongest reason for doubting the existence of. a general 
magma of heated rock, which on relief of pressure may flow as a liquid 
and rise into the crust and even to the surface, is the fact that vulcan- 
ism of the remote geological ages has apparently been no more active 
than in the present and recent past. Indeed, it is very doubtful 
whether at any geological period there has been greater volcanic 



VULCANISM 493 

activity than in the Tertiary period which immediately preceded the 
present. If vulcanism is due to the presence of a general heated 
magma which has been in existence during the earth's history, igneous 
activity would be expected to show a gradual diminution during the 
geological ages as the cold crust thickened by loss of heat. In order to 
account for the continuation of vulcanism, it would seem that there 
must be some cause for replenishing the past loss by radiation into 
space. This argument leads some to seriously doubt the existence of a 
general magma and looks to local causes for the generation of the heat 
which expresses itself in vulcanism. 

Further consideration of the problems presented by phenomena 
due to interior conditions is undertaken in a subsequent chapter. 



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494 COLLEGE PHYSIOGRAPHY 

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R. S. Holway. Recent Volcanic Activity of Lassen Peak, Univ. California Pub- 
lications in Geography, Vol. 1, 19 14, pp. 307-330. 

E. O. Hovey. Observations on the Eruptions of 1902 of La Soufriere, St. 
Vincent, and Mt. Pelee, Amer. Journ. Sci., 4th series, Vol. 14, 1902, pp. 
319-358; Bull. Amer. Museum Nat. Hist., Vol. 16, 1902, pp. 333-372; 
The New Cone of Mt. Pelee and the Gorge of the Riviere Blanche, Amer. 
Journ. Sci., Vol. 16, 1903, pp. 269-281 ; Ten Days in Camp on Mt. Pelee, 
Martinique, Bull. Amer. Geog. Soc, Vol. 40, 1908, pp. 662-679; Camping 
on the Soufriere of St. Vincent, ibid., Vol. 41, 1909, pp. 72-83; The 
Grande Soufriere of Guadeloupe, ibid., Vol. 36, 1904, pp. 513-530. 

E. Hull. Volcanoes: Past and Present, London, 1892, 266 pp. 

T. S. Hunt. Chemical and Geological Essays, Boston, 1875, 4^9 PP- 

J. P. Iddings. Origin of Igneous Rocks, Bull. Phil. Soc. Washington, Vol. 12, 
1892, pp. 89-124; Bysmaliths, Journ. Geol., Vol. 6, 1898, pp. 704-710; 
Yellowstone Park, Monograph 32, U. S. Geol. Survey, Part 2, 1899, pp. 
1-164, 215-440; Igneous Rocks, New York, Vol. 1, 1909, pp. 296-333; 
ibid., Vol. 2, 1913, pp. 343-657- 

T. A. Jaggar, Jr. The Evolution of Bogoslof Volcano, Bull. Amer. Geog. Soc, 
Vol. 40, 1908, pp. 385-400; Weekly Bulletin of Hawaiian Volcano Obser- 
vatory, Vol. 1, 1913, to date. 

T. A. Jaggar, Jr., and E. Howe. The Laccoliths of the Black Hills, 21st Ann. 
Rept., U. S. Geol. Survey, Part 3, 1901, pp. 163-303. 

D. W. Johnson. Volcanic Necks of the Mount Taylor Region, New Mexico, 

Bull. Geol. Soc. Amer., Vol. 18, 1907, pp. 303-324; A Recent Volcano in 

the San Francisco Mountain Region, Arizona, Bull. Geog. Soc. Phila., 

Vol. 5, 1907, pp. 6-n. 
H. J. Johnston-Lavis. The South Italian Volcanoes, Naples, 1891, 342 pp.; 

The Eruption of Vesuvius of April, 1906, Trans. Roy. Soc. Dublin, series 

II, Vol. 9, 1908. 
J. W. Judd. Volcanoes, New York, 1881, 381 pp. 
A. Lacroix. La M6ntagne Pelee et ses Eruptions, Paris, 1904; La Montagne 

Pelee apres ses Eruptions, Paris, 1908. 



VULCANISM 495 

G. D. Louderback. The Relation of Radioactivity to Vulcanism, Journ. 

Geol., Vol. 14, 1906, pp. 747-757. 
Sir Charles Lyell. Graham Island, Principles of Geology, nth edition, Vol. 2, 

I 873> PP- 58-63 ; Skaptar Jokul in Iceland, ibid., Vol. 2, pp. 48-53; 

Monte Nuovo, ibid., Vol. 1, pp. 606-616. 
E. de Margerie. Deux Accidents Crateriformes, Annales de Geographie, Vol. 

22, 1913, pp. 172-184. 
G. C. Martin. The Recent Eruption of Katmai Volcano in Alaska, Nat. Geog. 

Mag., Vol. 24, 1913, pp. 131-181. 
W. C. Mendenhall. The Wrangell Mountains, Alaska, Nat. Geog. Mag., Vol. 

14, 1903, pp. 395-407. 
G. Mercalli. I Vulcani Attivi della Terra, Milan, Vol. I, 1907, 421 pp. 
G. P. Merrill. The Meteor Crater of Canyon Diablo, Smithsonian Misc. 

Collections, Vol. 50, 1908, pp. 461-498. 

E. Ordonez. Le Jorullo, Guide Book n, Tenth International Geological 

Congress, 1906, 55 pp. 
R. D. Oldham. Lava Plateau of the Deccan, Medlicott and Blandford's Man- 
ual of the Geology of India, Calcutta, 1893, PP- 255-284. 

F. Omori. The Usu-san Eruption and the Earthquake and Elevation Phenom- 

ena, Bull. 5, Imperial Earthquake Investigation Committee, 191 1, pp. 

1-37; ibid., 1913, pp. 101-107. 
F. A. Perrett. Vesuvius : Characteristics and Phenomena of the Present 

Repose Period, Amer. Journ. Sci., Vol. 178, 1909, pp. 413-430. 
J. Phillips. Vesuvius, Oxford, 1859. 
Gaius Pliny. Pliny's Letters, Book 6, translation from the Latin in Shaler's 

Aspects of the Earth, pp. 50-56. 
W. E. Pratt. The Eruption of Taal Volcano, Philippine Journal of Science, 

Vol. 6, 191 1, pp. 63-83. 

F. L. Ransome. Some Lava Flows of the Western Slope of the Sierra Nevada, 

Cal., Bull. 89, U. S. Geol. Survey, 1898, 74 pp. 
H. H. Robinson. The San Franciscan Volcanic Field, Arizona, Prof. Paper 

76, U. S. Geol. Survey, 1913, 213 pp. 
I. C. Russell. Volcanoes of North America, New York, 1897, 346 pp. ; Lava 

Plateau of Columbia and Snake Rivers, U. S. Geol. Survey, Bull. 108, 

1893; Water Supply Paper 4, 1897; 20th Ann. Rept., Part 2, 1900, pp. 

129-134; Water Supply Papers 53, 54, 1901 ; Bull. 199, 1902, pp. 59-134; 

Bull. 217, 1903; Water Supply Paper 78, 1903; Bull. 252, 1905; Igneous 

Intrusions in the Neighborhood of the Black Hills, Journ. Geol., Vol. 4, 

1896, pp. 23-43. 
K. Sapper. Die Mittelamerikanischen Vulkane, Petermanns Mitteilungen, 

Ergansungsheft 178, 1913, .173 pp. 
D. Sato. Eruption of Mt. Usu, Bull. 23, Survey of Japan, 1913, pp. 1-13- 
K. Schneider. Die Vulkanischen Erscheinungen der Erde, Berlin, 191 1, 272 

pp.; Zur Geschichte und Theorie des Vulkanismus, Prague, 1908, 113 pp. 
P. Scrope. Geology of the Extinct Volcanoes of Central France, London, 

1858, 258 pp. 
S. Sekya and Y. Kikuchi. The Eruption of Bandai-San, Trans. Seismological 

Soc. Japan, Vol. 13, 1890, pp. 140-222. 
N. S. Shaler. Volcanoes, Aspects of the Earth, New York, 1904, pp. 46-97. 
N. S. Shaler and R. S. Tarr. Dikes of the Cape Ann District, Massachusetts, 

9th Ann. Rept., U. S. Geol. Survey, 1889, pp. 579-602. 

G. O. Smith. Geology and Physiography of Central Washington, Prof. 

Paper 19, U. S. Geol. Survey, 1903, pp. 1-39. 

G. J. Symons, J. W. Judd, and Others. The Eruption of Krakatoa and Sub- 
sequent Phenomena, London, 1888, 494 pp. 

R. S. Tarr. A Recent Lava Flow in New Mexico, Amer. Naturalist, Vol. 25, 
1891, pp. 524-527. 

T. Thoroddsen. Volcanoes of Iceland, Petermanns Geog. Mitteilungen, Vol. 
153, 1911, pp. 108-111; ibid., Vol. 51, 1905, pp. 1-5. 



496 COLLEGE PHYSIOGRAPHY 

S. von Waltershausen. Der Aetna, Leipzig, 1880, 2 vols., 371, 548 pp. 
Zeitschrift fur Vulkanologie, Naples, Vol. 1, 1914, to date. 

TOPOGRAPHIC MAPS 

Volcanoes 

Flagstaff, Ariz. Lassen Peak, Cal. Shasta, Cal. 

Livingston, Mont. Mt. Taylor, N.M. Marysville Buttes, Cal. 

Crater Lake Special, Ore. Island of Kauai, Hawaii Chitina, Alaska, 601 A 

Laccolites 
Henry Mountains, Utah Sturgis, S.D. Spearfish, S.D. 

Lava Plateau ' 
Spokane, Wash. Bisuka, Idaho. Ellensburg, Wash. 

Trap Ridges and Palisades 
Springfield, Mass. New Haven, Conn. New York City Special 



CHAPTER XIV 

PLAINS AND PLATEAUS 

Nature and Origin 

The simplest of land forms is the plain, and it is by far the most 
widespread topographic feature on the earth. Much the greatest 
portion of the ocean bottoms is occupied by plains, and a large propor- 
tion of the continent surfaces as well. 

A plain is a level or gently undulating portion of the earth's surface, 
and it is usually, though not always, underlain by horizontal or nearly 
horizontal strata. In origin plains are most commonly the result of 
deposition of sediment, usually in water, and often in ocean water. 
Plains that have been formed beneath the sea have often been brought 
to a position above sea level by one of those changes in relative level 
of sea and land already studied. Very often they have been raised 
high above sea level, and are then commonly called plateaus, though 
this term has no real scientific significance, and in popular usage has 
no commonly accepted meaning. In general a plateau is understood 
to be a high plain, though it is common to speak of the Great Plains 
west of the Mississippi River, even where they are over 5000 feet above 
sea level, while lower areas west of the Rocky Mountains are called 
plateaus. On the other hand, some of the deep-lying plains of the 
ocean bottom are often referred to as oceanic plateaus. 

Plains are formed by river, glacier, lake, and ocean deposits and by 
volcanic outflow ; they are also formed by denudation, as when a sur- 
face is worn to the state of old age, or when a river, swinging back and 
forth, planates the surface. Naturally, therefore, there are numerous 
differences among plains from the standpoint of their origin or struc- 
ture. There are also differences according to the process by which 
they are being modified, and their stage in the erosion cycle ; for, like 
all land surfaces, plains exposed to subaerial denudation undergo a 
cycle of dissection. Starting level, they may be sculptured into a hilly 
state, and then, with the approach of old age, they tend again toward 
levelness, 

Different Types of Plains 

River Plains. — In the chapter on rivers the nature and origin of 
river plains has been stated with such fulness (pp. 143-168) that we 
need merely call attention to the fact that they are to be classified 
in this group of land forms. The chief plains due to river action are 

2K 497 






498 



COLLEGE PHYSIOGRAPHY 



,1* 




PLAINS AND PLATEAUS 499 

floodplains (Fig. 328 and PL III), terraces, deltas, broad, flat 
alluvial fans, and deposits in partly or completely enclosed mountain 
valleys. Perhaps to these might be added the outwash gravel plains 
built up by streams issuing from glaciers (PI. IV). 

Glacial Plains. — Where glaciers have spread out over gently 
undulating surfaces their deposits have sometimes filled valleys and 
made the land more level. This is in places done (a) by deposits 
directly from the ice, making till plains, (b) sometimes by deposits 
made by water from the melting glacier, and very often by the two 
combined. Large tracts of country in the northern Central States 
have been levelled up in this way. In places the deposits are several 
hundred feet in depth, and were all the glacial drift removed, this 
region would be far less level than now. 

The ice of large glaciers also builds up plains or plateaus. Thus 
the piedmont Malaspina Glacier is a low-lying plateau; the Green- 
land ice sheet is a vast ice plateau ; and the Antarctic ice sheet is a 
still greater one. 

Lava Plains. — Lava flows, spreading out at the base of a volcano, 
or still more notably when flowing from fissures, may give rise to plains. 
This finds illustration in the Columbia River valley of Washington, 
where flow after flow of lava has not only filled valleys, but has even 
caused the burial of mountains, transforming a hilly and mountainous 
country to a level or undulating lava plateau of great extent. The 
plateau of the Deccan in India and the Icelandic plateau are other 
instances of lava plateaus. 

Volcanic ash, falling upon the country round about a volcanic vent, 
may also level up the surface, though wind and running water are aids 
in the building of such a plain. The country near Vesuvius is an 
instance of a plain of this origin. 

Lacustrine Plains. — Sediment settling upon a lake bottom tends 
to smooth over irregularities and gradually to form a level bottom. 
If such a lake disappears, a plain is left on its site, as in the case of the 
valley of the Red River of the North, in which the great Lake Agassiz 
formerly stood. If a lake persists, it will in time become completely 
filled and its site will then be occupied by a filled lake plain. Thou- 
sands of instances of this are found in the regions of former glaciation, 
where the filled lake plains are still so level that they are swamps. 

During the stages of lake filling, smaller plains are also formed 
around the shores. In some cases a narrow plain is cut by the waves 
as they eat into the land ; in others the shore is built outward, forming 
a swampy plain strip at or near lake level. Larger plains are formed 
by the filling of bays, and by the growth of deltas into the lake. 

Marine Plains. — Along ocean coasts there are also narrow plains 
formed by wave cutting, as in the submerged offshore platform that 
extends seaward from exposed headlands. There are filled bays also, 
and delta plains, and swampy coastal strips in protected parts of the 
shore, — the salt marsh plains. 



5 oo COLLEGE PHYSIOGRAPHY 

As in the case of lakes, the sediment borne to the sea is strewn over 
the sea floor, tending to level it by filling the depressions and by 
smoothing over the elevations. Far from the coast the settling of 
organic remains to the sea floor has had a similar effect. The tendency 
is, therefore, to make plains of deposit, even where the sea bottom has 
been roughened by diastrophic movement. It is partly because of this 
process that such vast areas of the ocean bottom are plains ; though 
doubtless here, as on the land, there are great tracts which have not 
been deformed by diastrophism or roughened by vulcanism. The 
sea floor is narrower, protected from the roughening effect of denuda- 
tion. 

Where a part of the ocean bottom is brought above sea level, 
unless this is accompanied by pronounced deformation, it is normally 
added to the land as a plain. This is why so many coasts in regions of 
recent uplift are bordered by coastal plains, as in the case of eastern 
North America south of New York. Uplift along the western coast, 
being accompanied by crustal deformation, has resulted in mountain 
formation ; but even here strips of coastal plain are present where the 
sea bottom has been raised without folding or faulting. 

Plains of Denudation. — As a land surface is worn down by denuda- 
tion, plains are caused not only by deposit but also by the direct attack 
of the agents of denudation. Instances of this have already been 
mentioned as a result of wave work along lake and sea coast. The 
lateral cutting by a river, as illustrated in terrace formation, is 
another. 

Plains also develop as denudation lowers a land surface. This is 
best illustrated in areas of nearly horizontal strata as a result of the 
fact that resistant beds are removed less readily than weaker ones. 
Therefore, when such a resistant bed is reached by subaerial denuda- 
tion, it tends to hold the surface at that level while the weaker strata 
are stripped away. Since the layer is horizontal the surface to which 
it gives rise is more or less level, that is, a plain. The nature of the 
process and the resulting topography are more fully considered in the 
discussion of the dissection of plateaus. These plains may be called 
plains of differential gradation. 

If a land surface continues to be reduced by denudation, it is ulti- 
mately worn down so near to baselevel that it approaches the condition 
of a plain, even though in its initial state it was mountainous land. 
Such a surface has been given the name peneplain, or almost a plain. 
Former peneplains (PI. X) , now uplifted or dissected, are found in many 
places. Some of them were at an earlier period interpreted as plains 
of marine denudation, but it is now believed that extensive plains of 
this origin are not common, if indeed they are present at all. The coast 
line appears to be too unstable to prevent their development, and 
though the zone of wave attack is one of great activity, it is confined 
to a very limited area, compared to that upon which subaerial denuda- 
tion is at work. 



PLAINS AND PLATEAUS 



5oi 



Constructional and Destructional Plains. — A simple classification 
of plains, upon the basis of origin, is to consider them as a result of 
(1) constructional processes, (2) destructional processes. Those 
plains made by deposit — nuviatile, glacial, volcanic, lacustrine, and 
marine — are constructional in origin. Those shaped by the degrada- 
tion of the land, as outlined in the preceding paragraph, are of destruc- 
tional origin. Of these the peneplain and the plain of differential 
gradation are most common and most important. 

The Life History oe Plains and Plateaus 

Dissection of Plains. — In its initial stage a plain has a level sur- 
face, often so level that water does not freely drain off. This is true on 





Fig. 329. 



The plain of the valley of the Red River cf the North, the bottom of 
glacial Lake Agassiz. 



parts of the coastal plain in southern Florida and on the Texas coast ; 
it is true of plains of other origin, as in the swampy plains of filled 
lakes and on deltas and floodplains. Such a plain is a young plain, 
and, because of its swampy nature, it is not suited to human occupa- 
tion and it is usually a wilderness of luxuriant plant growth. In the 
tropical zone the dampness favours the development of tropical diseases 
and the plains are even dangerous to cross. Rice culture in the South 
and cranberry culture in the North are about the only industries that 
are favoured by such conditions in United States, though forest 
products are obtained in some parts, and peat is taken from some 
of the swampy lake plains of the North. 

If such a plain is elevated high enough above baselevel so that the 
streams are enabled to cut along their beds, the surface begins to be 



$o2 COLLEGE PHYSIOGRAPHY 

dissected. First there are narrow young valleys, with broad, flat- 
topped divides between, as in the plains of the Red River of the North 
in North Dakota and Manitoba (Fig. 329). If the soil is good, such 
a surface may become the seat of successful agriculture, for the land is 
level and drainage is provided for by the stream courses. A little 
later, when steep-sided gorges are sunk below the plain level, condi- 
tions begin to be slightly less favourable. 

Thereafter, the plain will pass through the various stages of youth, 
maturity, and old age, if there is no accidental interference with the 
cycle of development. Exactly what the successive stages will be 
depends upon several factors of which three — rainfall, elevation, and 
rock structure — are the most important. Each of these is so funda- 
mental that it has a dominating influence on the land forms of the 
various stages in the cycle of dissection. 

Influence of Uniform Rock Structure. — Assuming the simplest 
conditions for the basal consideration of the cycle development we may 
start with a plain of uniform rock structure, at a moderate elevation, 
and in a region of moderate rainfall. Here the cycle of development 
consists first of the development of a few gorge-like valleys with 
intervening flat-topped divides ; then the increase in number of such 
valleys by gnawing back at the headwaters, thus narrowing the 
divides ; accompanying and succeeding this stage, a broadening of the 
valleys and a lowering of the inter-valley tracts, thence on to the stage 
of old age. Thus the plain passes (1) from a level surface, (2) to a 
level surface crossed by gorge -like valleys, (3) to an undulating, hilly 
country, and (4), by a lowering of the inter- valley tracts, back toward 
the condition of a plain in old age. 

No matter what the elevation, rainfall conditions, and rock struc- 
ture, the cycle of development of plain from youth to old age passes 
through essentially these stages, but with notable variations in topo- 
graphic form in accordance with their influence. 

Influence of Complex Rock Structure. — Considering first of all 
the influence of rock structure, we will assume exactly opposite con- 
ditions from those first stated ; that is, instead of uniformity of rock 
structure, a high degree of complexity, such as exists in a peneplain 
produced by the reduction of a mountain region to the condition 
of old age in a first cycle. Here rocks of various degrees of resistance 
stand at all angles. With the dissection of such a plain in the second 
cycle the surface form in the successive stages is influenced, not merely 
by the denudation, but also by the structure and attitude of the under- 
lying rocks. Instead, therefore, of a symmetrical series of valleys 
and hills, developed by the operation of denudation alone and unin- 
fluenced by variations in the underlying rock, a topography is developed 
in which the rock structure and attitude dominate the topography. 
Ridges and parallel valleys may develop and a truly mountainous 
topography may be etched out in the varying rocks, especially in the 
stage of maturity. In old age, however, when the influence of un- 



PLAINS AND PLATEAUS 503 

derlying rock becomes lessened, even such a surface tends toward 
the plain. 

Influence of Alternate Weak and Resistant Strata. — Intermediate 
between these two extremes there are all conditions of variability in 
rock structure beneath plains. By far the most widespread is the 
variation in degree of resistance in the nearly horizontal strata. 
Until rivers have cut into these layers this difference is not revealed 
and exposed to denudation, but when alternate layers of strata of 
different degrees of resistance are exposed, they commence at once to 
influence the rate of denudation and, therefore, the topography. This 
influence first expresses itself along the bottoms of the streams, giving 
rise to rapids and falls, and along the valley sides, to rock terraces. 
Each resistant layer wears back at a slower rate than the less-resistant 
strata, and, therefore, tends to stand out more boldly. 

Tablelands. — As such a plain passes from youth to maturity the 
differences in rock structure, if great enough, may dominate the topo- 
graphic form. This is brought about in two ways : (1) by retardation 
of vertical denudation, (2) by horizontal recession of cliffs. The first 
gives rise to level tops, the second to steep cliffs. The term tableland, 
often used as a synonym of plateau, is derived from these topographic 
features, which are commonly well developed in such high plains. 
The Spanish word mesa, which means table, refers to a specific table 
area in a land of table top forms. A mesa is a flat-topped surface 
terminated on some or all sides by a steep face or escarpment. A 
smaller area of similar form is called a butte. Both buttes and mesas 
so abound in many plateau lands as to have suggested the term table- 
land. 

In buttes and mesas, and in less well-defined level surfaces faced by 
escarpments, there is a general similarity of conditions. The level top 
is underlain by a resistant stratum nearly or quite horizontal in posi- 
tion, and it is so resistant to the general lowering of the surface by 
denudation that a table form results. If it is worn away, denudation 
will proceed more rapidly in the underlying weaker strata until another 
resistant layer is encountered lower down, when the rate of denuda- 
tion will once more be halted and a table top surface be developed at 
the lower level. Accordingly as the surface wears down, table forms 
appear at different levels. Thus in crossing a plateau one may go from 
one level to another, each time ascending or descending an escarpment 
that separates the two levels ; and one may be certain that when the 
surface of an upper level is lowered, it will be halted at the next lower 
level of resistant rock, and that, at an earlier stage, the site of the 
hi'gher levels was occupied by a table top at a greater elevation. 

The mesas, buttes, and other table top areas are wasting away, 
partly by a general lowering of their surfaces, but even more effectively 
by the wasting back of their bordering escarpments. The underlying 
weaker layers crumble on exposure to the air and to underground water, 
the edge of the overlying resistant layer is slowly undermined, and 



5°4 



COLLEGE PHYSIOGRAPHY 



fragments fall down the escarpment face, to be removed by running 
water or by wind, or perhaps, after further disintegration, by weather- 
ing. This process of undermining of an escarpment face has been 
spoken of as sapping. By the operation of sapping, cliffs are made to 
recede and thus the table top areas are steadily diminished. The 
recession naturally starts from the sides of valleys cut into the hori- 
zontal strata, and the escarpments may be pushed back many miles. 
Similar recession of cliffs may start from fault scarps as well as from 
stream-cut valleys. » 

The rate of recession of cliffs under the process of sapping will vary 
greatly with difference in resistance of the strata, thickness of strata, 
and rate of removal of the fragments that fall from the cliff face. In 
western United States, where such cliff recession is illustrated in a 
multitude of buttes, mesas, and other table top, escarpment-faced 
areas, some of the most striking instances are where lava rests upon 
unconsolidated or partly consolidated clays. There the escarpments 
are strongly developed, and at the base of the cliff are many blocks of 
lava, while the surface back of the escarpment is broken and fissured 
by cracks developed in the process of undermining through sapping. 
From such extremes there are all gradations to faintly developed 
scarps where layers are thin or where differences in resistance are 
slight. 

Belted Plains and Cuestas. — The strata underlying plains are often 
inclined at a low angle, thus bringing successive layers to the surface. 
This is well illustrated in the coastal plains of eastern United States, 

where, passing from the 
coast inland, one finds 
layers of different kinds 
rising to the surface and 
dipping gently seaward. 
This gives rise to belts 
of different soils, some 
sandy, some clayey, ex- 
tending roughly parallel 
to the coast. In conse- 
quence there are cultural 
belts also, dependent 
upon the soil conditions. 
Since these outcrop- 
ping strata also vary in 
resistance, they give 
rise to topographic belts 
also, as the surface is 
slowly worn down. The 
weaker strata wear fastest, leaving the more resistant layers at a higher 
level, and giving rise to a belted coastal plain (Fig. 330). Since 
the strata have a seaward dip, the surfaces of the more resistant layers 




Fig. 330. — Block diagram of a belted coastal plain. 



PLAINS AND PLATEAUS 505 

slope gently toward the sea. On the landward side there may be a 
steep slope, and the resistant layer may even end in a low escarpment, 
which is slowly receding in the direction of the dip by the operation 
of sapping. The name cuesta has been given to such a land form, with 
a steep face on one side and a gently sloping surface on the other 
(Fig. 331). Like the word mesa, this is a Spanish term. It is applied 




Fig. 331. — Cross-section to show the relation of cuestas to rock structure and the use of 
the terms escarpment and vale. (After Veatch.) 

in New Mexico to low ridges with a steep slope on one side and a 
moderate slope on the other. Cuestas are well developed in Louisiana, 
Alabama, the Paris basin of France, etc. 

The drainage of a belted coastal plain may be perceptibly influ- 
enced by the etching out of the layers. On a coastal plain the normal 
drainage is seaward, down the slope of the plain, some of the streams 
extending out over the plain from the land, others developing on the 
plain between these extended streams. As the surface wears down, 
however, subsequent streams develop in the valleys along the outcrop 
of the weaker layers, on the inner face of the cuestas. These subse- 
quent stream courses extend approximately at right angles to the direc- 
tion of the consequent streams, which they enter as tributaries ; and 
to them obsequent tributaries flow down the cuesta scarp, thus flowing 
in the direction exactly opposite to the original consequent course. 

A belted arrangement is also developed during the denudation of 
inland plains, some of them far from the sea and not to be classed as 
coastal plains, though in their inception far back in geological time 
they may have risen above the sea as coastal plains. In these ancient 
plains the strata are consolidated, and the differences in resistance to 
denudation may give rise to pronounced topographic forms. There 
is such an arrangement of belted uplands and lowlands in Wisconsin 
from the shores of Lake Michigan northwestward to the highland of 
crystalline rocks which occupies the northern part of the state. An- 
other case is in Ontario and western New York. To the north are 
crystalline highlands and, bordering it, a lowland in weak strata, in a 
part of which Lake Ontario lies. South of this is an escarpment where 
the Niagara limestone outcrops, and back of it another plain in which 
Lake Erie lies, while beyond this rises another escarpment, the northern 
edge of the Allegheny Plateau, or, as it has been called, the Allegheny 
cuesta. Neither this nor the Niagara cuesta has any persistent inclina- 
tion of the surface away from the escarpment, although the strata dip 



5 o6 COLLEGE PHYSIOGRAPHY 

that way. A second process has, however, operated in this region, 
the glacial agency following the normal stream agency. Similar belted 
plains and plateaus are very common in other parts of the world, as in 
eastern England and north of the Colorado River in Arizona. They 
grade into the tableland type of topography discussed above. 

Effect of Elevation. — In the dissection of a plain it is manifest that 
there must be great difference in the resulting form, according to alti- 
tude. A high plain, or plateau, offers opportunity for the develop- 
ment of gorges and canyons ; and the depth of dissection reveals many 
different strata, so that there is ample opportunity for the exposure 
of cliff-making strata. In the wearing down of such a surface, there- 
fore, the stages of youth and maturity are marked by the cutting of 
canyons, the recession of cliffs under the influence of sapping, and the 
development of buttes, mesas, and other table topped areas. 

Regions of lower altitude offer less opportunity for the production of 
such forms, which are really characteristic of plateaus ; yet they are 
not entirely absent. Gorge valleys, mesa-iike forms, miniature buttes, 
and well-defined escarpments are developed in the course of dissection 
of even low plains of variable horizontal strata. They are merely 
less numerous and less striking features in the dissection of a plain of 
low elevation. 

A high plain may be so cut up by dissection during the stage of late 
youth or early maturity that it is transformed to a maze of hills and 
valleys. It may even become so dissected as to simulate mountain 
topography, and win the name mountain. This is the case in the 
Catskill Mountains of New York, which are really nothing more than 
a much-dissected plateau of nearly horizontal strata. If the strata 
are not greatly different in their degree of resistance to denudation, the 
hilltops may become rounded, and, in the absence of pronounced 
cliff-making strata, the valley slopes may be fairly smooth. Then, 
as in the case of the Allegheny Plateau of central New York, the 
maturely dissected plateau may become a hilly region, with few flat- 
topped areas, no typical mesas and buttes, and few escarpments. In 
the plateau in question the only escarpment of prominence is that 
which forms its northern face, where relatively resistant limestone 
outcrops. This escarpment in one place has developed a slope so steep 
as to give rise to the local name Helderberg Mountain. The effect of 
the glacial accident in this plateau is not, as yet, well worked out. 

Where the strata are variable in degree of resistance, the effect of 
the cliff-making layers on the topography is very pronounced, and flat- 
topped areas and escarpments abound. 

Effect of Climate. — The angular forms developed by the excava- 
tion of canyons and by the recession of cliffs appear in far greater 
perfection in arid than in humid climates. This is because (i) weather- 
ing is less active, or mechanical disintegration more active, in arid 
regions, so that there is less tendency to round the angular forms : 
(2) the wind action in arid regions removes the disintegrated frag- 



PLAINS AND PLATEAUS 507 

ments and thus leaves the bed rock more exposed ; (3) vegetation cover 
is less extensive in arid climates, and thus exerts less influence in hold- 
ing the disintegrated fragments where they fall ; and (4) the general 
lack of vegetation obscures the angular forms less in arid than in 
humid regions. Thus a cliff fifty feet in height may be quite hidden 
from general view in a forest-covered region, while in an arid coun- 
try it would stand out with full prominence. It is true also that 
most high plateaus in the world, in which angular forms are most 
rapidly developed, are located in arid regions. 

Accordingly dissected plateaus in humid regions, while not lacking 
escarpments and table-topped areas-, are not so characterized by them 
as are dissected plateaus in arid regions. Rounded slopes are common 
and even dominant in the one case, and angular forms characterize 
the other, largely as a result of differences in the climatic conditions 
under which they have developed. 

Relation of Plains and Plateaus to Human Life 

Dense Settlement upon Plains. — It is upon the plains of the world 
that the greatest part of the human population is found. Because of 
the levelness, soil is not readily washed off, and it is, therefore, com- 
monly deep and often fertile. In many cases, too, the plains are made 
of transported sediment of fine grain and fertile character, as in the 
case of noodplains, deltas, and abandoned lake bottom plains. Such 
level surfaces often contain a large admixture of humus, often so 
much that they are quite black in colour, and consequently very fertile, 
the humus being due to the luxuriant growth of plants upon the plains, 
and often to swampy conditions on the level surface, as a result of 
which the humus is protected from loss by decay. 

Agriculture is encouraged also by the levelness of the surface, 
which makes farming operations easier, and aids in the construction 
of highways for transportation of products. Very often, too, plains 
are crossed by streams in which the slope is so gentle that they are 
navigable, thus further aiding in transportation. 

Plains occupied by an agricultural population are so numerous 
that it would be a long list if all were mentioned. Among them are 
included the plains of the Mississippi valley ; the plains of France, 
Belgium, Holland, and northern Germany ; the Hungarian plain ; 
the Russian plains ; the delta plain of the lower Nile ; and the deltas 
and noodplains of India and China. 

Kinds of Plains Unfavourably Situated. — Some plains are too 
swampy for occupation, as already stated ; some have a poor soil, like 
the sandy soils of parts of the Atlantic coastal plain ; and there are 
some with too dry a climate, or so far north that the climate is too cold, 
as in northern Canada' and Siberia. High plains are sometimes so 
lofty that their climate is unfavourable to settlement, as in parts of 
Tibet ; but elevation is at times beneficial. For example, the Colorado 



5 o8 COLLEGE PHYSIOGRAPHY 

plateau, in an arid region, rises high enough in places for forest growth ; 
and part of the Columbia lava plateau of Washington receives rainfall 
enough for agriculture because of its elevation. In tropical countries 
elevated plains and plateaus are often high enough to give rise to 
temperate conditions in a zone where otherwise the tropical heat would 
prevail. A large portion of the settlement of the tropical parts of the 
New World and of Africa is to be found upon high plains, above the 
level of torrid heat. 

Young plains, if so immature and so low that their drainage is 
retarded, are unfavourable to settlement. As the cycle of denudation 
proceeds, the surface becomes more irregular, and, if low, they may 
become and remain habitable throughout the cycle. If the plains 
are high, dissection may transform the surface to such a degree of 
ruggedness that density of population is discouraged. This finds 
illustration in the much-dissected plateau that fringes the western 
base of the Appalachian Mountains, a rugged, hilly region of sparse 
settlement, in the main, with little agricultural industry, still largely 
covered by forest, and now, as hitherto, a barrier to travel. 

Plains and Plateaus of the United States 

Typical Plains. — Plains occupy so much of the land surface, and 
their general characteristics are so alike in the different continents, 
that it does not seem necessary to enter into a consideration of the 
characteristics of plains in various parts of the world. A brief descrip- 
tion of the features of the plains and plateaus of the United States, 
which include all the main types, will give an adequate idea of the 
characteristics of plains in general. This description, which is, in 
part, in the nature of a summary, will commence with the eastern 
part of the country and proceed westward (Figs. 332, 339). 

The Coastal Plains. — Off the eastern coast of United States there 
is a level sea bottom plain, known as the continental shelf, sloping sea- 
ward at the rate of 5 or 6 feet per mile. If there should be an uplift 
of 600 feet, this very level plain would be added to the continent. 
The surface of the plain would be very level, and it would be underlain 
by unconsolidated sediments. 

Such has been the actual history of the region south of New York 
at a recent period ; for, by change in the relative level of the land and 
sea, a part of the sea bottom has been brought above sea level, form- 
ing the coastal plains which skirt the eastern coast from New York to 
Mexico and beyond (Fig. 333). These plains are, in general, level, they 
incline gently seaward, and they are underlain by nearly, or quite, 
unconsolidated sediments. At the coast line the plains extend with 
no noteworthy break beneath sea level, for they are continuous with 
the continental shelf. The line of separation is marked by sand bars 
and other coastal forms, and the coast line is somewhat irregular be- 
cause of recent slight subsidence, as in Chesapeake Bay. 



PLAINS AND PLATEAUS 



509 







5"> 



COLLEGE PHYSIOGRAPHY 



In parts this plain is so level that it is swampy, and some of the 
outer portions, as in southern Florida and between Galveston and 
Houston, Tex., have been upraised so recently that the fossils en- 
tombed in the sediments are of the same species as those still living 
in the neighbouring waters. Even the irregularities of the former 
sea bottom are preserved and give rise to lower ridges and depres- 
sions, the chief topographic features of the new land. Farther inland 
the plains have been longer exposed, they are higher, and, conse- 
quently, they are somewhat dissected by the extended and consequent 
streams which flow seaward over them. These streams have sluggish 
flow, and the larger ones are navigable ; their valley walls are pre- 
vailingly low and sloping, being composed of unconsolidated sediment ; 
and, in their lower courses, the tide enters, and the streams are bor- 
dered by swamp lands, through which they flow in meandering courses. 




Fig. 333- — The level coastal plain in Florida. 



These coastal plains are not densely settled. Part of the region 
is too swampy, especially along the coast ; but, here, there is some 
rice cultivation, and there are forests of cypress and other trees, adapted 
to growth in swampy land. Farther inland much of the soil is too 
sandy for successful agriculture, and the plains are covered by a pine 
forest, from which much lumber, as well as tar and turpentine, are 
obtained. There are belts of clay and other more fertile soils, and, 



PLAINS AND PLATEAUS 



5ii 






on these, cotton and other crops are raised. The inner margin of the 
coastal plain is ordinarily dissected into a low, undulating, hilly land, 
in places 400 to 600 feet above sea level, and a hundred miles or more 
from the sea. 

Still farther inland the plains end against the old land, a low hilly 
surface known as the Piedmont Plateau, an ancient mountain region 
now worn to a surface of low relief. Doubtless the source of much of 
the sediment of which the coastal plains are made was this old land, 
during the period of its reduction by denudation. The Piedmont 
Plateau from New Jersey to Texas is covered by a fertile residual soil, 
and is the seat of extensive cotton culture. 

At the boundary between the Piedmont Plateau and the coastal 
plain the streams are so commonly interrupted by rapids and falls 
that their line has been called the Fall Line (Fig. 334). The rapids and 
falls are due to the fact that the streams 
have been able to cut more rapidly into 
the unconsolidated sediment of the coastal 
plains than in the resistant crystalline 
rocks of the Piedmont Plateau. Because 
of the water power, and because naviga- 
tion is checked here, the Fall Line is the 
seat of a chain of towns and cities, in- 
cluding Trenton, Philadelphia, Baltimore, 
Washington, Richmond, Raleigh, Colum- 
bia, Augusta, Macon, and Montgomery. 
Thus the coastal plains are bordered 
by a chain of towns and cities along the 
inner margin, and back of them is a fer- 
tile agricultural region. While the plains 
themselves are, in the main, sparsely set- 
tled, traffic across them by rail and stream 
from the Piedmont Plateau to the sea is important, and, over such a 
level surface, railroads are easily built. Because of this traffic the 
coastal plains are bordered on the outer side by a chain of sea coast 
towns, including Norfolk, Wilmington, Charleston, Savannah, Mobile, 
and Galveston, situated on harbours that are, in general, poor and 
partly obstructed by sand bars. Most of these harbours are shallow 
bays, formed by the slight submergence of the level land, admitting 
the sea into the coastal plain valleys. 

Allegheny Plateau. — Bordering the Appalachian Mountains on 
the west is a plateau (Fig. 335), extending from the Hudson River south- 
ward to Alabama. Near the mountains it is high, and usually higher 
than the mountains themselves ; but its surface descends toward the 
western and northern margins, merging into the plains of the Missis- 
sippi valley, and terminating in an escarpment on the east, and in 
New York, on the north. The plateau is so high and so rugged that 
it has been called the Allegheny Mountains in the central part, the 




Fig. 334. 



The Fall Line and its 
cities. 



5i2 



COLLEGE PHYSIOGRAPHY 



Catskill Mountains on the northeastern end, and the Cumberland 
Mountains on the southern end. The names Allegheny Plateau and 
Cumberland Plateau are preferable. The surface rises to an ele- 
vation of 2000 or 3000 feet, and in the Catskill Mountains to an eleva- 
tion of over 4000 feet. 

In this plateau the strata are nearly or quite horizontal, quite in 
contrast to the complexly folded strata of the Appalachian Mountains. 
Standing high above baselevel, there has been ample opportunity for 




Fig. 335. — Allegheny Plateau along the New River, W. Va. (Hillers, U. S. Geol. Survey.) 



the streams to sink their channels deeply into the plateau, and, there- 
fore, valleys 1000 to 2000 feet in depth have been cut. But, since the 
stage of denudation has passed that of early youth, the valleys are 
not prevailingly steep-walled gorges and canyons, though locally 
there are precipitous slopes and even gorges. In general the slopes 
have wasted back so that, although steep, they are usually capable 
of supporting fairly continuous forest growth, and in many parts 
have been cleared for pasture or for tillage, especially in the glaciated 
northern part of the plateau in New York and Pennsylvania. 

There are flat-topped uplands, often cleared for farming, while the 
valley slopes are left in forest ; but butte and mesa topography is 
not typical of the region. Here and there some unusually resistant 
layer stands out as a cliff, traceable along the valley walls, but there 
is no such angularity of topography as characterizes arid plateaus. 
This is due to a combination of several causes : (1) the advanced stage 



PLAINS AND PLATEAUS 513 

of dissection, (2) the humid climate, (3) the protecting and obscuring 
influence of forest cover, (4) the uniform consolidation of the layers 
and the scarcity of exceptionally resistant beds. 

In early days the Allegheny Plateau was an even more important 
barrier to travel from the coast to the interior than the Appalachian 
Mountains proper. It still presents serious obstacle to road and rail- 
way building. A large part of the plateau is still forest-covered, and, 
even where cleared, farming is limited in amount and value. Many 
of the slopes are too steep for farms, and the more level uplands are 
separated from the valleys by such steep slopes that roads are poor, 
difficult to maintain, and hard to draw loads over. Even the valleys 
are ordinarily sparsely settled, and upland areas are often remote 
from markets. These conditions find their best expression in the 
plateau of eastern Tennessee and Kentucky, where there are people 
who have had such slight contact with the outside world that they 
preserve customs of the early day when settlers first occupied the 
plateau land. They still wear homespun, they resist the government 
laws against illicit distilling, and they take the law in their own hands 
in settling disputes and feuds. Farther north there is a less degree of 
isolation. 

Were it not for the fact that this region must be crossed by west- 
bound railways, and that valuable mineral wealth exists in the hori- 
zontal strata, the Allegheny Plateau would be much more isolated 
and sparsely settled than it is. Coal is found in many places, often 
revealed in the sides of the deeply cut stream valleys ; petroleum 
and natural gas are also found ; and iron ore is present in some beds. 
The exploitation of these products has caused influx of people in parts 
of the plateau. 

The only large city in the Allegheny Plateau is Pittsburg, whose 
growth has been due to mineral wealth and to a situation at the junc- 
tion of the Allegheny and Monongahela rivers, which unite to make 
the navigable Ohio River. Other centres of industry are located 
on the Ohio and on the railway lines that cross the plateau, especially 
in the broader valleys. 

The Mississippi Valley Plains. — From the Allegheny Plateau the 
surface slopes downward gradually, and fairly regularly, to the Missis- 
sippi, then westward it slopes up to the Great Plains and the base of 
the Rocky Mountains. This great area of plains, one of the largest 
in the world, is interrupted only by the low mountain areas of central 
Texas, Oklahoma, and Arkansas, the highland region around the 
western end of Lake Superior, and the Black Hills. 

It is not a single plain, of single origin, but such a complex of plains 
that it will not be possible here to refer to more than a few of the 
more important divisions. In the south it merges into the coastal 
plains ; along the rivers it is crossed by strips of floodplain ; while 
there are delta plains at the river mouths, notably the Mississippi. 
In southern Missouri and northern Arkansas the plains rise to form 

2L 



5 H COLLEGE PHYSIOGRAPHY 

the low Ozark Plateau, which has been interpreted as a trans-Missis- 
sippi extension of the Allegheny and Cumberland plateaus, which it 
resembles in important respects. Toward the west the plains become 
gradually more and more arid, and rise to true plateau elevation in 
the Great Plains (Fig. 336). In the southwest they may have been 
partly modified by wind work. In the north their topography is 
modified and often even entirely made by glacial deposits or by de- 
posits made in front of the glacier, for instance, the lacustrine silts 
of the Red River of the North, laid down in the bed of the ice- 
dammed Lake Agassiz. 

The underlying strata of this great series of plains are nearly hori- 
zontal sediments, consolidated into hard rock layers, but worn down 
to a condition of low relief, during a long and varied series of erosion 
cycles. It has been classed as an ancient coastal plain, or series of 
coastal plains, added to the continent in long-past geological time, 
and greatly denuded. Low escarpments still stand out and a belted 
arrangement of outcropping strata locally forms cuestas, as in Wis- 
consin, and affects both soil and topography. Where highest, as in 
the Allegheny Plateau, in the Great Plains, around the Lake Superior 
highland, especially in the Driftless Area, and in the Ozark Plateau, 
the plains are so dissected that the topography is hilly; but in the 
Great Plains the dissection has assumed the arid land type of angular 
form. Here one finds frequent escarpments, canyon-like valleys, 
and mesa forms, especially near the Rocky Mountains. Some areas 
of the Great Plains near the base of the Rocky Mountains have been 
levelled up by deposit of sediment washed down from the mountains 
and spread out by the streams at their base. 

Within the area of the Mississippi valley plains, the deposits, 
deepest in the valleys, have in general tended to still further level 
this portion of the plains. In fact, over considerable areas these 
deposits have formed so level a surface that water did not drain off 
naturally, and, therefore, extensive tracts were too swampy for tree 
growth when first seen by white men. These prairies, now drained, 
sometimes have a black, fertile soil because of the abundant organic 
matter, and are among the finest agricultural lands. It is probable 
that other prairie areas were formed, or extended, by fires set by the 
aborigines. 

As far west as the 100th meridian, the larger part of the Mississippi 
valley plains is occupied by an agricultural population, making this 
one of the leading farming regions of the world. It is the granary 
of the United States. Only limited areas are too hilly or too swampy 
for cultivation. With abundant coal and other mineral resources, 
and with excellent transportation facilities, it has developed varied 
manufacturing industries, and is the seat of large and flourishing 
cities. West of the 100th meridian these advantages are lacking, and 
the arid plains are given over to ranching, and only small towns are 
found here and there. Agriculture is confined mainly to the valleys 



PLAINS AND PLATEAUS 



5i5 




THE HIGH PLAINS 

Fig. 336. — A portion of the Great Plains. (W. D. Johnson.) 



Si6 COLLEGE PHYSIOGRAPHY 

where irrigation is possible, or to those spots where artesian water 
can be obtained; but in the north, the agricultural belt extends a 
little west of the iooth meridian. 

The Columbia Plateau. — A hilly and even mountainous land 
between the Rocky Mountains and the Cascade Ranges in Washington 
and Oregon and parts of neighbouring states was, as we have seen 
(p. 480) , flooded with successive lava flows until the surface was built up 
into a lava plateau, far more level than the original surface. The lava 
sheets still retain their nearly horizontal position, though tilted locally. 
In these lava plains streams have cut their way, forming canyons, 
in some places of considerable depth (Fig. 337). Residual soil, formed 
by the disintegration of the lava, covers extensive areas of this plateau, 
and in parts of it there is rainfall enough for successful agriculture, 
notably wheat raising. On this plateau, and in the midst of the 
agricultural region, the city of Spokane has grown, the largest city 
between the eastern base of the Rocky Mountains and the Pacific 
coast. 

The Columbia Plateau is a type of numerous similar, though smaller, 
lava plains in western United States. 

The Colorado Plateau. — In Utah and Arizona is a great area of 
tableland, with elevations up to 7000 or 8000 feet. It consists of a 
series of plateau surfaces, ending in escarpments. The strata of the 
plateau are essentially horizontal, of varying degrees of resistance, 
and exposed to long and complex denudation. There has been fault- 
ing and some local tilting of layers, and volcanic action has built upon 
the plateau surface a large number of volcanic cones, one of which is 
the large extinct volcano known as San Francisco Mountain. Some 
of the volcanic activity has been very recent. 

In the course of the denudation, deep canyons have been cut into 
the strata, and their walls are terraced by the differential denudation 
of the strata. Cliff-forming strata, revealed by faulting or by denuda- 
tion, have receded and are still receding, giving rise to escarpment 
faces which separate the different plateau levels. There is a multi- 
tude of butte and mesa forms, especially in the neighbourhood of the 
canyons. It is a wonderfully sculptured land, a typical tableland, 
with the angular topography developed in such regions by denu- 
dation. Over most of the plateau the climate is so arid that 
forest growth is impossible, but in places the elevation is sufficient 
for the growth of an open pine forest. There are broad tracts with 
little soil and vegetation, but, over much of the surface, there is 
grass enough for cattle or sheep raising, though very often these in- 
dustries are rendered impossible by the scarcity of water. 

Into this plateau is -sunk the Colorado River, in a canyon unsur- 
passed for grandeur among the valleys of the world (PL VIII). The 
Colorado, rising in the Rocky Mountains, receives an abundant 
water supply, which enables it to flow across the entire plateau region 
and the desert, to the Gulf of California. For 1000 miles of this dis- 



Plate VIII 



10' •% 






i 



Cheops 
Pyramid 






0* 



Jj£. 



{Spring 



Dana 
Butte 



m 






t*. 



Spring 



Indian ^/'' 
Garden 

/Spring 



si 



Hopi 2 Point 



lohave 
D oint 




LL 1'.:, E'. ii-.A-. IN i Cv.. 



GRAND CANYON OF THE COLORADO 

Colorado Plateau with part of the Grand Canyon near the Bright Angel or Cameron Trail. Con- 
tour interval 50 feet. (From Bright Angel Quadrangle, United States Geological Survey.) 



PLAINS AND PLATEAUS 



5i7 




Fig. 337- — Lakes in the Grand Coulee, an abandoned channel of the Columbia River. An 
intermontane lobe of ice diverted the Columbia southward at A . It followed the channel 
A—B long enough to cut a deep canyon . There is no w an abandoned waterfall at Coulee 
City, and another (Fig. 69) at C. Moses Coulee seems to have been eroded entirely by 
glacial waters. 



518 COLLEGE PHYSIOGRAPHY 

tance it flows between steeply rising canyon walls, the grandest por- 
tion of which is the so-called Grand Canyon, which is over 200 miles 
in length. Here the walls in places rise 6000 feet above the river, and 
with such precipitousness that descent into the canyon is, in most 
places, impossible. No large streams join the Colorado in its canyon 
area, though there are numerous tributary canyons through which 
water sometimes flows. 

The canyon form varies from one part of the course to another, 
according to the nature of the enclosing rock, in some parts where the 
rock is fairly uniform being narrow and precipitous, in others, where 
the strata are more variable, flaring toward the top and being bordered 
by a series of rock terraces. In one place the canyon widens so that 
the distance across at the top is 10 miles or more. Throughout most of 
its course the canyon is cut in nearly horizontal sedimentary strata, 
but in parts of the Grand Canyon the river has cut down to a worn- 
down, buried, mountain area, in the highly folded and complex strata 
of which the bottom of the canyon is sunk. The river is here super- 
imposed upon the mountain structure that is hidden from view beneath 
thousands of feet of sedimentary strata. 

The Colorado Plateau is thus bisected by a great gash, forming an 
impassable barrier to travel across it. Moreover, there are a multitude 
of minor canyons, for there are a number of large tributary canyons, 
and, as Powell says, " every river entering these has cut another 
canon ; every lateral creek has cut a canon ; every brook rises in a 
canon ; every rill born of a shower, and born again of a shower, 
and living only during the showers, has cut for itself a canon ; so that 
the whole upper portion of the basin of the Colorado is traversed by a 
labyrinth of these deep gorges." 

Not only has there been this deep dissection of the plateau by 
canyons, but the evidence is clear that thousands of feet of strata 
have been removed from the plateau surface by long-continued denu- 
dation, the present canyons, scarps, and table top areas representing a 
late stage in this long denudation history (Fig. 338). 

Impressive as the Colorado Canyon is as a scenic feature, it is even 
more impressive for the lesson that it gives of the vastness of the 
changes by which the earth's surface is moulded. The buried moun- 
tain area in the canyon bottom tells of a period of deposit in the sea, 
followed by one of folding and then by long subaerial denudation by 
which the mountains were worn to a condition of low relief. Then 
comes submergence and the deposit of thousands of feet of sedimentary 
strata, completely covering the peneplained mountain area. Follow- 
ing this was uplift and a long, complex denudation history, with 
accompanying faulting, minor folding, and volcanic activity. During 
this denudation thousands of feet of strata have been removed, and 
the plateau has been traversed by a series of canyons, one part trench- 
ing the strata to a depth of over a mile. Such a history, which is 
only fragmentary, testifies eloquently to the vast duration of geological 



PLAINS AND PLATEAUS 



5i9 



time and the complexity of the processes by which the topography 
of to-day has been evolved. 

Plains of the Great Basin. — Between the low, short, mountain ranges 
of the Great Basin (Fig. 339) are depressions of various origins into which 
streams and wind have carried sediment, which, strewn over the valley 



Fig. F.— The folding 




Fio. G.- 


- The first faulting. 




'''•'"• v'- : .° 


• rty '.■?: \°. : : ':": •.'?. v°V__ 




--r.r-r_ 


>~~I-I-Z-Z-~^J&i : 


•0' • *'■ b '• '■ '° - o : * "' '-' >'Y° : . '-° 


^^^$^^^^>>>>>r->r~:-:-: 






ssssssg^aaggi 




J-r-L 


', ' 1 ' 1 lQ 


n±r 


.' 1 ! 1 '. 1 \C 


Z^JUUU 



Fig. H. — The great denudation. 








Fig. M. — The third faulting. 




Fig. 338. — Eight stages in the complex history of folding, faulting, denudation, emission of 
lava, canyon cutting, etc., by which the Colorado Plateau has reached its present form. 
(D. W. Johnson.) 



bottoms, has formed deposits varying in extent and degree of regularity. 
In some cases plains of considerable size have been formed, and in 
others the gentle-sloping surfaces of alluvial fans have sufficient level- 
ness to be classified as plains. One of the largest stream deposits is in 
the very southern portion near the head of the Gulf of California, 
where the Colorado River has a great, fan-like delta deposit. 

In many of the depressions in the Great Basin, lakes have formerly 
stood, where now only shallow salt lakes, or salinas and alkali flats, 
exist, as we have already seen was the case near Great Salt Lake. 
During these stages of higher lakes, the deposits formed lake bottom 
plains, which now are exposed to the air by evaporation of the waters. 
Among the most extensive of these lake bottom plains are those around 
the Great Salt Lake, but there are many other similar plains in the 
Great Basin. 



520 



COLLEGE PHYSIOGRAPHY 













>- IE o 






ttl'ol J J 



St 



SfHg 







PLAINS AND PLATEAUS 521 

Where not saline or alkaline, these level surfaces are well adapted 
to agriculture if water can be brought for irrigation. This can be 
done especially well on the alluvial fans, and on the plains near the 
mountains from which streams issue, and such spots form oases in the 
general desert, as near Salt Lake City. 

The Great Valley of California. — The broad valley between the 
Coast Ranges and Sierra Nevada is essentially a plain, sloping upward 
toward each mountain base, with the largest slope toward the Sierra. 
This plain undulates in a longitudinal direction because it consists of 
a series of coalescing alluvial fans, which develop strength of form 
near the mountains ; but toward the valley axis the alluvial fan charac- 
ter becomes more indistinct. The moderate slope of the surface and 
the fertile soil give to this broad plain great agricultural possibilities, 
which are realized in the north where there is sufficient rainfall, and in 
the arid southern part wherever water can be obtained from the 
alluvial-fan-building streams for use in irrigation. The Willamette 
valley in Oregon and the Puget Sound lowland in Washington are 
smaller basins, between the Cascades and Coast Ranges, but with 
less alluvial filling than in the valley of California. 

The Pacific Coast. — No broad coastal plain borders the Pacific 
coast ; but, for most of the distance, mountains rise from the coast 
line. Here and there, however, there are narrow strips of coastal 
plain, uplifted above the sea and fringing the mountain base. At 
stream mouths, too, there are delta deposits of small extent. On 
such a coast there is small chance for settlement, communication 
with the interior is interfered with by the mountains, and travel 
along the coast is difficult. It reminds one of the coast of Italy, 
where the railways from the coast, as at Genoa, must at once 
tunnel into the mountains, while those along the coast pass through 
a succession of tunnels. Coming out of one tunnel and revealing 
a vista of the sea, and of a narrow delta plain occupied by a 
village, the train almost at once tunnels into the next spur, and so 
on for miles. On the Pacific shore, coastwise railways have not yet 
been built, except in part of the distance between San Francisco and 
Los Angeles, nor is travel by road possible along most of the coast. 
It offers a striking contrast to the low,, flat, coastal plains of the At- 
lantic coast, over which roads and railways can be built anywhere, 
excepting where swamp lands interfere. 



References to Literature 

C. Abbe, Jr. Physiography of Maryland, Md. Weather Service, Vol. 1, 1899, 

35 PP- 

H. H. Barrows. Geography of the Middle Illinois Valley, Bull. 15, 111. Geol. 
Survey, 1910, 128 pp. 

Isaiah Bowman. Physiography of the United States, Forest Physiography, 
New York, 191 1 : Atlantic and Gulf Coastal Plain, pp. 498-553; Appa- 
lachian Plateaus, pp. 685-720; Prairie Plains, pp. 460-497; Great Plains, 



522 COLLEGE PHYSIOGRAPHY 

pp. 405-459; Columbia Plateaus, pp. 192-206; Colorado Plateaus, pp. 

256-299; Arizona Highlands, pp. 246-255; Lower Colorado Basin, pp. 

236-245; Great Basin, pp. 210-235; Pacific Coast Valleys, pp. 177-191. 
M. R. Campbell and A. C. Mendenhall. Geologic Section along the New 

and Kanawha Rivers in West Virginia, 17th Ann. Rept., U. S. Geol. 

Survey, Part 2, 1896, pp. 473-511. 
W. B. Clark and E. B. Matthews. The Physical Features of Maryland, Md. 

Geol. Survey, Vol. 6, 1906, pp. 26-259. 
Collier Cobb. North Carolina, Journ. School Geog., Vol. 1, 1897, pp. 257-266, 

300-308. 
G. E. Condra. Geography of Nebraska, Lincoln, 1906, 192 pp. 
N. H. Darton. Geology and Water Resources of Nebraska West of the One 

Hundred and Third Meridian, 19th Ann. Rept., U. S. Geol. Survey, Part 

4, 1898, pp. 719-785; Underground Waters of a Portion of Southeastern 

Nebraska, Water Supply Paper 12, U. S. Geol. Survey, 1898. 
W. M. Davis. The United States of America, Mill's International Geography, 

1899, pp. 710-773; The Drainage of Cuestas, Proc. Geol. Assoc, Vol. 16, 

1899, pp. 75-93 ; The Development of Certain English Rivers, Geog. 

Journ., Vol. 5, 1895, pp. 127-146; Kiistenebenen, Ebenen, und Hoche- 

benen, Erklarende Beschreibung der Landformen, Leipzig, 191 2, pp. 197- 

245 ; Excursion to the Grand Canyon of the Colorado, Bull. Mus. Comp. 

Zool., Vol. 38, 1901, pp. 107-201 ; Excursion to the Plateau Province of 

t Utah and Arizona, ibid., Vol. 42, 1903, pp. 1-50. 
."A. Dietz. The Fall Line, Journ. Geog., Vol. 4, 1905, pp. 244-248. 
, S. Diller. A Geological Reconnaissance in Northwestern Oregon, 17th 
Ann. Rept., U. S. Geol. Survey, Part 1, 1896, pp. 441-520. 

C. R. Dryer. Studies in Indiana Geography, Terre Haute, 1897, 113 pp. 

C. E. Dutton. Geology of the High Plateaus of Utah, Powell's U. S. Geog. and 
Geol. Survey, 1880, 307 pp.; Tertiary History of the Grand Canon Dis- 
trict, Monograph 2, U. S. Geol. Survey, 1882, 264 pp. and atlas. 

F. V. Emerson. Geography of Missouri, Bull. Univ. Missouri, Educational 
Series, Vol. 1, 1912, 74 pp. 

N. M. Fenneman. Physiography of the St. Louis Area, 111. Geol. Survey, 
Bull. 12, 1909, 83 pp. 

Henry Gannett. The United States, Stanford's Compendium of Geography 
and Travel, North America, Vol. 2, London, 1898. 

L. C. Glenn. South Carolina, Journ. School Geog., Vol. 2, 1898, pp. 9-15, 85-92. 

H. E. Gregory. Physiography of the United States, Physical and Commercial 
Geography, Boston, 1910, pp. 58-65. 

Arnold Guyot. Physical Structure and Hypsometry of the Catskill Mountain 
Region, Amer. Journ. Sci., 3d series, Vol. 19, 1880, pp. 429-451. 

C. W. Hall. The Geography and Geology of Minnesota, Minneapolis, 1903, 
299 pp. 

A. Hague and S. F. Emmons. Great Basin, King's Report of the Geological 
Exploration of the Fortieth Parallel, Prof. Papers of the Engineering 
Dept. U. S. Army, Vol. 2, Descriptive Geology, 1877, pp. 311-890. 

A. Heilprin. The Catskill Mountains, Bull. Amer. Geog. Soc, Vol. 39, 1907, 
pp. 193-201. 

R. T. Hill. The Geography and Geology of the Black and Grand Prairies, 
Texas, 21st Ann. Rept., U. S. Geol. Survey, Part 7, 1901, pp. 1-666; Phy- 
sical Geography of the Texas Region, Folio 3, Topographic Atlas of 
the United States, U. S. Geol. Survey, 1900. 

F. M. Hodge. The Enchanted Mesa, Nat. Geog. Mag., Vol. 8, 1897, pp. 
273-284. 

W. D. Johnson. The High Plains and their Utilization, 21st Ann. Rept., U. S. 
Geol. Survey, Part 4, 1900, pp. 601-741; ibid., 22d Ann. Rept., Part 4, 
1902, pp. 631-669. 

J. F. Kemp. Ore Deposits of United States and Canada, Eng. and Min. Journ., 
New York, 1893. 



PLAINS AND PLATEAUS 



523 



L. Lesquereux. On the Origin and Formation of the Prairies, Worthen's 

Geol. Survey of Illinois, Vol. 1, 1866, pp. 238-254. 
W J McGee. The Geology of the Head of Chesapeake Bay, 7th Ann. Rept., 

U. S. Geol. Survey, 1888, pp. 537-646; The Lafayette Formation, ibid., 

12th Ann. Rept. 1891, pp. 347-521. 
H. J. Mackinder. Britain and the British Seas, New York, 1902, 377 pp. 

C. F. Marbut. Physical Features of Missouri, Mo. Geol. Survey, Vol. 10, 

1896, pp. n-109. 

Lawrence Martin. The Physical Geography of Wisconsin, Bull. Wis. Geol. 
Survey (in press). 

G. C. Matson and F. G. Clapp. Geology of Florida, 2d Ann. Rept., Florida 
Geol. Survey, 1908-1909, pp. 25-49; G. C. Matson and S. Sanford, 
Water Supply Paper 319, U. S. Geol. Survey, 1913, 445 pp. 

H. R. Mill and Others. International Geography, New York, 1899. 

J. W. Powell. Exploration of the Colorado River of the West, Washington, 
1875, 291 pp. ; Physiographic Regions of the United States, National 
Geographic Monographs, New York, 1896, pp. 65-100. 

H. Reusch. The Norwegian Coast Plain, Journ. Geol., Vol. 2, 1894, pp. 347- 

349- 

H. Ries. Economic Geology of United States, New York, 1907, 451 pp. 

I. C. Russell. North America, New York, 1904, 435 pp. 

R. D. Salisbury. The Physical Geography of New Jersey, N. J. Geol. Survey, 
Vol. 4, 1898, 200 pp. 

S. Sanford. The Topography and Geology of Southern Florida, 2d Ann. 
Rept., Florida Geol. Survey, 1908-1909, pp. 177-231. 

N. S. Shaler. United States of America, New York, 1894. 

G. B. Shattuck. Coastal Plain, Pliocene and Pleistocene, Md. Geol. Survey, 
1906, 137 pp. 

E. A. Smith. Report on the Geology of the Coastal Plain of Alabama, Geol. 
Survey of Alabama, 1894, 759 pp. 

J. R. Smith. Plateaus in Tropical iVmerica, 8th International Geographical 
Congress, Washington, 1905, pp. 829-835. 

J. E. Spurr. Descriptive Geology of Nevada South of the Fortieth Parallel, 
Bull. 208, U. S. Geol. Survey, 1903, 229 pp. 

R. S. Tarr. Physical Geography of New York State, New York, 1902, Chap- 
ter I, Physiographic Features; Chapter III, Plains and Plateaus; Chap- 
ter XII, Influence of Physiographic Features upon Industrial Devel- 
opment; Economic Geology of United States, New York, 4th edition, 
1903. 

W. S. Tower. Plateau Province, Regional and Economic Geography of Penn- 
sylvania, Bull. Geog. Soc. Phila., Vol. 4, 1906, pp. 204-217, 271-281. 

A. C. Veatch. Long Island, Prof. Paper 44, U. S. Geol. Survey, 1906, pp. 28- 
32; Louisiana- Arkansas, ibid., Prof. Paper 46, 1906, pp. 14-69. 

O. D. von Engeln. Effects of Continental Glaciation on Agriculture, Bull. 
Amer. Geog. Soc, Vol. 46, 1914, pp. 241-264, 336-355. 

R. H. Whitbeck. Economic Aspects of Glaciation in Wisconsin, Annals Assoc. 
Amer. Geographers, Vol. 3, 1913. 

D. E. "Willard. The Story of the Prairies, Chicago, 1907, 377 pp. 



TOPOGRAPHIC MAPS 



Coleman, Tex. 



Buttes 
Mt. Carrizo, Colo. 



Bisuka, Idaho 



Madison, Wis. 
Jefferson City, Mo. 



Central Plains 

Marion, Iowa 
Lacon, 111. 



Butler, Mo. 
Ottawa, 111. 



5 2 4 



COLLEGE PHYSIOGRAPHY 



Winterville, N.C. 
Atlantic City, N J. 



Higbee, Colo. 



Marshall, Ark. 
Skaneateles, N.Y. 



Hollow Springs, Tenn. 



Wichita, Kans. 
Great Falls, Mont. 
Lexington, Neb. 



Lassen Peak, Cal. 
Fargo, N.D. 
Niagara Gorge, N.Y. 



Boise, Idaho 



Brownwood, Tex. 
Watrous, N.M. 



Coastal Plain 

Leonardtown, Md. 
Barnegat, N J. 



Pt. Lookout, Md. 
Norfolk Special 



Dissected Arid Plateau 
Kaibab, Ariz. Mt. Taylor, N.M. 

Dissected Humid Plateaus 



Centre Pt., W.Va. 
Ovid, N.Y. 

Escarpments 
Niagara Gorge, N.Y. 

Great Plains 

Lamar, Colo. 
Palo Pinto, Tex. 
Syracuse, Kan. 

Lake Plains 

Sierraville, Cal. 
Toole Valley, Utah 
Hamlm, N.Y. 

Lava Plains 
Modoc Lava Bed, Cal. 

Mesas 

Higbee, Colo. 
Mt. Taylor, N.M. 



Pikeville Special, Tenn. 
Kaaterskill, N.Y. 



Fond du Lac, Wis. 



Coleman, Tex. 

Denver and Vicinity, Colo. 

Kearney, Neb. 



Disaster, Nev. 
Salt Lake, Utah 
Rochester Special, N.Y. 



Mt. Taylor, N.M. 



Kaibab, Ariz. 
The Dells, Wis. 



CHAPTER XV 
MOUNTAINS 

The Term Mountain 

Mountains, Hills, and Plateaus. — In common usage the term 
mountain applies to any unusual elevation (Fig. 340). Thus on the 
Texas plains, a butte 200 feet high may be called a mountain ; the dis- 
sected Allegheny Plateau, where it rises above the Hudson valley, is 




Fig. 340. — The Alps in Austria, rising above the snow line. 

known as the Catskill Mountains, and the escarpment bordering this 
plateau on the north is known as Helderberg Mountain. On the 
other hand, an integral part of the Appalachian Mountain system is 
commonly called the Berkshire Hills, and another, lower part, the 
Piedmont Plateau. 

Folded Structures in Mountains. — In this book the term moun- 
tain is used in a more restricted sense, referring to those parts of the 

525 



526 COLLEGE PHYSIOGRAPHY 

earth's crust which have been so disturbed by diastrophic movement as 
to notably influence the topographic forms, either directly by uplift 
or indirectly by denudation working upon the disturbed strata. 
In the plain or plateau the strata are essentially horizontal, even 
though higher than many mountains; in the mountain the strata 
diverge from the horizontal to an appreciable degree. 

A hard-and-fast line cannot be drawn between plateau and moun- 
tain, for there is every gradation from horizontal to inclined strata, 
and plateaus are locally broken by faults, and deformed by folds. 
Moreover, plateaus may be so dissected as to simulate rugged moun-' 
tain topography, as in the Catskills, and mountains may be worn to 
such low relief as to resemble a plain, as in the Piedmont Plateau. 

Volcanic Mountains. — Volcanic peaks are not here included under 
mountains, for they are distinctly the product of vulcanism. Yet 
they occur among mountains and form noteworthy peaks in mountain 
chains, and vulcanism in various forms is intimately associated with 
mountain formation, while volcanic rocks make up a large proportion 
of many mountain masses. 

Here as elsewhere in the study of physiography, gradation of phenom- 
ena is found to be the rule. A topical study is not warranted by the 
phenomena of nature, for everywhere there is intergradation ; it finds 
its only excuse in the demand of simplicity of exposition. Mountains, 
plains, volcanoes, rivers, and weathering are not phenomena set off 
by themselves ; they are complexly interrelated. 

Mountain Types 

Relation to Folding and Faulting. — The disturbance of strata, 
forming mountains, may be brought about either (i) by folding, 
(2) by faulting, or (3) by combined folding and faulting; and either 
the folding or the faulting may be very simple or very complex. 
The strata involved may be sedimentary, igneous, or metamorphic, or 
a combination of these. The disturbance may take place with or 

without visible igneous 
activity, though it is 
probable that subterra- 
nean intrusion occurs in 
connection with most 
extensive mountain 
formation. 

Fault Block Moun- 
l ig. 341 . — Fault block mountains. tains. — A simple type of 

mountain results from 
the tilting of strata on one side of a fault plane, forming the fault 
block mountain. In this case a ridge is formed, with an escarpment face 
on the side toward the fault, and a gentle slope in the opposite direc- 
tion, the inclination of this slope depending on the dip of the inclined 




MOUNTAINS 527 

strata (Fig. 341). This type of mountain is found in the Great Basin 
region of southern Oregon, and many of the ridges in the Great Basin 
farther south have been assigned to the same cause. Some of the Basin 
Ranges of Oregon are 10 to 40 miles long and over 1000 feet high. 
Similar faulted blocks are often developed in plateau uplift, and often 
the inclination of the strata is very slight, so that there is every grada- 
tion from the broken, tilted fault block to the broken, untilted blocks, 
both faced by escarpments. At times the faults merge into monoclinal 
folds, and thus there is gradation from tilted fault blocks to escarp- 
ments due to folding. Ridges due to monocline folding are common 
in the plateau region of Utah and Wyoming. 

Between the fault block ranges sinking may take place ; and either 
this movement or continued uplift of the fault blocks is still in progress. 



Fig. 342. — Block diagram of the Uinta Mountains, with original mountain arch in the 
background and the present erosion forms in the foreground. (Powell.) 

This is proved by the faulting of alluvial fan deposits and the preva- 
lence of earthquakes, showing the recency of origin of the Basin 
Ranges. 

The Arched Mountain Type. — A second mountain type of simple 
form is that caused by the updoming of a surface with little or no 
faulting, and with no complex folding, — merely a gentle dip of the 
strata from the centre of the domed area. When dissected, such a dome 
may develop the rugged topography of mountains. The Black Hills 
are of the arched mountain type. The mountain dome is about 50 by 
100 miles, and now rises to a height of between two and three thou- 
sand feet. It is surrounded by concentric ridges and valleys, related 
to resistant and weak strata. Before it was unroofed, this dome 
must have risen at least 6000 feet above the adjacent plains. 

Simple arching may rear the strata much higher and give rise to 
more pronounced mountain topography. Powell named such a 
mountain form the Uinta type, after the Uinta Mountains of Wyoming 
and Utah, where he found it developed. This mountain range is a 
broad, flat arch, fully 150 miles long and 50 miles broad, rising 10,000 
to 11,000 feet above sea level and 5000 to 6000 feet above the surround- 
ing plateau. The strata are nearly horizontal along the crest, but 



528 



COLLEGE PHYSIOGRAPHY 



dip steeply at the margins and then quickly resume their horizontal 
position (Fig. 342). The present surface has been developed by long- 
continued denudation, in the course of which, it is estimated, 3J 
miles of strata have been removed from the plateau-like crest of the 
arch. 

Laccolitic Mountains. — Roughly circular or elliptical domes 
may be formed by the intrusion of laccolites beneath strata, raising 
them so that they dip outward with approximate uniformity from the 
centre of the dome. This type of mountain was first recognized by 
Gilbert in the Henry Mountains of Utah, a group of five dome-shaped 
mountains, the highest of which rises 5000 feet above the surrounding 
plateau. This type of dome mountain, which may be called lacco- 




Fig. 343. — Block diagram of the Henry Mountains as they now are, the back of the dia- 
gram showing the dome before it was eroded and unroofed. (Gilbert.) 



litic mountains, has since been recognized in other places. Like the 
Uinta Mountains, these have been greatly denuded and the lacco- 
litic core is revealed (Figs. 324, 343). 

Symmetrical Mountain Folds. — From such simple types of folding 
there is every gradation to great complexity, and commonly among 
mountains there is not a single fold, but a number, side by side. In 
some cases the strata are thrown into a succession of roughly parallel 
waves, in which the layers dip away from the crest of each wave at 
fairly uniform angle, and toward the troughs (Fig. 344) . Thus a given 
layer undulates up and down with the regularity and symmetry of the 
waves of the ocean, each trough or syncline forming a valley, each 
crest or anticline a ridge. This type of symmetrical mountain fold is 
well illustrated in parts of the Swiss Jura, but, even here, denudation 
has stripped off some of the folded layers and partly destroyed the sym- 
metry of form, though the undestroyed layers preserve the symmetry 
of folding. The Appalachian Mountains resemble the Jura in their 



MOUNTAINS 529 

symmetrical folding, but they are much older and have been so long 
denuded that the original folds no longer dominate the topography, 
which is now determined by the relative resistance of the folded strata. 
The Jura Mountains are the most youthful folded mountains in the 
world, in stage of the cycle, as the fault blocks of Oregon are probably 
the most youthful faulted mountains. 

Normal Mountains. — In mountain folding the strata are very 
commonly thrown into far more complex position than in the cases 
so far considered. There are unsymmetrical folds, with an inclination 
of the strata greater on one side than on the other ; there are closed 
folds, overturned folds, fan-shaped folds ; there are faults of various 
kinds, inclinations, and degrees of throw; and there is a complex 
relation of sedimentary, igneous, and metamorphic strata. Among 
great mountain ranges this is the ordinary condition ; so much so that 
one might call them normal mountains (Figs. 345/346), and the others 




Fig. 344. — Symmetrical mountain folds of the type developed in the Jura, with two stages 

of erosion. 

mere intermediate stages between the plain and the mountain. It 
would doubtless be possible to classify mountains of this complex 
character, but the attempt does not seem profitable, for there is almost 
infinite variety in the complexity. This class of mountain might be 
called the Alpine, or Himalayan, or Andean, or Rocky Mountain type, 
if a name were needed. s 



Distribution of Mountains 

The Two Great Mountain Belts. — Most of the really lofty moun- 
tains of the world, and the ones in which the evidence of present growth 
is most noticeable, are arranged in two great belts, the one nearly sur- 
rounding the Pacific, the other along an east-west circle north of the 
equator (Fig. 347) . These are the belts already noted (pp. 417 and 478) 
as the earthquake and volcanic belts — both phenomena associated 
with growing mountains. It is further noteworthy that the lofty 
mountains of these belts are mainly marginal to the continents, though 
some rise off the continent edge or island chains, and some back from 
the continent edge, as in southern central Asia, and in western United 
States. Plateaus are commonly associated with these mountain 



53° 



COLLEGE PHYSIOGRAPHY 




NEW ENGLAND ALPS OF THE LATE PALEOZOIC 





close: of the triassic basin 







BLOCK MOUNTAINS OF THE EARLY JURASSIC 



Fig. 345. — Normal mountains in central Connecticut. 

Four stages in the development of a mountainous region. Still earlier stages of folding, 
faulting, vulcanism, and denudation preceded the first shown here, for which more pre- 
cise data as to the topography of the Paleozoic and pre-Paleozoic are lacking. (Barrell, 
Geol. Survey of Connecticut.) 



MOUNTAINS 



53i 




CLOSE OF THE TERTIARY PERIOD 




DURING THE GLACIAL PERIOD 



>-^£& ^3^ jj^frdsb* *£&> 



We&rern Highland 








Eastern Highland 








\ aC? : ' : '" 





PRESENT GEOLOGIC TIME 



fill Continental ice sheet, glacial period. Wm Paleozoic sediments, metamorphosed to schists and quartzites. 
EpiJ Triassic sediments and laws. i/ilA Pre-Paleozoic complex gneisses. 



~l Paleozoic intrusive granite-gneisses. scale in miles, horizontal and vertical. 10 



Fig. 346. — Normal mountains in central Connecticut. 

Four additional stages in the history of diastrophism and erosion in southern New England. 
The clouds suggest climatic conditions and furnish a rough vertical scale, the cumulus 
clouds being about a mile above the earth's surface. (Barrell, Geol. Survey of 
Connecticut.) 



532 



COLLEGE PHYSIOGRAPHY 



uplifts, and very often the plateaus occupy more area than the folded 
mountain uplifts. 

Besides these two mountain belts there are individual chains here 
and there, both on the land and in the sea. Of the former the moun- 
tains of western Africa are an instance ; and in the Pacific and Indian 
oceans are many chains rising from the sea floor. These are all 
mountains of recent or present growth, but there are many chains 
which rose in a former time and have since been exposed to denuda- 
tion, with little or no regrowth. Such mountains, which are not in 
the two belts of recently elevated chains, have often been so worn 
down that they are no longer classed among the lofty mountains of 




Fig. 347. — Distribution of present-day mountains in the world. 



the world ; and some are reduced to such low relief that they do not 
commonly pass for mountains. Among the ancient mountains, 
now greatly reduced, may be mentioned the Appalachians of eastern 
United States, the Brazilian Highlands, a large part of the British 
Isles, the Scandinavian Peninsula, and parts of Germany and France. 
Mountain Folding of Various Dates. — Many mountains have been 
subjected not merely to one period of uplift and folding, but have 
suffered disturbance again and again. The Appalachian Mountains, 
the Rocky Mountains, and the Alps have had such complex history, 
but the Appalachians were subjected to their latest period of folding 
in long past geological time, while the Alps and Rocky Mountains have 
suffered recent regrowth (Fig. 348). Thus it is evident that a line 
along which folding has once taken place may be the seat of subse- 
quent disturbance ; or the later foldings may not affect these regions 



MOUNTAINS 



$33 



but occur along entirely new lines near by, or remote from them, as 
the case may be. 

The denudation of mountains and plateaus supplies great quantities 
of sediment for removal to lower levels, and the repeated uplifts 
tend to continue the supply. Because of these facts mountains 
have been called the backbones of continents, connected by a tissue of 
sediment supplied by their denudation. Out of the detritus thus 
furnished have been built many of the plains that stretch between 
the mountains ; and even a large portion of the strata in the moun- 
tains have been derived in similar manner, and later folded to form 
parts of mountains. The regularly folded Appalachians, for example, 




Fig. 348. — Map of the world to show the distribution of mountains of Tertiary age 
The arrows show supposed directions of crustal movement in the mountain making. 
(Taylor.) 

are composed of sedimentary strata, first deposited in the sea at the 
western base of the older Appalachians of the Piedmont Plateau, 
from which the sediment came, and then folded into mountain form. 
Some of the important facts regarding the mountains of the world 
are summarized in the following table, in which the elevations of a few 
of the higher plateaus are added for comparison. 



Feet 

Abyssinian Plateau 6-9,000 

Aconcagua, Andes, Argentina (highest in 

South America) 22,860 

Apo, Mindanao, Philippines .... 10,312 

Ararat, Turkey in Asia 17,325 

Mt. Blanc, France (highest in Alps) . . 15,781 

Bolivian Plateau 10-13,000 

Brazilian Plateau 2-3,500 

Chimborazo, Andes, Ecuador .... 20,498 
Cotopaxi, Andes, Ecuador 19,613 



Feet 

Elbruz, Caucasus, Russia 18,200 

Erebus, Antarctica 12,365 

Etna, Sicily 10,870 

Everest, Himalayas, Nepal (highest known 

in world) 29,002 

Fremont Peak, Rocky Mountains, Wyo. 13, 79° 

Fujiyama, Japan 12,365 

Hekla, Iceland 5, no 

Kenia, Africa i9,i99 

Kilimanjaro (highest known in Africa) . I9>7i7 



534 



COLLEGE PHYSIOGRAPHY 



Feet 
Kosciusko, Australia (highest in Australia) 7,336 

Kunchinjunga, Himalayas 28,156 

Logan, St. Elias Range (highest in 

Canada) 19, 539 

McKinley, Alaska (highest in North 

America) 20,464 

Marcy, Adirondacks, New York . . . 5,344 

Matterhorn, Alps 14-780 

Mauna Kea, Hawaiian Islands .... 13,805 
Mauna Loa, Hawaiian Islands .... 13,675 
Mayon, Luzon Island, Philippines . . . 8,900 

Mexican Plateau 5-6,000 

Mitchell, Appalachian Mts., N. C. (highest 

in eastern United States) 6,711 

Orizaba, Mexico (highest in Mexico) . . 18,314 
Pico del Turquino, Cuba 8,600 



Feet 

Pike's Peak, Rocky Mountains, Colorado 14,111 

Popocatepetl, Mexico 17,798 

Rainier, Cascade Mountains, Washington 14,408 

Ruwenzori, Africa 16,815 

St. Elias, Alaska 18,025 

San Francisco Mountain, Arizona . . . 12,611 

Shasta, Cascade Mountains, California . 14,380 

Tibet Plateau 10-15,000 

Tina, Haiti 10,300 

Vesuvius, Italy 3,880 

Washington, White Mountains, N. H. 

(highest in northeastern United States) 6,279 
Whitney, Sierra Nevada, California (high- 
est in United States) 14,502 

Yunque, Porto Rico 3,609 



Mountains of Eurasia. — Next to plains, mountains are the most 
widely distributed and most extensive of land forms. They form a 




Fig. 349. — View in the Caucasus with snow-covered slopes and cloud-filled valleys. 



large proportion of the area of some continents, notably Asia. Here, 
in addition to the fringing mountain islands — the Japanese, Philip- 
pine, and East Indian Islands, and those of the peninsulas, there is 
the great complex of mountains in central, eastern, and southern Asia, 
together forming the greatest mountain area of the earth, and includ- 
ing the highest peak, Mount Everest, 29,000 feet in elevation, towering 
even above the Plateau of Tibet, which is 15 to 16 thousand feet high. 
The mountains of southern Asia extend east through the Caucasus 
(Fig. 349) and Asia Minor, to the Mediterranean region, whose north- 
ern shore is mainly bordered by mountains, while mountain spurs pro- 
ject to form the Balkan and Italian peninsulas. The Spanish peninsula 
includes the Pyrenees and Sierra Nevada ranges in addition to other 
shorter and lower ones. North of the Alps are the worn-down 
mountains extending from central France eastward through Germany 



MOUNTAINS 535 

into Austria and the Balkan Peninsula. The low Urals extend 
north and south along the eastern boundary of Russia, and an ancient 
mountain range extends from northern Scandinavia, through the Brit- 
ish Isles to Brittany in France. North of Europe is the mountainous 
Spitzbergen and other Arctic islands. Thus Eurasia has a great 
number of mountains, extending in all directions, forming a great 
complex, and in various stages of development, some very old, some 
even now rising. 

Mountains of Africa. — Africa is far less mountainous, for it is 
mainly a plateau, somewhat broken around the edge ; but it is not 
sufficiently explored for an exact mapping of its mountains. The 
two principal ranges are the Atlas Mountains in the north, and Cape 
Mountains in the south. In central eastern Africa some of the peaks, 
which are volcanic, attain an elevation of nearly 20,000 feet. 

Mountains of Australia and Antarctica. — In Australia the principal 
range is along the east coast, but there are shorter ranges in other 
parts, none, not even the east coast mountains, being very lofty. 
New Zealand is part of a mountain range in the sea, and there are 
scores of others in the Indian and Pacific oceans. The Antarctic 
continent is too little known to state its condition, though such parts 
as are explored are mountainous in character. 

Mountains in the Americas. — In the New World there is a con- 
tinuous mountain chain from the southern tip of South America to 
the northern part, where it spreads apart fan-shaped, one branch 
going into the Isthmus of Panama, others northeastward through the 
Caribbean. This Andean system broadens in the centre, especially 
in Peru and Bolivia, and includes extensive plateaus between the 
nearly parallel chains. Here is found Aconcagua, the loftiest moun- 
tain in the New World, about- 23,000 feet, and from the coast 
the slope goes on down 15,000 feet or more to the deep sea. The 
Brazilian Highlands are an ancient, worn-down mountain area; 
and the Venezuela Highlands are another and higher area of the same 
nature. 

North of South America are the West Indian or Antillean Mountains, 
rising from the sea floor at depths of 16,000 feet or more to elevations 
of 5000 or 10,000 feet above sea level, forming therefore a really 
imposing mountain system, though mainly beneath the sea. Short 
mountain ranges occur in Central America and southern Mexico. 
Then begins the series of chains of the North American Cordillera, 
with intermediate valleys and plateaus, which stretch northward to 
Alaska, and curve westward toward Asia through the Aleutian 
Island chain. In the United States the ranges of this broad area of 
north-south mountains are, from east to west, the Rocky Mountains, 
the Basin Ranges, the Sierra-Nevada-Cascade Ranges, and the 
Coast Ranges. These mountains attain their culminating height in 
Alaska, where St. Elias rises 18,025 feet, Logan 19,539 f eet > an d Mc- 
Kinley 20,464 feet. 



536 COLLEGE PHYSIOGRAPHY 

The Appalachians of eastern United States and Canada are a worn- 
down mountain chain, and there is a great area of reduced mountain 
land in northern and central Canada, besides some low mountain 
masses in Oklahoma, Texas, and Arkansas. The islands of the Arctic, 
including Greenland, are mainly reduced mountain land. 

All Mountains not Lofty. — From this summarized survey of the 
mountain areas of the world, from which oceanic mountains have 
been in the main excluded, it is evident that, in considering the dis- 
tribution of mountains, attention cannot be confined to those moun- 
tains which are lofty. There is, perhaps, as much mountainous 
area of low relief as is included in the well-recognized mountain 
chains. Such mountains are old — they have had their period of im- 
posing elevation, but have lost relief under the steady and long- 
continued attacks of denudation, and have not been notably renewed 
by recent uplift and folding. 

The Growth of Mountains 

Mountain Growth not Rapid. — In the chapter on diastrophism 
it has been shown that mountain growth is in progress in parts of the 
earth — in the St. Elias Range, the Coast Ranges of California, the 
Andes, and Japan, for example. This growth is not rapid ; it consists 
of intermittent movements, with long periods of rest, or of such slow 
movement as to have escaped detection. There is no evidence that 
mountain growth in the past has proceeded with great rapidity, though, 
so far as any proof to the contrary goes to show, it may have been 
more rapid. All that we can be certain of is that, on the whole, 
the mountain formation has been much more rapid than the levelling 
processes of denudation, and that, consequently, lofty ranges have 
been reared. 

Cessation of Mountain Growth. — Some areas, where mountains 
were formed in very early geological ages, have not subsequently been 
notably disturbed by mountain folding. For example, the peneplained 
and buried mountain mass in the bottom of the Grand Canyon of 
the Colorado has not been subjected to refolding in all the ages 
required to lower it by denudation and in all the subsequent time re- 
quired for the deposit of thousands of feet of sedimentary strata and 
for the great denudation since these were uplifted out of the sea. 
This time is to be reckoned in millions of years, for it spans many 
geological periods. The Lake Superior-Hudson Bay Highland has 
had a similar history, and there are many other known cases of areas 
long ago reared to mountain conditions, and since then immune. 

Recurrence of Mountain Growth. — In other cases, as we have seen, 
there has been recurrent growth, during which the earlier mountains 
have been much denuded, so that later sediments partly overlap 
them with notable unconformity. These later sediments are then 
involved in a subsequent folding, which affects not only them, but 



MOUNTAINS 537 

the old mountain rocks also. Along certain belts there has been note- 
worthy recurrence of mountain folding during the geological past; 
in other places there has been absence of mountain formation during 
most of geological time, as in the greater part of the area of the Missis- 
sippi valley plains. 

Uplift in Mountains. — When extensive mountain growth takes 
place there is, in the first place, definite uplift, often involving a 
broad area most of which escapes with slight disturbance of the 
strata, giving rise to plateaus. Here and there in the plateau 
there may be faulting, or monoclinal folding, or doming or other 
form of moderate flexure, some parts rising, others sinking ; and 
lava extrusion, either from fissures or from volcanic vents, may take 
place. But along certain belts there develops notable faulting or 
folding, or both, and these belts rise as mountain chains of complex 
structure. From them also lava may outflow, and the wearing down 
of such mountains by denudation reveals the fact that much igneous 
rock was intruded beneath them, often in great batholites. 

Down Folding in Mountains. — In more or less close association 
with such mountain uplift, there is commonly, perhaps universally, 
notable depression. Linear depressions of unusual depth lie close by 
some of the oceanic mountain chains, for example near Porto Rico 
and near Guam; deep oceanic water lies off the South American 
coast ; the plain of Lombardy and valley of the river Po lies at the 
southern base of the Alps ; the valley of northern India at the 
southern base of the Himalaya Mountains ; the Great Valley 
of California between the Coast Ranges and the Sierra Nevada; 
the Puget Sound- Willamette lowland between the Cascades and the 
Coast Ranges ; and the Death Valley, below sea level, at the eastern 
base of the Sierra. Within the ranges themselves are smaller basins 
due to down folding as in the parks of the Colorado Rockies. These 
depressions are often so filled with sediment, washed into them from 
the mountains, that their true depth is masked. Many deep bays 
and seas fringing the mountain coasts of the continent are evidently 
down sunken portions of the crust — the Gulf of Mexico, the Carib- 
bean Sea, the Mediterranean, and the Red Sea, for example. It has 
already been shown that the Mediterranean is still subsiding along 
fault planes. 

Horizontal Orogenic Movements. — Not only is there great subsi- 
dence and notable elevation accompanying mountain growth, or 
orogeny; there are also extensive horizontal movements. It was 
long ago pointed out that if the strata involved in the folded Appala- 
chians were stretched out to the horizontal position in which they were 
originally deposited in the sea, they would occupy many miles more 
area than at present. Evidently, therefore, this part of the earth's 
crust has been shortened by a shove from one side, which threw the rock 
layers into folds, as one may fold the leaves of this book by pushing 
at their margin. In the Appalachians the lateral thrust apparently 




538 COLLEGE PHYSIOGRAPHY 

operated from the Atlantic side. In the Alps, likewise, Heim believes 
that a lowland 375 to 750 miles wide has been converted by compres- 
sion and folding into a mountain chain averaging not quite 100 miles 
in width. 

Now recently it has been discovered that rocks are not only folded, 
and folds overturned by lateral thrust, but great thrust faults are de- 
veloped, by which slices of the 
crust are moved bodily over 
nearly horizontal or gently- 
inclined planes for many miles 
(Figs. 350, 351). Many moun- 
tains are traversed by such thrust 
~ ., . , , ,. , , ... faults, involving a series of hori- 

Fig. 350. — Overthrust folding and faulting ' ° . . 

with minor crumpling. (Heim.) zontal movements and pushing 

older rocks over later ones. This 
thrust faulting, clearly shown by the work of Peach and Horn 
in the ancient mountains of the Scottish Highlands, is a common 
feature in mountain regions, as in the Front Range of the Rockies 
in Montana and southern Canada, described by Willis and by 
McConnell, and in the southern Appalachians. It is of fundamental 
importance in mountain history. Some of the extremely complex 
mountain structures of the Alps, originally explained by Heim on the 
basis of double or fan-shaped folds (Fig. 352), are now thought to be 
decken, or rock sheets, determined by horizontal movements of great ex- 
tent by which older strata have slipped out over younger for distances 
of many miles. The brilliant work of Bertrand, Lugeon, and others 
shows this clearly, as is now recognized by Heim (Fig. 352). Follow- 
ing such horizontal movements by (a) thrust faulting or (b) decken 
folding, or (c) movements upward, as well as forward and back, 
during two periods of thrust faulting, or (d) the folding back 
of recumbent older layers on younger, it is possible for erosion to 
form isolated peaks which may be bold in form, because made of resist- 
ant older rocks, and give a striking contrast to the mild topography 
of the weak younger rocks upon which they now rest, as " mountains 
without roots." Chief Mountain, Montana (Fig. 353), and the group 
of peaks of the Mythen northeast of Lake Lucerne, Switzerland, are 
excellent illustrations of such isolated klippen, or drong mountains. 



&. 



Fig. 351. — Thrust faulting in the southern Appalachians. (Keith.) 

Still a third evidence of horizontal movement is in the plan of 
mountain chains, best illustrated in the mountains fringing the 
Asiatic coast. These chains form a series of scallops, or loops, 
bowed outward (Fig. 348) to a greater or less degree, as in the 
Himalayas, the Japanese Islands, and the Aleutian Islands. These 



MOUNTAINS 



539 



loops, to which Suess has called attention, have the appearance of a 
forward gliding of parts of the crust from a polar direction, with 
accompanying folding and faulting forming mountain ranges. The 
complex of mountains of eastern and southern Asia would find explana- 
tion on the assumption of such an outward movement from this great 
land mass. It is difficult to explain on any other basis. This point 
is further considered in later pages (pp. 605-6, 620, 623). 

Flowage of Rocks. — In mountains in which denudation has re- 
moved the upper layers, the rocks are often found to be not merely 
bent, but greatly contorted. It is evident that there has been what 
amounts to flowage of the rocks, though there is no reason for believ- 
ing that they were melted. Adams has reproduced such flowage in the 
laboratory, and there can, therefore, be no doubt that rocks, under 




Fig. 352. — A. The Glarner double fold, as interpreted by Escher and Heim from 1870 
to 1902. B. The Glarner rock sheet, as interpreted by Bertrand in 1883, Suess in 
1892, and Heim in 1903. 

such great pressure as accompanies mountain growth, will flow. In- 
deed, under great pressure cavities are closed and breaking is im- 
possible, so that the rocks yield to pressures by flowage instead of by 
breaking. At the surface they yield by breaking. It is probable that 
faults, visible at the surface and now forming there, are but surface 
expressions of stresses which, deeper in the earth, are forming folds ; 
and probably mountain folds, now revealed by denudation, were over- 
lain by beds near the former surface in which faulting occurred. 
Whether rocks under stress will break or bend depends upon (i) the 
depth and consequently the pressure they are under ; (2) the rate at 
which the stress is applied, a rapidly-applied stress causing breaking, 
whereas the same stress more slowly applied causes bending ; (3) the 
nature of the rocks, some being far more brittle than others. Probably 
also other factors have influence, such as temperature and the amount 
of interstitial water in the rocks. 



Names Applied to Mountains 

Typical Forms. — Mountains are usually a complex of elevations 
and depressions, some of the elevations being elongated, others more 



540 



COLLEGE PHYSIOGRAPHY 



or less conical, like great hills. The latter are commonly called 
mountain peaks, such as Pike's Peak, or just mountain, or mount, 
as Mount St. Elias or Mont Blanc. In the Alps some very sharp 
peaks are called needles (French aiguille), and certain pyramidal peaks 
are known as horns — the Matterhorn, for example. Others more 
dome-shaped are sometimes called domes. The elongated elevations 
are known as ridges, and these are sometimes very long and narrow, and 
often steeper on one side than on the other. Ridges are well developed 
in the Jura and in the Appalachian Mountains. Both ridges and peaks 




F J G. 353- — The overthrust to the east in the Front Range of the Rocky Mountains in 
Glacier National Park, with the Algonkian and Paleozoic (white) on top of the Cre- 
taceous (oblique lines). Chief Mountain is a klippen, or drong mountain. (Willis.) 



are commonly products of denudation, acting upon the inclined 
mountain rocks. There are many valleys of erosion, some broad and 
U-shaped, as where broadened and deepened by glacial erosion, 
others narrow stream gorges. The longitudinal valleys extend 
parallel to the ridges, while the transverse streams cross the ridges 
by water gaps, which are one form of mountain pass (Fig. 371). 
Wind gaps, however, are also passes, and in general any depression 
in mountains across which travel is possible, is a pass. Passes are 
often valleys caused by erosion, but not necessarily so. 

Ranges, Systems, and Cordilleras. — A group of mountain forms, 
usually including numerous peaks or ridges with intervening valleys, 



MOUNTAINS 



54i 



is known as a mountain range. The terms mountain system and 
mountain chain are often employed as synonymous with range, 
though it is more com- 
mon to use these terms 
to mean something more 
extensive than range. 
For example, the Rocky 
Mountains consist of a 
number of ranges, such 
as the Front Range, the 
Big Horn Mountains, 
etc., together making 
the Rocky Mountain 
System. But there is 
much confusion in the 
common usage of these 
terms. A group of 
mountain systems may 
be called a cordillera; 
for example, the Cordil- 
lera of western United 
States, which includes 
the Rocky Mountain 
System, the Basin 
Ranges, the Sierra Ne- 
vada-Cascade System, 
and the Coast Range 
System. 

Ranges, systems, and 
chains are constructional 
forms, due to uplift of 
those portions of the 
crust, or to down sinking 
of the areas on one or 
both sides, or to both of 
these movements com- 
bined. Such change of 
level usually affects lin- 
ear portions of the sur- 
face and, therefore, these 
forms are commonly 
elongated. There are 
often long valleys be- 
tween them, such for ex- 
ample as the Great Valley of California, between the Sierra Nevada 
and the Coast Ranges, and the Swiss Plateau or Alpine Foreland 
between the Alps and the Jura, The plateau and basin area be- 




542 COLLEGE PHYSIOGRAPHY 

tween the Rocky Mountains and the Sierra Nevada-Cascade System 
is another instance of a depression between mountain chains. Very 
often such great depressions are basins of interior drainage, because 
the mountains on one side so cut off the vapour-laden winds that the 
climate is too arid for the streams to find their way across, and even 
for the running water to fill the depressions and transform them to 
lakes. There are areas of such interior drainage in the Andes and 
north of the Himalayas, as well as in western United States. 

Complexity or Mountain Structure 

Contrasts with Plains and Plateaus. — With folding and faulting 
among the primary causes for mountain formation, it follows that the 
strata will be predominantly inclined, and often inclined at a very 
high angle, up to the vertical. This fact alone offers a striking con- 
trast to the structure of plains and plateaus (Fig. 355), where the 
strata are predominantly horizontal, or nearly so. The degree of 



Fig. 355. — Folded rocks on the right, as in the Appalachians, grading into horizontal 
structures on the left, as in the Allegheny Plateau. 

complexity of rock position is further increased in many mountains 
because of the fact of repeated mountain growth, often involving not 
only the strata of the older mountains, but also later strata, deposited 
upon them and, by the regrowth, folded or faulted into the ancient 
mountain mass. 

The mountain movements serve to indurate rock strata, and, in 
places of complex folding, even to alter or metamorphose them, fre- 
quently to the extent of completely destroying their original charac- 
teristics. Metamorphic and sedimentary strata may, therefore, be 
side by side and in all positions and relationships. By the movements, 
too, the rocks are jointed far more than the horizontal strata of plains 
and plateaus, and the extent of the development of joint planes varies 
greatly from place to place, thus introducing another element of vari- 
ation in the nature of the rock and in its power of resistance to denu- 
dation. 

Finally, igneous rocks are often complexly involved in moun- 
tain structure. There frequently are lava flows and ash deposits; 
there are dikes and sills ; and there may be laccolitic intrusions, and 
even huge batholites of coarsely crystalline granitic rock in the moun- 
tain core. These affect the mountain rocks (1) by their own charac- 
teristics, (2) by the disturbance of the strata which their intrusion 
brought about, (3) by metamorphism of the strata near the contact. 
The igneous rocks, like the sedimentary strata, may be subjected to 
folding, faulting, jointing, and metamorphic action during the moun- 
tain movements. 



MOUNTAINS 



543 



Ruggedness due to Erosion of Complex Structures. — For these 
reasons a mountain system is a zone of extraordinary complexity of 
rock structure and attitude, contrasting absolutely with the simplicity 
of conditions in plains and plateaus. Sedimentary, metamorphic, 
and igneous rocks occur in the mountain mass in great variety, with 
marked difference in degree of resistance to denudation, in all atti- 
tudes, and an infinite series of relationships. Naturally, therefore, 
mountains, when acted upon by denudation, assume a degree of 
ruggedness and variety of form quite unknown in simpler land forms. 

Sculpturing of Mountains 

Results of High Altitude. — That the ruggedness for which moun- 
tains are noted is not the result merely of elevation is proved by the 




Fig. 356. — Just above timber line in the Rocky Mountains. 



fact that some plateau areas are higher than mountains which are 
noted for their ruggedness. It is due primarily to the action of 
denudation operating upon rocks varying in kind and in altitude. 
Elevation is, however, a factor in the development of this ruggedness 
because of (1) original height, (2) original differences in elevation, 
and (3) the greater scope for the activities of the agents of denudation 
with elevation. 

Timber Line. — Peaks and ranges are often high enough to limit the 
growth of trees (Fig. 356), so there is a timber line at an altitude where 
the mean annual temperature is only 2 or 3 below the freezing point. 
Gannett has shown that in the United States the altitude of the timber 
line varies from 4000 feet on Mount Washington in New Hampshire 
to 12,000 feet in the Colorado Rockies, and from 5500 feet in the 



544 



COLLEGE PHYSIOGRAPHY 



Cascades of Washington to 11,700 feet in the mountains of southern 
California. In arid regions there is also a lower timber line determined 
by drought. 

High-altitude Weathering. — Mountains are the seat of exceed- 
ingly active denudation. Because of their elevation, the temperature 
is lowered, or the day and night extremes are so great that frost action 

is vigorous, and ex- 
posed rock surfaces 
are broken by it. 
Mountain surfaces 
are often covered 
with broken rock 
fragments, where 
the slope is not too 
steep for them to 
remain ; while, from 
the steeper slopes, 
frost-riven frag- 
ments are fre- 
quently falling. 
The abundant steep 
slopes that develop 
in mountains, es- 
pecially in the 
mountain desert 
above timber line, 
give much oppor- 
tunity for the work 
of weathering, by 
keeping the bare 
rock exposed to the 
weather. The vari- 
able nature of the 
rocks and the abun- 
dant jointing also 
favour the rapid 
work of weathering. 
Probably in no one 
part of the earth's 
surface is weathering more active than in lofty mountains. One 
cannot be among them long without seeing the fall of rock fragments 
from the cliffs, and, now and then, great masses descending as 
avalanches (PL I). 

Stream Erosion. — Lofty mountains are also commonly the seat 
of heavy precipitation, often in the form of rain or of snow, which, 
upon melting, gives rise to large volumes of running water. The 
water entering the rocks aids in weathering by its direct attack 




Fig. 357. — The Royal Gorge of the Arkansas in the Colorado 
Rockies, a stream-cut canyon half a mile deep. 



MOUNTAINS 



545 



through solution or chemical change and by frost action. The 
streams which run off at the surface have high velocity, because of 
the steep slope and large volume, and they are also supplied with abun- 
dance of cutting tools. Consequently, they readily cut their valleys 



Scale o£ Miles 




Fig. 358. — Cirques, arretes, and U-shaped valleys in the mountains of Glacier National 
Park, Montana. (Chief Mountain Quadrangle, U. S. Geol. Survey.) 



in the mountain rock, and, thereby, expose more rock to weathering. 
Gorges and other steep-walled valleys are common phenomena in 
lofty mountains ; and they may be cut to great depth, because the. 
mountain surface lies high above sea level (Fig. 357). 

2 N 



546 COLLEGE PHYSIOGRAPHY 

Glacial Erosion. — The altitude of lofty mountains sometimes re- 
sults in their rising above snow line, which varies from 18 or 20 thou- 
sand feet near the equator to sea level in the polar regions. The 
snowfall serves as a protecting cover to the mountain on which it 
lies, though when it descends in avalanches it tears off rock fragments 
by its friction, and bears them along. Changing to ice, the snow slowly 
moves down the valleys as glaciers, grinding the valley bottoms and 
sides. Glacial sculpturing is, as we have seen, a significant factor in 
the shaping of mountain topography (Pis. V, IX, Fig. 358). In the 
Alps, for example, a considerable share of the valleys and the sharpened 
ridges, or arretes, are due directly or indirectly to ice erosion. 

Wind Work. — Wind work is also very important among high 
mountains. They are exposed to high winds, the winds sweep about 
in eddies, often concentrated into almost hurricane force, and, wherever 
bare rock is exposed, loose fragments, even of the size of small pebbles, 
are driven before them. 

Rapidity of Denudation. — Altogether, the denudation of moun- 
tains is so favoured that it proceeds with comparative rapidity and with 
complex results. Elevation gives opportunity for rapid work, and com- 
plex rock conditions favour the development of varied form. Conse- 
quently high mountains are ordinarily rugged in the extreme, — a 
maze of peaks and valleys of various forms and sizes. These, though 
characteristic of such mountains, are not inherent in the mountains, 
but are developed in the mountain elevation by the processes of 
denudation. 

Forms Sculptured in Mountains 

So complex are the forms of mountain sculpture that a complete 
analysis is here quite out of question. Only a few of the most note- 
worthy types will be considered. 

Ridges. — Where a stratum of resistant rock outcrops, it tends to 
be left behind in the general wearing down of the surface. If the 
stratum has a linear outcrop, as is often the case with folded or faulted 
sedimentary strata, the tendency is for a ridge to develop, with a 
depression or linear valley along the line of the weaker, underlying 
stratum. The height of the ridge will depend upon the extent to which 
the differential denudation lowers the surface ; its width will vary with 
the width of the ridge-making stratum ; the strength of its develop- 
ment will depend upon the degree of resistance of the ridge-making 
stratum ; and its length will depend upon the extent of the outcrop. 

If the strata are vertical, the ridge will have approximately the same 
slope on the two sides, and ridges will occur parallel to one another 
at intervals wherever a resistant layer outcrops. If the strata are 
inclined, as is most common, one side will have an inclination approxi- 
mately that of the dip of the ridge-making stratum, while the other 
side will have a steep slope. This is a monoclinal ridge or hogback. 



MOUNTAINS 



547 



This form varies with the dip, grading down to the horizontal position 
of strata, in which a steep face on one side rises to a table top area, a 
form which characterizes plateau topography. 

As denudation proceeds, the removal of 'the weaker underlying 
stratum, by sapping, in a manner similar to that observed in plateaus, 
causes recession of the cliff, and the ridge migrates in the direction of 
the dip. It also becomes lower at the same time, but will not become 
lower in relation to the surrounding surface if denudation is freely 
at work there; indeed, the ridge may even be etched into greater 
prominence at the same time that it is being lowered and caused to 
recede. Since ridges, being etched into relief, commonly form divides, 
there is a migration of 
divides as the ridge cliff 
recedes. This process 
of recession has been 
called monoclinal shift- 
ing (Fig. 359). 

Ridges are naturally 
most perfectly devel- 
oped among mountains 
of sedimentary strata, 
in which folding or fault- 
ing have been of a fair 
degree of regularity so 
as to permit linear out- 
crops, as in the Jura 
and in the Appalachian 
Mountains. In the latter, for example, there are ridges many miles 
in extent, rising to nearly uniform elevation, and extending in straight 
or curved lines, sometimes zigzagging across the country, almost 
diametrically outlining the position of the resistant beds and their 
variation in dip. 

Peaks. — The resistance to denudation of less regular beds gives 
rise to peaks of infinite variety of form. Sometimes a row of peaks 
is really a ridge, dissected transversely by more effective denudation 
along joint plane areas or because of some Other favourable condition. 
More commonly the rocks are locally resistant, or local erosion has 
removed the rocks round about, giving rise to the peak form. At times 
the variations in rock resistance are in such limited areas that the peaks 
are needle-like, or horn-like ; but, on the other extreme, they may be of 
sufficient area to give rise to dome-like peaks. There is every grada- 
tion in these residual forms of elevation, from the dome to mere 
pinnacles a few feet across at the base and a few feet in height. 

Dome-like peaks are very commonly due to the presence of coarsely- 
crystalline, granite rocks, parts of the batholitic intrusion into the 
core of mountains. Being durable, such rocks resist denudation far 
more than most rocks and especially sedimentary strata. Thus, when 




Fig. 359. — While the surface is being worn down from 
BB to CC, the monoclinal ridge A shifts some distance 
to the right. 



548 



COLLEGE PHYSIOGRAPHY 



these weaker rocks are stripped away, single peaks, or groups of peaks, 
remain standing above the general level to which the weaker rocks 
have been lowered — as in the cases of the Adirondack Mountains of 
New York and the Black Forest of Germany. Hundreds of mountain 
peaks are underlain by granitic rocks — Pike's Peak, Mount Washing- 
ton, Mount Mitchell, and neighbouring peaks are instances in the 
United States, while the Scandinavian upland and the Scottish High- 




Fig. 360. 



Erosion forms similar to the Dolomites. Rocky Mountains of Glacier 
National Park, Montana. 



lands furnish instances in Europe. Similar cases abound in other 
mountain areas of the world. 

In these regions of granitic rock, stratification planes are absent, but 
joint planes serve as guides to the work of denudation. Their in- 
fluence is very clearly seen in the Sierra Nevada, especially in and near 
the Yosemite Valley (PL IX). The granite is crossed by two sets of 
nearly vertical joint planes, irregularly spaced, but crossing each other 
at approximately right angles. There is a third set more nearly hori- 
zontal, but gently curved, so that the rock is traversed by a series of 
concentric planes. As a surface wears down by denudation, these 
joint planes exert a profound influence on the topography. Weather- 
ing, running water, and former glaciers have all been at work modifying 
the mountain form, and all have been guided in their work through the 
weakness introduced by the joint planes. The granite has peeled 



Plate IX 



11U°36' 

S 



; ". \ « b m 



4/ 



Yos'emite Falls 



>» 

c ' 



b m /Yosemite Pt. /w 

Castle £ c 

Cliffs 



% B M V 

i 1 

[ /. 

■d\ Lower 

"£« f Yosemite Falls 




Glacier Pt. A 



Sentinel Rock 



119?3G' .0 



1 Mile 



ILLIAMS ENGRAVING CO., 



YOSEMITE VALLEY 



mountain valley in the Sierra Nevada of California which was deeoened and had its walls over- 
steepened by glacial erosion. Yosemite Falls at lip of hanging valley. Contour interval 50 feet. 
(From map of Yosemite Valley, United States Geological Survey.) 



MOUNTAINS 549 

off along the concentric joint planes, as the layers of an onion may 
be peeled off, and curved outlines and dome-like peaks have resulted. 
Indeed, some of the most prominent topographic forms of the Yosem- 
ite Valley region are called domes. There are also half domes, 
where a dome is bisected by the stripping away of the rock along one 
of the vertical sets of joint planes. There are also great precipices, 
determined by the removal of rock along these vertical planes of weak- 
ness; and there are notches excavated where a number of vertical 
joint planes close together have permitted more rapid denudation. 
The whole topography is determined by the massiveness of the rock 
and the joint plane weakness, worked upon by weathering, stream 
erosion, and glacial erosion. 

In regions of generally horizontal structure of well-jointed sedi- 
mentary rocks the peaks are apt to develop a castellated form. This 
is typical of the eastern limestone Alps, which are known as the Dolo- 
mites. Such castellated forms are also found in the mountains of the 
Glacier National Park in Montana (Fig. 360), and their northward 
continuation in the portions of the Rockies and Selkirks along the 
Canadian Pacific Railway. 

There are a multitude of influences at work determining mountain 
peak form, only a few of which are outlined in the preceding paragraphs. 
They are all denudation forms, but there are various combinations 
of denudation and rock structure and position and, consequently, an 
almost infinite variety of peak form. With the exception of volcanic 
cones, mountain peaks are not of constructional origin, but are a 
phase of the destruction of mountains. Elevation has not caused 
them, excepting in so far as it has given the opportunity for the agents 
of denudation to sculpture the elevated complex. 

Mountain Valley Forms. — Perhaps the most characteristic feature 
of valley form in lofty mountains is the gorge, with its associated 
precipices. This characteristic is due to the fact that the land is high, 
thus giving rise to steep slopes, while water and sediment load are 
abundant. The elevation above baselevel, which furnishes the op- 
portunity for gorge cutting, is due to the recency of the uplift, which, 
as we have seen, is still in progress in many lofty mountains. Such 
mountains are, therefore, young land forms and the streams are 
in the stage of youth and busily at work in the attempt to reach grade. 

From the gorge stage there is every gradation among mountains 
to the broadly open valley with moderate slopes ; but by far the 
greater number of slopes among mountains are steep, because of the 
great mass of elevated land that must be removed by denudation. 
Therefore, even in mountains that have long since ceased to rise, valley 
sides are steep ; and in young mountains they are prevailingly steep 
and even precipitous. 

Glacial erosion has sculptured valleys in the high mountains of all 
parts of the earth, and has given rise to a series of topographic forms 
so characteristic that they are easily recognized. In the higher parts 



550 COLLEGE PHYSIOGRAPHY 

amphitheatre-like valleys or cirques have been excavated, bordered 
by very steep walls ; and, by the recession of cirque heads (Fig. 358), 
sharp arretes have been developed. Farther down U-shaped valleys 
have been excavated, bordered by steep walls, frequently precipitous 
on both sides, but sometimes steeper on one side, against which the 
glacier was cutting most effectively. Such valleys are often straight 
and canal-like, where powerful glacial abrasion has smoothed off all 
the valley side irregularities, and even worn off the projecting spurs 
that are normal to stream-made valleys. The steepened slope of 
glacial erosion origin grades upward into the more irregular and, 
often, less steep slopes of the upper valley walls, where glacier scouring 
did not reach. 

The floors of main glaciated valleys often have giant steps which 
are evidently due to differential glacial erosion. Where two glaciated 
valleys of about the same size come together there may be a step up to 
the mouth of each one. This is called a confluence step. 

Tributary valleys to these troughs of glacial erosion commonly 
enter at levels well above the trough bottom. From these hanging 
valleys the water descends either through narrow gorges, with torren- 
tial velocity, as in so many cases in the Alps, or by direct fall down the 
steepened slope, as in the Yosemite Valley, in parts of the Alps, and 
in all other mountain regions of former vigorous glacial action. Along 
the flattish floor of the glaciated troughs exist many lakes, some of 
them behind barriers caused by glacial deposit, others in depressions 
locally scoured out by glacial erosion, even in rock basin depressions. 
The Italian Lakes On the south side of the Alps are instances of such 
lakes in depressions deepened by glacial erosion, though dammed also 
by glacial deposit ; and in the Alps there are many small lakes which 
are true rock basins. 

Mountain Passes. — While resistant rocks are left as ridges and 
peaks, weaker rocks are worn down to form depressions. If the weak 
rock has considerable linear exposure, the resulting depression is a 
linear valley ; if it is more localized, the depression is more restricted. 
A great number of depressions in mountains, which, because of their 
lowness, offer a route, or pass, across them, are due to the local lowering 
of the surface where weaker rock occurs. It may be a thin-bedded 
stratum in the midst of more massive and more durable strata that 
gives rise to a pass ; or a weak igneous rock, crossing massive granitic 
rock; or an area of abundant joint planes or of crushing; or some 
other structural weakness. 

There are other causes for passes, often effective because of local 
weakness of the mountain rock (Fig. 361) . For example, a glacier, flow- 
ing over a low portion of a mountain ridge, may so lower the moun- 
tain along its course as to leave a pass on the disappearance of the ice. 
A river crossing a mountain ridge or range, perhaps along a zone of 
weakness, forms a valley which may serve as a pass. Again, a moun- 
tain river, gnawing at its headwater, may push the divide back and 



MOUNTAINS 



55i 







552 COLLEGE PHYSIOGRAPHY 

gradually encroach upon the valley of a stream flowing on the opposite 
side of the divide. By such headwater erosion the upper tributaries 
of the opposing stream may even be captured, and the divide pushed 
back to the opposite side of the ridge or range, thus forming a notable 
gap in the mountain which becomes a good pass. The Maloja Pass 
in Switzerland has been explained in this way, the stream on the 
southern or Mediterranean side, because of its steeper, more direct 
course, having eaten its way back and captured headwaters of the 
Inn River, so that a very low, flat-bottomed pass exists, with a steep 
slope on the Mediterranean side. In many mountains the pushing 
back of headwaters, especially along zones of weak strata, has gone 
so far that the stream source is pushed across the mountain and its 
valley has become a pass. There are also passes which follow the 
valleys of antecedent streams across mountains. 

Mountain Deposits. — The waste from the wearing down of moun- 
tains is mainly distributed far and wide by the streams that radiate 
from them ; out of this waste are built intermont plains and other 
deposits, and extensive deposits in lakes and ocean. Some of it, 
however, comes temporarily to rest within the mountains, giving 
rise to characteristic local topographic features. At the cliff base 
are extensive talus deposits, some steep and bare of vegetation, others 
more gently sloping and forest-covered. The talus slope is one of 
the significant features in mountain landscape ; it is a curve or slope 
of deposit, often contrasting strikingly with angular outlines above 
where sculpturing is in progress. 

Avalanche Deposits. — Here and there are avalanche deposits, 
great streams of rock fragments, sometimes hummocky in topography. 
When freshly fallen they are barren belts, forming great blotches 
in the landscape, perhaps in the midst of fields or forests through 
which they plunged in their destructive downward course. In such 
cases, too, there is, on the mountain face, the fresh scar, caused by 
the avalanche downfall. In time the avalanche becomes clothed 
with vegetation, and the mountain side scar is partly obscured by 
new growth of vegetation. Mountain sides reveal many such ava- 
lanche scars in various stages of healing and at the base of the moun- 
tain slopes are to be seen the rock streams and avalanches that de- 
scended from them. 

Alluvial Fans. — Alluvial fans, often with steep grade, are com- 
mon in the broader mountain valleys, where the mountain streams 
emerge with high velocity and abundant sediment load, some of which 
must be dropped on the gentler slopes of the main valley. Such allu- 
vial fans exist by the thousands in the Alps and in other mountains ; 
and their graded slopes are often the sites of villages. 

Glacial Deposits. — Where the mountain valleys have been occu- 
pied by glaciers, their sides and bottoms may be veneered with ground 
moraine, and dotted with boulders of rock varieties common higher 
up the valleys. Lateral moraines may fringe the valley wall, and 



MOUNTAINS 553 

terminal moraines with hummocky topography may sweep across 
the valley in crescentic curve. Outwash gravel plains may occupy 
the valley bottom, raising and levelling its surface, and perhaps 'carved 
into terraces by stream erosion, subsequent to the time of deposit 
when the glacial streams were flowing. 

Glacial deposits, and sometimes avalanches, have caused obstruc- 
tion to mountain drainage and, thereby, given rise to lakes, some 
merely pools, others of good size. Such lakes, as well as those in 
rock basins, become the seat of deposit of sediment transported by 
the mountain streams. In a region of such active denudation, lakes 
are commonly filled with rapidity, as such work goes. Thus it is 
that there are many flat-surfaced meadow areas where lakes once 
existed, and many others where lakes are partly rilled. Extensive 
fan-shaped deltas are built out into the sides of the deeper ones, while, 
at their heads, the inlet streams are commonly bordered by extensive 
delta flats and marshy lands, contrasting strikingly with the rugged 
topography of the encircling mountain walls. Frequently these 
delta areas are the only level land in the neighbourhood, and upon them 
the villages of the region have grown. 

Cycle of Mountain Development 

Sculpturing during Uplift. — During the period of active growth, 
mountains continue to rise differentially, and faster than denudation 
can lower them. They become steadily higher, though there seems 
to be a limit beyond which they cannot be reared. During such 
growth some parts are raised higher than others by folding or faulting, 
and probably neighbouring parts are lowered. Earthquakes are de- 
veloped during the movements and volcanic outbursts may occur 
in association with the uplift. The result is the formation of a 
range or a system of individual ranges and ridges, with valleys 
between. Associated with the localized folding or faulting may be 
broad uplift without notable disturbance of the strata, giving rise 
to a plateau, and the plateau uplift may even be the grand feature 
of the uplift, while the mountain range is but a local disturbance 
in it. 

If there were no denudation on the earth, the mountain form thus 
produced would be notably irregular as the direct result of differential 
folding and faulting ; with denudation its irregularity is greatly in- 
creased, though its elevation is diminished. Since mountain growth 
is slow, while the activity of denudation is increased by the elevation, 
the result of denudation is to greatly sculpture the mountain form, 
even during the period of its uplift. Thus it is that lofty mountains 
are so rugged, — a combination of original elevation and irregularity 
with the sculpturing by denudation superimposed upon it. All 
lofty rugged mountains are in the stage of youth, and most, if not all, 
of them are still growing. 



554 COLLEGE PHYSIOGRAPHY 

Young and Mature Mountains. — Such mountains are character- 
ized by peaks and rugged ridges; and by precipices and gorge-like 
valleys. They are the seats of active denudation, and, if growth 
ceases, the denudation operates here, as on all other land forms, (a) 
to reduce the valley bottoms to grade ; (b) to broaden the valleys and 
lessen the slopes of the valley walls ; and (c) to remove the interstream 
areas. Thus the mountain elevation diminishes, its ridges and peaks 
are lowered, and its valleys are broadened. It passes through the 
cycle of maturity and into old age. The mountains of Scotland, of 
Scandinavia, the Black Forest and the Vosges, the mountains of New 
England and the Adirondacks, are all in the stage of topographic 
maturity. They are not low, with moderate slope, because made that 




Fig. 362. — Geological cross-section of the peneplain in New Hampshire near Mt. Monad- 
nock, showing the indifference of topography to structure during old age in the mountain 
cycle. (Hitchcock.) 

way by uplift, but because reduced to that condition by long-con- 
tinued denudation. 

Mountains in Old Age. — Further lowering may continue, theoret- 
ically, until the former mountain is reduced to the level condition of 
a plain, in which the influence of the underlying complexity of rock 
structure no longer exerts appreciable influence upon the topography. 
There are no known instances of this extreme, which has probably 
rarely, if ever, been attained over any considerable area of the earth. 
But an approach to this stage of extreme old age has been reached by 
numerous mountain regions during their past history. Such an old 
mountain, worn down to such low relief as to approach the condition 
of a plain, is a peneplain. On the peneplain, the surface swings up 
and down, with the sites of the more resistant layers still marked 
by low swells, or by much reduced peaks. Such reduced peaks, or 
hills, rising above the peneplain level, have been called monadnocks, 
after Mount Monadnock, which is interpreted as such a residual, 
rising above the ancient New England peneplain (Fig. 362 and 
Pl.X). 

Illustrations of Peneplains. — Old-age mountains, in the peneplain 
stage, are well represented in the United States in the Lake Superior 
region (Figs. 363, 364) , the Piedmont Plateau, and the low, hilly country 



MOUNTAINS 



555 



extending northeastward past Washington, Philadelphia, New York, 
and Boston. From the structural features of the rocks along this belt 
the inference is warranted that at an earlier geological period there 





Fig. 363. — The peneplain of the Lake Superior region, with Jasper Peak, a monadnock. 

arose here a truly lofty mountain range, now so reduced that it is the 
seat of an abundant population, and of some of our largest cities. 
Other peneplains now uplifted and dissected have been described from 
numerous regions. One of the best instances of these is in the Rhine 




Fig. 364. — Map of the peneplain of the Lake Superior region, with monadnocks rising 
above the general level, and monoclinal ridges and mesas carved in the slightly dissected 
upland. 

valley of Germany, where the strata are in the characteristic position 
and condition of mountain rocks, but the upland surface is a moder- 
ately undulating plateau, with some elevations of greater height 



556 COLLEGE. PHYSIOGRAPHY 

where the strata were more resistant. Recent uplift has permitted 
the Rhine and other rivers to sink their valleys into the peneplain 
and it is now beginning to be redissected, but extensive tracts of the 
ancient peneplain remain. The more mountainous parts of New 
England, and in fact the entire Appalachian belt, are interpreted as 
an uplifted and much more dissected peneplain, and the same inter- 
pretation, with greater or less degree of probability, has been given to 
many mountain areas. 

Revived Mountains 

The Effect of Uplift. — If a mountain is greatly reduced, even 
though to the state of a peneplain, subsequent uplift without accom- 
panying folding and faulting gives opportunity for the development 
of mountainous topography. Ridges may be etched out again, peaks 
may develop, and, if the uplift is great enough, a topography of such 
great ruggedness may be carved that it is difficult or even impossible 
to distinguish the revived or rejuvenated mountain from a young 
mountain of recent uplift. Some inherited features may remain, 
such, for example, as remnants of the former peneplain surface, or 
rock terraces marking the sites of former valley bottoms, or incised 
meanders, developed when the streams swung in curving course over 
the peneplain surface. The forms developed in the revived moun- 
tain will vary in nature and in intensity, according to (i) the amount 
of the uplift, (2) the nature of the rocks, (3) the length of time that 
denudation has been at work in sculpturing the area. 

Illustration from Germany. — The uplifted peneplain of central 
Germany may be taken as a typical instance of an early stage in re- 
vival of a mountain region by uplift. In it the dissection has gone 
far enough for the development of gorges along the major streams, and 
some sculpturing of the complex mountain rocks near them. But 
between the streams is a broad, swinging upland surface, level enough 
for farming, and the broad bottoms and general slopes of the old 
valleys of the peneplain stage are still traceable, while the streams 
now sunk in the peneplain are flowing in meandering course, giving 
perfect illustration of incised meanders. 

The Second-Cycle Appalachians. — The Appalachian mountain 
system will serve as a typical instance of a revived mountain range 
of more mature dissection. These mountains were upraised in early 
geological ages, forming a very lofty range, and the strata were folded, 
crumpled, and faulted in intricate manner, while batholitic intrusions 
rose into the mountain core, and volcanic rocks poured out at the sur- 
face. There were subsequent uplifts also, but the last one, at the 
close of the Carboniferous Period, involved not only the ancient moun- 
tains, but extended westward and raised in a series of folds with some 
faulting, a great thickness of sedimentary strata. Thus the Appa- 
lachian system includes two quite opposite types of mountain struc- 



MOUNTAINS 



557 



ture, one, in the west, with moderately folded sedimentary strata, 
the other, in the east, a complex of sedimentary, metamorphic, and 
igneous rocks. 

Both these areas were worn to a condition of slight relief, and it 
was from a study of a part of this region that Davis conceived the 
idea of the peneplain. Subsequent to the peneplain stage there has 
been uplift, with some warping, and probably with some folding and 
faulting, so that the upraised surface is higher in some parts than in 
others. Thus, the Piedmont Plateau is still low and fairly even, 
while the neighbouring Blue Ridge rises to mountainous height. 

Denudation has cut into the uplifted peneplain, but there is still 
a fair degree of uniformity of the crests, which are interpreted as rem- 




FlG. 365. — 'At the close of the first cycle the ridges were worn so low that the streams 
crossed them, but after an uplift (right-hand diagram.) the ridges were etched out into 
strong relief in the second cycle, the streams still maintaining their courses across 
them. Convergence of ridges due to inclination, or pitch, of axes of folds. 



nants of the peneplain. The crystalline rocks have been greatly 
sculptured, and there is a maze of peaks and intervening valleys, but 
the stage of reduction in the main cycle has gone so far that the slopes 
are reduced to the condition of early maturity. The mountains are, 
therefore, not notably rugged. 

In the western folded Appalachians, the evidence of the former 
peneplain condition is even more clear. The ridges etched into relief 
rise to a remarkably uniform elevation, in some cases extending for 
miles in straight course with little break in the evenness of the skyline. 
Between the ridges are linear valleys of considerable breadth along 
the belts of weaker strata, and, in some of these, the streams have the. 
meandering course of entrenched meanders, a condition which could 
only be inherited from a former state of floodplained valley bottom 
such as would exist on a peneplain surface. 

Many of the streams, even small ones, cross the ridges, flowing in 
narrow gorge-like valleys, known as watergaps. These valleys have 
been sunk in the ridges by the downcutting of the streams, and time 
enough has not elapsed for the valley walls to broaden out, as has 



55S COLLEGE PHYSIOGRAPHY 

been the case in the weaker strata. Some of these courses across 
ridges, notably of the smaller streams, are apparently inherited from 
the earlier peneplain stage when the ridges stood out in less relief, 
and the streams flowed over resistant rock layers, later discovered 
by denudation and etched out into relief, while the superimposed 
streams continued their course across them, cutting the gorge-like 
gaps (Fig. 365). 

The even-crested Appalachian ridges, etched into relief in the second 
cycle, are determined by resistant rock layers in pitching folds. The 
pitch of these folds, therefore, carries some of the pairs of ridges, de- 
termined by the outcrop of the same resistant stratum on the two sides 
of a syncline or anticline, below the present baselevel, so that the ridges 
die out. This is where the ridges are diverging. Where they con- 
verge, as in a pitching synclinal fold, two ridges will come together, 
forming a canoe-shaped valley (Fig. 365). The zigzag character of 
the Appalachian ridges is, therefore, due to the fact that erosion is. 
etching into a series of pitching folds. 

Rocky Mountains. — Even the Rocky Mountains of Colorado, 
and possibly other parts as well, are revived mountains, though in 
this case probably accompanied by a greater measure of differential 
uplift than in the cases already described. Portions of the old, re- 
duced surface still remain as upland plateaus, separated by broad 
valleys or basins of downfolding, in the Parks ; but the streams have 
sunk deep, canyon-like gorges, such as the Royal Gorge of the Arkan- 
sas (Fig. 357) ; weathering has roughened some of the upland; and 
glacial erosion has introduced its characteristic elements of sculptur- 
ing, notably the cirques and the U-shaped valleys. The same condi- 
tion is found in the Cascades and many other mountains now in their 
second cycle. 

Mountain River Valleys 

Complexity of Mountain Drainage. — If a river flowing upon a 
surface which is raised into mountains persists in its course, it is an 
antecedent stream (p. 191) ; or, if a stream in its downcutting dis- 
covers a mountain structure, as at Grand Canyon, it is a superim- 
posed stream (p. 184). Otherwise, mountain drainage is developed as a 
result of mountain form or structure and has characteristics dependent 
upon these conditions. Naturally, in a region of such complex struc- 
ture and such irregularity of form the drainage characteristics are 
varied and complex. Only part of the elements involved, and a few 
of the characteristic results can be considered here. 

Radial Drainage. — From a dome-shaped mountain, such as a 
laccolite, the consequent drainage is radial, as on a volcano. Radial 
drainage of subsequent origin also develops in connection with the 
sculpturing of peaks. This fact tends to preserve the peak form, for, 
entirely aside from its extra resistance to denudation, since it is a 
watershed, stream erosion is reduced to a minimum. 



MOUNTAINS 



559 




WILLIAMS ENGJtAVINS CO.. 



Fig. 366. — Topographic maps to show types of drainage in the Appalachian Highland. 
Upper map shows longitudinal and transverse valleys and trellis drainage, while lower 
map shows insequent dendritic drainage. (Monterey, Va., and Charleston, W. Va., 
Quadrangles, U. S. Geol. Survey.) 



560 



COLLEGE PHYSIOGRAPHY 



Longitudinal and Transverse Drainage in Mountains. — In linear 
mountain forms the drainage normally finds its way by short courses 
down the slopes into and along linear valleys occupied by longitudinal 
streams ; while here and there escape is found across a low portion 
of a ridge or around the end of a ridge to another valley. Such trans- 
verse streams usually form a small proportion of the total drainage, 
though they are often conspicuous because of the deep gorges they 
cut across the ridges. Consequent drainage of this type is well de- 
veloped in the Jura Mountains of Switzerland and France. It con- 
sists (i) of numerous short streams of steep grade from the valley 
sides, (2) longer, roughly parallel streams in the valleys between the 
ridges, and (3) occasional transverse streams in gorges (Fig. 366). 

Subsequent stream courses of similar habit also develop during 
the denudation of mountains, as is typically illustrated in the western 



TACO n ic 

Berlin- Stephentotun . 
jBoonic R. 1 Valley 

Srg- Sb., 



RANGE 



MT. GREYLOCK 




Fig. 367. — The synclinal, schist mountains and anticlinal, limestone valleys of the Appa- 
lachians of western Massachusetts. (Dale.) 



folded Appalachians. Here the ridges etched out in the resistant 
rocks are left as divides, from which short streams descend to longi- 
tudinal valleys etched out in the weaker strata. These short streams 
have a steeper course on the outcrop side of the ridge-making rock 
and 'a longer slope on the opposite or dip side, the length of the slope 
depending, in large measure, on the inclination of the strata. A 
similar condition of drainage, of consequent type, occurs on fault 
block mountains, where the steep slope is on the side of the fault, 
and the greater slope on the side toward which the block inclines. 

Adjustment to Rock Structure in Mountain Drainage. — Whatever 
may be the original consequent course of drainage in a mountain region, 
there is, as denudation lowers the mountain, a constant tendency for the 
streams to adjust themselves to the rock structure discovered, and, 
therefore, to develop subsequent courses, as already explained 
(pp. 184-185). There is every reason to believe that, if one could ob- 
serve the succession of events during the reduction of a mountain 
mass, one of the most striking features would be the steady shifting 
of drainage lines as variations in the rock structure are being revealed. 
The extent to which such changes may occur can best be understood 
by considering two cases, both finding illustration in the folded 
Appalachian Mountains, which have gone through one cycle of denu- 
dation and are now well along on the second. 



MOUNTAINS 



S6i 



Synclinal Mountains. — In the Appalachians, and in other parts 
of the world, there are mountains whose summits are synclines, and 
valleys whose structural features are anticlines. This is a complete 
reversal of the normal condition as typified in the Jura, where the 
anticlines are the ridges, and the synclines the valleys. Such reversal 
is apparently due to the operation of the law of monoclinal shifting 




Fig. 368. — Block diagrams to show the evolution of synclinal mountains (B, D). 



(p. 547) ; that is, that a divide located upon an inclined stratum will 
migrate in the direction of the dip as the surface is lowered. 

This may be illustrated in its application to the formation of syn- 
clinal mountains, by considering a simple, theoretical case (Fig. 368). 
Assume a syncline bordered by two anticlines, the syncline being a val- 
ley, the anticlines ridges. Assume, further, resistant surface layers on 
both syncline and anticlines and weaker layers beneath. If for any 
reason, such as fissuring along the tops of the anticlines or because 



562 



COLLEGE PHYSIOGRAPHY 



denudation operated more rapidly there, on account of superior ele- 
vation or any other cause, a longitudinal stream should begin to cut 
into the top of the anticline, the underlying weak layers would be 
exposed to denudation on the anticline crest before they were in the 
syncline depression. When the resistant upper layers were thus 
breached, the anticline crest would be transformed to two ridges, 
each inclining away from the crest. Between them would be a valley 
whose bottom was in the weaker layers and whose sides were the steep, 
outcrop faces of the resistant, ridge-forming layers. As the surface 
lowered the ridge crests would migrate in the direction of the dip, 
that is, toward the syncline valley ; and this lowering would be accel- 
erated by sapping as the weaker, loose layers were removed. Thus 
the syncline valley is made to steadily lose drainage area, while the 
anticlinal crest valley makes a corresponding gain. With a continua- 




Fig. 369. — Stream diversion and the migration of a divide because the river pirate has a 
short course to the sea. 

tion of the process the synclinal valley is narrowed, and its stream 
loses power, while the anticlinal valley broadens, its stream gains 
more volume and power, and it is cut deeper. The ultimate result 
may be, what is commonly observed in nature, a synclinal mountain 
and an anticlinal valley, — water flowing along the line of former 
elevation, and a mountain extending where in the earlier stages water 
flowed in a linear valley. The beginning of such anticlinal valleys 
is to be seen in the Jura, where valleys are opening along the crests 
of arched folds. Fully developed synclinal mountains are found in 
the Appalachians (Fig. 367). 

Migration of Divides in Mountains. — A second feature illustrated 
in the Appalachian mountains is that of migration of drainage by 
headwater erosion. There can be little doubt but that, when the 
Appalachian Mountains were elevated, the main drainage extended 
from some medial portion of the mountains, some eastward, some 
westward. The exact site of this ancient divide cannot now be deter- 
mined ; but it seems quite improbable that it lay even approximately 
along the line of the present divide, for some of the streams head 



MOUNTAINS 



563 



far back in the mountains, and others even in the plateau on the west- 
ern side ; such, for instance, is the case with the Susquehanna River. 
Proof of exactly what happened to establish the present divide is 
lacking. Doubtless the process was both a long and a complex one ; 
but it is exceedingly probable that headwater erosion was an impor- 
tant element in the migration of the divide. 

Again, to illustrate the process, we may revert to an hypothetical 
case (Fig. 369). Assume that streams flowing eastward descended by 
short courses to the ocean, while those flowing westward reached the 
ocean only after passing over long, roundabout courses. The former 
evidently would have a great advantage and might be expected to push 
their divides westward, robbing the weaker streams on that side. A 
similar result would be brought about if there were heavier rainfall 




Fig. 370. 



Block diagrams to show the origin of the barbed tributaries of certain streams 
in the Catskills. 



on the eastern side, thus giving those streams greater power to eat 
back at the headwaters. 

Diversion of Mountain Streams. — The robbing of streams by head- 
water erosion is not purely a matter of theory, for numerous instances 
are now known. The case at the Maloja Pass in Switzerland, where 
the streams descending by short course to the Mediterranean have 
robbed the Inn of headwaters that formerly pursued a long round- 
about course to the Danube, has already been mentioned (p. 552). 
Another instance of such a river pirate is found in the eastern slope 
of the Catskill Mountains, where they rise above the Hudson valley. 
Two small streams, the Kaaters Kill and the Plaaters Kill, descend- 
ing this steep slope, have evidently robbed an opposing stream of its 
headwaters because it could not compete with them for drainage 
which it had been carrying many miles to baselevel. As a result, 
the upper course of these small streams receives tributaries pointing 
westward toward the direction in which they formerly flowed instead 
of eastward as the lower tributaries do. The tributaries enter in 
barbed fashion (Fig. 370). 



564 



COLLEGE PHYSIOGRAPHY 




Contour Interval 33>^ feet. 
Datum is mean Sea Level. 



Fig. 371. — Topographic map showing water gaps of the Susquehanna River in resistant 
sedimentary rock of Blue Mountain, Second Mountain, and Peters Mountain, with 
broader valleys between in weak rock. Incised meanders in southwest corner of map. 
(Harrisburg Quadrangle, U. S. Geol. Survey.) 



MOUNTAINS 



56S 




566 



COLLEGE PHYSIOGRAPHY 



River Piracy in Mountains. — The robbery of drainage by an op- 
posing stream or river piracy seems to be common among mountain 
regions. There are other causes for it besides those mentioned above ; 
in fact, anything that accelerates headwater erosion on one side of a 
divide, as compared with that on the other side, gives opportunity for 
the pushing back of the divide and the possible capture of headwaters, 
or even of good-sized streams, by the successful river pirate. Mono- 
clinal shifting of divides is such a cause, for since the divide migrates 
in the direction of the dip, the stream on the dip side is open to success- 
ful robbery. The presence of a resistant stratum across a stream 
is another source of weakness in the contest for headwaters. Earth 
movements also may weaken one stream or strengthen its opponent ; 
and the diversion of streams to other systems may be brought about 




Fig. 373. — Block diagrams to show the origin of a wind gap, as at Snicker's Gap in the 

Blue Ridge. 



or assisted by glacial action, by volcanic deposits, and even by ava- 
lanche deposits. 

There are thus ample reasons for the migration of divides ; indeed 
the divide that remains in one position as the surface wears down must 
be the exception, for this assumes a delicate balance of conditions on 
the two sides. With migration of the divides comes opportunity for 
river piracy, and it is conceivable that, under favouring conditions, 
divides may slowly march onward, crossing ridge after ridge even to 
the opposite side of the mountains, as seems to have been the case 
in the Appalachian Mountains. 

Water Gaps in the Appalachians. — In the Appalachians there are 
numerous gorge-like valleys where the streams cross the ridges. These 
water gaps are typically illustrated by the Delaware Water Gap (Fig. 
372) or the gap at Harper's Ferry. They may be due to more than a 
single cause, but the majority are apparently due to the fact that the 
rivers flowed across the ridges at that earlier stage when the surface was 



MOUNTAINS 567 

a peneplain and since the uplift which gave the streams opportunity 
to cut into the ridge, there has not been time for the valleys in the 
resistant strata to broaden. This illustrates a phenomenon very 
common among mountains, for valleys frequently broaden and nar- 
row as they extend from weak to resistant rocks. The same val- 
ley may have the gorge form of youth in the resistant rocks and 
be broadened out to the form of maturity where the strata are weak. 
This is true of the Connecticut River, and Appalachian longitudi- 
nal valleys and transverse gaps illustrate this contrast very clearly 
(Fig. 371). 

The resistance to river erosion by the resistant, ridge-forming strata 
of the Appalachian Mountains has been so great that streams flowing 
across them have in some instances been captured by more favourably 
located longitudinal streams. After their capture the gaps through 
which they flowed still persist, though, by the deepening of the neigh- 
bouring longitudinal valleys, as in the Shenandoah valley, often left high 
above the bottom of these valleys as wind gaps ; like Snicker's Gap 
(Fig. 373), and many others in the Appalachians. 

Relation of Mountains to Man 

Mountains are, generally speaking, regions of sparse settlement; 
yet in various respects they have and have had very important in- 
fluence on the human race. The nature of this influence will be con- 
sidered under the following headings. 

Unfavourableness to Agriculture. — Old mountains may have so 
level a surface and such a deep cover of disintegrated rock that, like 
other level lands, they are the seat of extensive agriculture. This 
is the case in the Piedmont Plateau, a region noted for its cotton and 
tobacco culture. Young, much-dissected mountains, on the other 
extreme, are so irregular, there is so much steep slope, and there is 
so little soil, that agriculture is necessarily limited; and a part of 
the surface may rise above the zone in which crops can be raised. 
Here agriculture is confined to the broader valleys, chiefly the longi- 
tudinal valleys, and to such slopes as are suited to pasturage (Fig. 
374). In mountains between these two extremes are intermediate 
conditions of agricultural industry; but in none excepting very old 
mountains is it prominent. 

In both the United States and Europe the mountainous sections 
are predominantly sparsely settled, even where surrounded by dense 
settlement ; and this is primarily because of the absence of the agri- 
cultural basis for settlement. This is true, for example, of so reduced 
a mountain area as the Highlands of Scotland, and the even more 
reduced mountain area of New England. Local areas, especially 
the broad valley bottoms, are given over to farming and the slopes 
to pasturage, but these areas form only a small percentage of the 
whole, and give basis for only a limited population. 



568 COLLEGE PHYSIOGRAPHY 

Importance of Lumbering. — In the United States the name Black 
Hills was given because of the dark colour of the low, forest-covered 
mountains in the midst of the brown plains ; and the name Green 
Mountains is derived from the colour of the forest verdure. The 
Schwartzwald, or Black Forest, of Germany, gives another indication 
of the fact that mountain slopes are commonly forest covered. In the 
Jura, for example, while there is settlement and farming in the longi- 
tudinal valleys, and the more gentle slopes are cleared for pasture, 
the steeper slopes are prevailingly left in forest. 

In some parts of the world the mountain forests have been almost 
completely cleared away, as in Italy; in others the climate is too 




Fig. 374. — High summer pasture on the slopes of the Jungfrau in Switzerland, above 
timber line and close to the glaciers. 

arid for forest growth, as in some of the low Basin Ranges of western 
United States ; and in lofty mountains there are slopes too steep for 
forest growth, and slopes that rise above the timber line. But, in 
general, forests clothe extensive areas in mountain ranges. While 
in places the forest occurs here because the mountains rise high enough 
to cause sufficient rainfall, in general the presence of forests is not due 
to any especially favourable condition in the mountains, but rather to 
the fact that the forest is left here (1) because the land does not pay 
to clear, (2) because of the difficulties in the way of removing the 
forest products to regions where needed. 

That the first of these conditions is important is indicated by the 
fact that the Germans, the most scientific foresters of the world, 
artificially maintain the forest on the low mountain slopes, where 
the forest products are found more valuable than agriculture. The 



MOUNTAINS 569 

importance of the second condition is illustrated in newer lands, like 
the United States. For a long time the mountains of New England, 
the Appalachian Mountains, and the old mountain region around the 
western end of Lake Superior were our chief sources of forest products. 
These sources have now been partly exhausted, though forests still 
exist in the more remote and more rugged portions. Now the north- 
ern Rockies and the Cascade Ranges are being exploited. 

The mountains as a natural forest reserve have been of high im- 
portance to the United' States ; and, since they can be continued as 
a reserve on land that is of less value for other purposes, it is of high 
importance that the lessons taught by the Germans are now being 
applied in this country. To maintain and protect the forest means 
not merely a continuation of the supply of forest products, but also 
regulation of the streams that drain the mountains and, thereby, a 
diminution of floods, and a checking of denudation in the mountains, 
by which weathered rock fragments are removed from slopes, where 
they are needed, to be deposited in lower areas, perhaps where not 
wanted. 

Importance of Mining. — The name of one of the low, much- 
worn German mountains is the Erzgebirge, a word meaning ore moun- 
tains. The same relationship is indicated by the German word 
Bergwerk, or mountain work, which means mining, clearly indicating 
how closely mining in that country is related to mountains. Through- 
out the world there is a close relation between mountains and some 
sorts of mineral wealth. Some kinds of valuable mineral may be 
found away from mountains, such as coal, clays, and certain building 
stones ; but with few exceptions, the great mining regions of the world 
are in mountain areas, the chief exception being certain of the 
deposits of coal. Sometimes the association with mountains is with 
young mountain areas, but more commonly, with older, much-worn 
mountains, or with mountains that having passed through one cycle 
of denudation have started upon another. 

The United States, equally with Germany, shows the relationship 
of mining to mountains. Iron, coal, petroleum, granites, and lesser 
quantities of other mineral products are found in the Appalachian 
Mountains and the Adirondacks. The worn-down mountains about 
the western end of Lake Superior are the seat of the most important 
iron mining industry in the world, and one of the leading copper de- 
posits. In the western mountains, which rank high among the great 
mining regions of the world, are produced the bulk of our gold, silver, 
and copper, besides lead, zinc, and other metals. This mountain mining 
belt extends northward through Alaska and southward far into Mexico. 

Similar relationship is seen in other parts of the world, and one of 
the chief reasons for settlement among mountains, and the chief rea- 
son for towns and cities is the mining industry and the related manu- 
facturing in reducing the ores. Were it not for mining, for example, 
most of the mountain towns of western United States would not exist 



57° 



COLLEGE PHYSIOGRAPHY 



and the population would become greatly reduced. The proof of 
this statement has been given on numerous occasions when the min- 
eral wealth has given out and populous towns have become nearly or 
quite deserted. 

The reasons for the relation of ores to mountains are as follows : 
(i) lava intrusions or outflows bring to the surface minute quantities 
of metal, (2) the lava furnishes the heat necessary for water to extract, 
transport, and deposit these metals in concentrated form in veins, (3) 
the mountain movements furnish the necessary crevices in which the 
vein deposit can be made. In some cases not all three of these causes 
are needed, as in certain iron deposits; and, in some cases, metallif- 
erous deposits can be made away from these favouring conditions, 
as in the lead and zinc regions of southwestern Wisconsin, southern 
Missouri, and northern Arkansas. But these are exceptions whose 
explanation is not difficult, and they do not affect the truth of the 
general statement. The presence of such mineral deposits in worn- 
down mountains is due to the fact that denudation has worn the sur- 
face down to the zone in which the veins have been formed. Doubt- 
less similar veins lie unrevealed in some mountains, while others that 
once existed have been worn away as the mountains were lowered. 

Limited Manufacturing. — Because of the limited raw products 
and the sparseness of population, manufacturing in mountain regions 
is ordinarily not of great importance. Such manufacturing as exists 
is mainly connected with the leading raw products — those of the 
forests and the mines. Smelters, stamp mills, and lumber mills are 
commonly found in mountain regions. Occasionally, too, as in the 
Appalachians, coal occurs, which gives the fuel basis for other forms 
of manufacturing. 

In the manufacturing, water power from the mountain streams is 
often of basal importance. Frequently, therefore, a mountain base 
is fringed with manufacturing towns, as, for instance, in the Adiron- 
dack^, in England along the slopes of the Pennine Chain, and along 
the base of the Alps in Switzerland and in northern Italy. Now that 
electricity generated by water power can be conducted economically 
by wire, the extent of the influence of the mountain streams is being 
extended. It is by no means improbable that in the future the great 
power which now runs to waste in the mountain torrents will prove 
a resource of higher and higher value. 

Mountains as Resorts. — Mountain scenery is an asset of moun- 
tains upon which it is impossible to place a direct money value. At- 
tracted by the fresh air, by the grand scenery, by the invigorating 
climbs, by the hunting and fishing, by the mineral springs of me- 
dicinal properties, people resort to mountains in great numbers, espe- 
cially in Europe and America. From their sojourns, these visitors 
reap benefits of various kinds, not the least of which is the mental 
inspiration which the grandeur of the mountain scenery must give 
to even the least imaginative. 



MOUNTAINS 571 

The attraction exerted by mountains in the respects mentioned has 
led to so many going to the more accessible of them, that the moun- 
tain as a resort becomes really a valuable asset, as in the case of the 
Adirondack and White Mountains of the United States, the High- 
lands of Scotland and Wales, and numerous mountains on the con- 
tinent, especially the Alps. One of the great sources of income to 
the Swiss people to-day is obtained from the tens of thousands of 
tourists who flock to the Alps each summer. Paths, roads, and even 
railways are built to remote and inaccessible points, and hotels are 
to be found almost anywhere that numbers choose to go. The tourist 
industry has led to the opening up of much of the Alpine region from 
France to the far end of the Tyrol, and in summer this mountain 
range is without doubt the most densely settled, high mountain region 
of the world. One can scarcely go to any part of the region without 
encountering others driving, walking, or climbing. 

Throngs go to the Hartz Mountains, the Black Forest, the Vosges, 
and the Jura; but other, more inaccessible, mountains are far less 
visited. In western United States, for instance, it is only in a few 
sections that the asset of scenery has as yet become of importance, 
though one cannot doubt that this is one of the future resources of 
many parts, as yet known only to the few. Even more true is this 
of Alaska, whose mountain scenery, excelling in grandeur that of the 
Alps, is now known to very few, excepting explorers and prospectors. 

Mountains as Retreats. — Because of inaccessibility, mountains 
have, from the very earliest times, served as places of retreat of weaker 
people before the inroads of stronger ones. Sometimes, too, people 
are left untouched among mountains by invading hordes who overrun 
the surrounding lands, but avoid the difficulties of the mountains. A 
handful of people, knowing the country, can perhaps defend it against 
a far stronger force, and, if need be, escape by difficult routes unknown 
to the invaders. The mountain regions of the world furnish numerous 
instances of the protecting influence of mountains, operative especially 
in earlier days, but now far less effective than formerly because of 
the improvement of highways. 

In the British Isles, for instance, the Welsh people long resisted 
successfully the invaders who swept over the more level portions of 
Great Britain, and they still preserve their primitive language. The 
Scottish Highlanders also held out against invaders, and were them- 
selves divided into groups, or clans, favoured by the difficulties of access 
to the deep-set glens and mountain fastnesses. They too have pre- 
served to this day languages and customs of a bygone period, including 
even peculiarities of- dress. Similarly one finds peculiar customs on 
the continent, — in the Black Forest and the Tyrol, for instance ; and 
also languages of peculiar kind, as in the Alps where Rhaeto-Romansh 
dialects are found in one part, and in the Pyrenees, where the Basque 
language still survives, — a language wholly unlike any other known 
to-day in Europe. 



572 COLLEGE PHYSIOGRAPHY 

The large number of separate small countries and different languages 
and dialects in central Europe in earlier days is partly accounted for 
by the complex mountains there. The greatest number of small 
states in Germany, for example, are found in the region of worn-down 
mountains, while the largest states, Prussia and Bavaria, are primarily 
plain and plateau, respectively. The influence of the mountains in 
this respect is partly because of the natural boundaries they present, 
partly because of the ease of defending the mountains or of retreating 
to the less accessible parts, and partly because mountain land is often 
not valued highly enough to warrant serious effort to take possession 
of it and to hold it. 

The former importance of mountains in encouraging the existence of 
independent states is seen to-day in the large number of German states, 
the different peoples in the Austrian Empire, and even in some sur- 
vivals of this former state of independence, as Liechtenstein in the 
Alps, Luxembourg in the old mountain plateau between France and 
Germany, and Andorra in the Pyrenees. Similarly, in the Himalayas, 
the mountain state of Afghanistan retains its independence, though 
encroached upon from the south by the British and from the north 
by the Russians. 

Mountain people are justly credited with high bravery and strong 
love of liberty, and to these qualities are often ascribed, in part at least, 
their success in maintaining themselves in their mountain retreats. 
Such qualities are surely to be expected of people who dwell amidst the 
grandest of nature's scenery and whose lives are spent in a battle for 
existence, often against odds, and often amid appalling danger. 
They must gain a love for their wild mountain home, a strength of 
mind and body, and a knowledge of their power which would help to 
rally them in defence of their land. One cannot go much among the 
mountains without feeling their moral influence. 

Mountains as Barriers. — That mountains are barriers to travel 
is one of the points set forth in the preceding section ; otherwise they 
could not become good retreats. Young transverse valleys and 
mountain gorges are often too narrow for a path. The frequent use of 
mountains as boundary lines between nations, as in the case of the 
Pyrenees between France and Spain and the Andes between Chile and 
Argentina, shows what excellent barriers they are. They rarely 
form perfect boundaries, however, for they are pierced by valleys, 
crossed by passes, and even by streams which head on one side and 
flow across the entire range, as in the case of the Chilean Andes. As a 
result, the mountains are often encroached upon from different sides, 
as the Alps are by France, Italy, Austria, and Germany, besides the 
central country of Switzerland, whose irregular boundary touches each 
of these countries, and often along a most artificial line. 

Even the Alps have, again and again, proved an ineffectual barrier 
since the days of Hannibal ; and the Himalayas, though they separate 
two quite distinct ethnic types, and two floras and faunas, have been 



MOUNTAINS 



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574 COLLEGE PHYSIOGRAPHY 

crossed again and again along the weakest part, bringing hordes of 
invaders into India. Yet, though capable of being broken, and, 
therefore, not absolute barriers, mountains have seemed greatly to 
retard the movements of people, and the migrations of plants and 
animals from the earliest days to the present time. Their ruggedness, 
the coldness of lofty passes, the forest barriers, and the mountain 
dwellers have all helped in this interference with migration. 

To-day, mountains are far less effective as barriers than they have 
even before been. Roads, cut in the mountain sides, and built up on 
steep slopes, rising to the passes in a series of zigzags, now extend in 
all directions in the Alps. Railways, rising by steep grades, even by 




Fig. 376. — The Copper River and Northwestern Railway in the Chugach Mountains of 
Alaska, where it traverses the stagnant, moraine-veneered, vegetation-covered termi- 
nus of Allen Glacier for si miles. There is ice beneath the rails and ties in the fore- 
ground. 

rack-and-pinion, and piercing the mountains in tunnels many miles in 
length, connect Austria, Germany, and France with Italy across the 
Alps. Engineering skill is able to confront even the most serious of 
mountain obstacles, if the object is sufficiently strong to warrant the 
expense (Figs. 361, 375). 

Yet, how great the barrier is, may be seen from a few examples, 
from regions where the incentive for human conquest has been less 
great. No railway has as yet been laid across the Himalayas; and 
until 1910, no railroad completely crossed the Andes, though several 
had been built across the outer range. In Alaska the coast ranges 
form so serious a barrier that the development of the mineral resources 
has been greatly retarded. A railway recently built through them 
crosses a large river between two huge glaciers (Fig. 128) which discharge 



MOUNTAINS 



575 



icebergs into the river from vertical ice cliffs, over 200 feet high and 
three miles or more in length ; and, a little farther, extends over the 
stagnant end of a glacier (Figs. 376, 377), for 5I miles. 

Where mountains rise from the seacoast, they may increase the 
effectiveness of the barrier by presenting a straight coast line, as in the 
Andean coast. Or, on the other hand, if subsidence has taken place, 
or if glacial erosion has worn the mountain valley below sea level, the 
coast may be greatly indented, as in southern Alaska, British Columbia 
and Washington, in Scotland, Scandinavia, and Greece. 

Even low mountains, especially if young, may be serious obstacles 
to travel. The Coast Ranges of western United States, for instance, 
though of no great height, are effective barriers. The coast is pre- 
vailingly straight and harbours 
are few. There is settlement 
along some of the longitudinal 
valleys, and lines of travel fol- 
low them; but there is almost 
complete absence of railways 
across them, and none along 
their seaward face. In the 
much higher Andes, also, there 
is travel and settlement along 
the valleys, but only at long in- 
tervals are there railroads con- 
necting the 'valleys with the 
sea. The mountain barrier has 
greatly retarded the development 
of the Andean countries, and 
even valuable mining districts 
are reached only by rude trails. 

It might seem that a worn-down mountain region would present little 
obstacle to travel, but this is far from being the case ; for, even when 
the valleys have broadened out to early maturity, there is still sufficient 
ruggedness to interfere with travel ; and it is often the case that forest 
tracts also aid in the effectiveness of the barrier. The Appalachian 
Mountains and the Allegheny Plateau will illustrate the nature of such 
a barrier (Fig. 378). The barrier long seemed to hem the colonists in 
along the coast lands ; and when the barrier was crossed and settlers 
occupied the country beyond, they were still so separated from the 
parent colonies that there was serious effort to set up an independent 
transmontane country. Now the barrier is crossed by railways at 
several points, but still there exist in the more inaccessible parts a 
people who are strikingly out of touch with the modern progress of the 
surrounding regions. 

Effect of Mountains on Climate. — Mountains rising above the 
general level have a cooler climate than surrounding lowlands. The 
decrease in temperature with altitude amounts to one degree for every 




Fig. 377. — A mountain railway in Alaska, 
passing between two great glaciers and over 
the terminus of a third. 



576 



COLLEGE PHYSIOGRAPHY 



300 feet. This is equal to going 30 to 60 miles poleward. As the 
winds rise to pass over mountains they are caused to precipitate some 
of their vapour, either as rain or snow. This has an important effect, 
both directly on the mountains, and indirectly on the region near the 
mountains. The effects on the mountains are (1) the influence of the 
precipitation on erosion, (2) the development of snow fields and 
glaciers, and (3) the supply of water necessary for forest growth. The 
influence on the surrounding country includes (1) the washing down of 
sediment, which may spread out at the mountain base, (2) the supply 




Fig. 378. — The Appalachians as a low but effective mountain barrier. (Willis.) 

of water to the surrounding country, and (3) the influence on the 
climate of the country. 

The climatic influence of mountains upon the neighbouring country 
is effective in several ways, but without doubt the most important is 
in the reduction of the rainfall. Winds that precipitate vapour on 
rising over the mountains have less for precipitation in the lee of the 
mountains, and they may have so little that the country is a desert. 
The Cascade Ranges of Washington, for example, have abundant 
rainfall on the western slopes and crests, but are bordered by a desert 
strip along their eastern base. The central Andes, on the other hand, 
reached by winds from the east, are clothed with forest on that side, 
but are a desert on the west. The Himalayas have their rainfall on 
the south and the desert to the north. The Alps, not lying athwart 



MOUNTAINS 577 

a prevailing wind system, have no desert, but abundant rainfall on all 
sides and on all slopes. 

Even low mountain ranges over which prevailing winds blow pro- 
duce an important influence on the rainfall. In the British Isles, for 
example, the mountains of Wales, the English Lake District, and the 
western highlands of Scotland have far heavier rainfall than the 
country to the east, and in places, in the east, as in the neighbourhood 
of Cambridge, there is very light rainfall. Were these mountains lofty 
and continuous from Scandinavia to Brittany, there would doubtless 
be desert conditions to the east, as there evidently have been in earlier 
geological ages. 

The water that falls in the mountains may serve many uses as it 
flows out over the surrounding country. The Rhine, supplied from the 
rains and snows of the Alps, and regulated in flow by the mountain 
lakes, is useful for navigation. A multitude of mountain streams 
serve as a source of water for power, and for municipal purposes ; 
and, in arid lands, the mountain streams give to the soil by irrigation 
a part of the water of which the mountains have robbed it. This 
finds illustration in many places in western United States, as at the 
Roosevelt Dam in Arizona. Also, large streams may flow long dis- 
tances, crossing and irrigating deserts far away from the mountain 
source of the water, as the Nile does in Egypt and the Colorado in 
California. 

Advantages and Disadvantages to Man. — It would be difficult to 
strike a proper balance between the beneficial and injurious effects of 
mountains, for there are many factors involved. Mountains greatly 
decrease the area of habitable land (i) by their own ruggedness and 
inhabitability, (2) by the aridity to which they give rise. On the other 
hand, they are a source of valuable mineral products, and out of their 
elevation and denudation has come a large part of the habitable land 
of the earth. They are the source of great rivers ; but such rivers also 
develop on plains. They increase the forest area in arid regions, but 
they also cause such aridity as to prevent forest growth where other- 
wise it would doubtless occur. They are barriers to travel and trade, 
but they are also retreats. During the general European war in 1914 
the mountains were a positive influence in some respects. During the 
German campaign against France at the beginning of the war, for ex- 
ample, the mountains helped bring about the invasion of Belgium 
because the ancient highland of Ardennes is lower and more easily 
crossed there than in Luxembourg. 

Of one point, however, we may be certain ; as land forms, the moun- 
tains are to be ranked among the least attractive for human occupation. 
Everywhere, throughout the world, whether lofty or moderately sub- 
dued, mountains are prevailingly regions of sparse settlement and 
relative inaccessibility. They offer a striking antithesis to the level 
plains, teeming with population, the seat of industry and progress, and 
the highways of busy trade. 



578 COLLEGE PHYSIOGRAPHY 



References to Literature 

J. Barrell. Central Connecticut in the Geologic Past, Proc. Wyoming Hist, and 
Geol. Soc, Vol. 12, 191 2, pp. 25-54; Bull. Geol. Surv. Conn, (in press). 

Isaiah Bowman. Physiography of the Central Andes, Amer. Journ. Sci., Vol. 
178, 1909, pp. 197-217, 373-402 ; Results of an Expedition to the Central 
Andes, Bull. Amer. Geog. Soc, Vol. 46, 1914, pp. 161-183; Physiography 
of the United States, Forest Physiography, New York, 1911, — New 
England, pp. 636-684; Piedmont Plateau, pp. 623-635; Appalachian 
Mountains, pp. 585-684; Adirondack Mountains, pp. 578-584; Lake 
Superior Highland, pp. 572-578; Laurentian Plateau, pp. 554-572; 
Rocky Mountains, pp. 298-404 ; Basin Ranges, pp. 218-228 ; Blue Moun- 
tains, pp. 207-209; Cascade and Sierra Nevada Mountains, pp. 149-176; 
Coast Ranges, pp. 127-148. 

A. H. Brooks. Geography and Geology of Alaska, Prof. Paper 45, U. S. Geol. 
Survey, 1906, 327 pp.; Mount McKinley Region, Alaska, ibid., Prof. 
Paper 70, 1911, 234 pp. 

M. R. Campbell. Geographic Development of Northern Pennsylvania and 
Southern New York, Bull. Geol. Soc. Amer., Vol. 14, 1903, pp. 277-296. 

T. N. Dale. Taconic Physiography, Bull. 272, U. S. Geol. Survey, 1905, 52 pp. 

R. A. Daly. Physiography of Acadia, Bull. Mus. Comp. Zool., Vol. 38, 1901, 
pp. 73-103; Accordance of Summit Levels Among Alpine Mountains, 
Journ. Geol., Vol. 13, 1905, pp. 105-125. 

J. D. Dana. On the Origin of Mountains, Amer. Journ. Sci., 3d series, Vol. 
5, 1873, PP- 347-35o; ibid., pp. 423-434, 474-475; ibid., Vol. 6, 1873, 
pp. 6-14, 104-115, 161-172, 381-382; Characteristics of Some Typical 
Mountain Ranges, Manual of Geology, 4th edition, New York, 1896, 
PP- 353-3 6 9; Orogenic Work, ibid., pp. 380-396. 

N. H. Darton. Shawangunk Mountain, Nat. Geog. Mag., Vol. 6, 1894, pp. 
23-34; Examples of Stream Robbing in the Catskill Mountains, Bull. 
Geol. Soc. Amer., Vol. 7, 1896, pp. 505-507. 

A. Daubree. Etudes Synthetiques de Geologie Experimental, Paris, 1879, 
828 pp. 

W. M. Davis. Topographic Development of the Triassic Formation of the 
Connecticut Valley, Amer. Journ. Sci., 3d series, Vol. 37, 1889, pp. 423- 
434; The Rivers of Northern New Jersey, Nat. Geog. Mag., Vol. 2, 1890, 
pp. 81-110; The Folds of the Appalachians, Goldthwaites Geog. Mag., 
Vol. 3, 1892, pp. 251-255; ibid., pp. 343-350; Physical Geography 
of Southern New England, National Geographic Monographs, New York, 
1896, pp. 269-304; The Mountains of Southernmost Africa, Bull. Amer. 
Geog. Soc, Vol. 38, 1906, pp. 593-623; 10th Census of United States, 
Vol. 15, 1886, pp. 697-712; Mountain Ranges of the Great Basin, Geo- 
graphical Essays, Boston, 1909, pp. 725-772; The Wasatch, Canyon, and 
House Ranges, Utah, Bull. Mus. Comp. Zool., Vol. 49, 1905, pp. 15-58; 
The Rhine Gorge and the Bosphorus, Journ. Geog., Vol. n, 191 2, pp. 
209-215; Gebirge, Erklarende Beschreibung der Landformen, Leipzig, 
1912, pp. 246-315; Colorado Front Range, Annals Assoc. Amer. Geog- 
raphers, Vol. 1, 1911, pp. 21-83. 

G. M. Dawson. Later Physiographic Geology of the Rocky Mountain Region 
in Canada, Trans. Roy. Soc. Canada, Section IV, 1890, pp. 3-74; ibid., 
Bull. Geol. Soc. Amer., Vol. 12, 1901, pp. 57-92. 

J. S. Diller. Tertiary Revolution in the Topography of the Pacific Coast, 
14th Ann. Rept., U. S. Geol. Survey, Part 2, 1894, pp. 397-434; Topo- 
graphic Development of the Klamath Mountains, Bull. 196, U. S. Geol. 
Survey, 1902, 69 pp. 

S. F. Emmons. Orographic Movements in the Rocky Mountains, Bull. Geol. 
Soc. Amer., Vol. 1, 1890, pp. 245-286. 

S. F. Emmons and Others. Geological Guide Book of the Rocky Mountain 



MOUNTAINS 579 

Excursion, 5th International Geological Congress, Washington, 1893, pp. 

255-487- 
H. W. Fairbanks. The Physiography of California, Bull. Amer. Bureau of 

Geography, Vol. 1, 1901, pp. 232-252, 329-353. 
A. Geikie. Physiographic Geology, Text-Book of Geology, New York, 1903, 

Vol. 2, pp. 1363-1388 ; Continental Elevation and Subsidence, Proc. Geol. 

Soc, London, Vol. 60, 1904, pp. lxxx-civ. 
J. Geikie. Fragments of Earth Lore, Edinburgh, 1893, pp. 36-61; Earth 

Sculpture, New York, 1898, pp. 92-149; The Architecture and Origin of 

the Alps, Scottish Geog. Mag., Vol. 27, 1911, pp. 393-417; Mountains, 

Edinburgh, 1913, 311 pp. 
G.K.Gilbert. Mountain, Universal (Johnson's) Encyclopedia, Vol. 8, 1900, 

pp. 282-284; Orology, Wheeler's Geographical Surveys West of the 100th 

Meridian, Vol. 3, Washington, 1875, PP- 21-62; Origin of the Physical 

Features of the United States, Nat. Geog. Mag., Vol. 9, 1898, pp. 308-317. 
A. Hague. King's U. S. Geological Exploration of the 40th Parallel, Vol. 2, 

Descriptive Geology, 1877, pp. 11 2-1 29. 

F. V. Hayden. Rocky Mountains, U. S. Geol. Survey of the Territories, Vol. 

1, 1867-1869, to Vol. 11, 1877. 
C. W. Hayes. Physiography of the Chattanooga District in Tennessee, 
Georgia, and Alabama, 19th Ann. Rept., U. S. Geol. Survey, Part 2, 1899, 
pp. 1-58; The Southern Appalachians, National Geographic Mono- 
graphs, New York, 1896, pp. 305-336. 

C. W. Hayes and M. R. Campbell. Geomorphology of the Southern Ap- 

palachians, Nat. Geog. Mag., Vol, 6, 1894, pp. 63-126. 
A. Heim. Der Bau der Schweizeralpen, Neujahrsblatt no, Naturforschenden 

Gesellschaft, Zurich, 1908, 26 pp. ; Untersuchungen iiber den Mechanis- 

mus der Gebirgsbildung, Basel, 1878, 2 vols, and atlas, 346 pp. 
A. Heim and E. de Margerie. Les Dislocations de l'Ecorse Terrestre, Zurich, 

1888, 154 pp. 
W. H. Hobbs. The Origin and the Forms of Mountains, Earth Features and 

their Meaning, New York, 1912, pp. 435-447; Tectonic Geography of 

Eastern Asia, Amer. Geol., Vol. 35, 1904, pp. 69-80, 141-151, 214-226, 

283-291, 371-378. 
W. Joerg. The Tectonic Lines of the Northern Part of the North American 

Cordillera, Bull. Amer. Geog. Soc, Vol. 42, 1910, pp. 161-179. 

D. W. Johnson. Block Mountains in New Mexico, Amer. Geol., Vol. 31, 

1903, pp. 135-139. 
A. Keith. Geology of the Catoctin Belt, 14th Ann. Rept., U. S. Geol. Survey, 

Part 2, 1894, pp. 285-395. 
J. F. Kemp. The Physiography of the Adirondacks, Pop. Sci. Monthly, 

Vol. 68, 1906, pp. 195-210. 
C. King. Mountaineering in the Sierra Nevada, New York, 1902 ; U. S. 

Geological Exploration of the 40th Parallel, Vol. 1, Systematic Geology, 

Washington, 1878, 803 pp. 
A. de Lapparent. De la Classification des Montagnes, Lecons de Geographie 

Physique, Paris, 1898, pp. 701-718. 
A. C. Lawson. Sketch of the Geology of the San Francisco Peninsula, 15th 

Ann. Rept., U. S. Geol. Survey, 1895, pp. 399-476. 
J. Le Conte. On the Structure and Origin of Mountains, Amer. Journ. Sci., 

Vol. 38, 1889, pp. 257-263; Theories of the Origin of Mountain Ranges, 

Journ. Geol., Vol. 1, 1893, pp. 543~573- 
W. Lindgren. A Geological Reconnaissance Across the Bitterroot Range, 

Prof. Paper 27, U. S. Geol. Survey, 1904, 123 pp. 

G. D. Louderback. Basin Range Structure of the Humboldt Region, Bull. 

Geol. Soc. Amer., Vol. 15, 1904, pp. 289-346. 
M. Lugeon. Les Grandes Nappes de Recouvrement des Alpes du Chablais et 
de la Suisse, Bull. Geol. Soc. France, Vol. 4, 1901, pp. 723-825; Les 
Nappes de Recouvrement de la Tatra et l'Origine des Klippes des Karpetes, 
Bull. Soc. Vaudois Sc. Nat., Vol. 39, 1903, pp. 146-197. 



580 COLLEGE PHYSIOGRAPHY 

J. E. Marr. The Scientific Study of Scenery, New York, 1900, pp. 55-112. 

Lawrence Martin. Physical Geography of the Lake Superior Region, Mon- 
ograph 52, U. S. Geol. Survey, 191,1, pp. 85-117. 

E. de Martonne. Recherches sur l'Evolution Morphologique des Alps de 
Transylvanie, Revue de Geogr. Vol. 1, 1897, 279 pp. 

H. Newton. Geology of the Black Hills, U. S. Geol. and Geog. Survey of the 
Rocky Mountain Region, Washington, 1880, 222 pp. 

B. N. Peach and J. Horn. Geological Structure of the North- West Highlands 

of Scotland, Memoirs Geol. Survev of Great Britain, Glasgow, 1907, 
668 pp. 

A. Penck. Die Gebirge, Morphologie der Erdoberflache, Part 2, Stuttgart, 
1894, pp. 327-438. 

J. W. Powell. Types of Orographic Structure, Amer. Journ. Sci., Vol. 12, 
1876, pp. 414-428; Physiographic Features, National Geographic Mono- 
graphs, New York, 1896, pp. 33-64; Physiographic Processes, ibid., pp. 
1-32; Physiographic Regions of the United States, ibid., pp. 65-100; 
Geology of the Eastern Portion of the Uinta Mountains, Geol. and Geog. 
Survey of the Territories, Washington, 1876, 218 pp. 

R. Pumpelly, J. E. Wolff, and T. N. Dale. Geology of the Green Mountains 
in Massachusetts, Monograph 23, U. S. Geol. Survey, 1894, 206 pp. 

A. C. Ramsay. The Physical Geology and Geography of Great Britain, 
London, 1864, 1894, 421 pp. 

T. M. Reade. Origin of Mountain Ranges, London, 1886, 359 pp.; Evolution 
of Earth Structure, New York, 1903, 342 pp. 

W. N. Rice. The Classification of Mountains, 8th International Geographical 
Congress, Washington, 1905, pp. 185-189. 

H. D. Rogers. The Geology of Pennsylvania, 2 vols., Philadelphia, 1858. 

I. C. Russell. Fault Blocks in the Great Basin, 4th Ann. Rept., U. S. Geol. 
Survey, 1884, pp. 435-464; North America, New York, 1904, 435 pp. 

W. B. Scott. Mountain Ranges, Introduction to Geology, New York, 1907, 
pp. 503-515- 

E. C. Semple. Mountain Passes: A Study in Anthropogeography, Bull. 
Amer. Geog. Soc, Vol. 33, 1901, pp. 124-137, 191-203; Influences of 
Geographic Environment, New York, 191 1, pp. 557-606. 

N. S. Shaler. Spacing of Rivers with Reference to the Hypothesis of Base 
Leveling, Bull. Geol. Soc. Amer., Vol. 10, 1899, pp. 263-276; Broad Val- 
leys of the Cordilleras, ibid., Vol. 12, 1901, pp. 271-300. 

G. O. Smith and F. C. Calkins. Geological Reconnaissance across the Cas- 
cade Range, Bull. 235, U. S. Geol. Survey, 1904, 103 pp. 

J. E. Spurr. Origin and Structure of the Basin Ranges, Bull. Geol. Soc. Amer., 
Vol. 12, 1901, pp. 217-270. 

E. Suess. Das Antlitz der Erde : de Margerie's Translation in French, 5 
vols.: Sollas's translation in English, 4 vols., Oxford, Part 1, The 
Movements in the Outer Crust of the Earth, Vol. 1, 1904, pp. 1-179; 
Part 2, The Mountain Ranges of the Earth, ibid., pp. 180-604; Part 4, 
The Face of the Earth, Vol. 3, 1908, 400 pp.; Part 4, Continued, Vol. 4, 
1909, 673 pp. 

R. S. Tarr. The Peneplain, Amer. Geol., Vol. 21, 1898, pp. 351-370; The 
Mountains — Subequality of Level, The Yakutat Bay Region, Alaska, 
Prof. Paper 64, U. S. Geol. Survey, 1909, pp. 28-29; Physical Geography 
of New York State, Chapter III, New York, 1902. 

C. Thomas. Hayden's Geol. and Geog. Survey of the Territories, Washing- 

ton, 1871, pp. 211-216. 

W. S. Tower. Regional and Economic Geography of Pennsylvania, Bull. 
Geog. Soc. Philadelphia, Vol. 4, 1906, pp. 57-76, 1 13-136, 193-204. 

W. Upham. A Classification of Mountain Ranges according to their Struc- 
ture, Origin, and Age, Appalachia, Vol. 6, 1892, pp. 191-207. 

C. R. Van Hise. Relations of Rock Flowage to Mountain Making, A Treatise 
on Metamorphism, Monograph 47, U. S. Geol. Survey, 1904, pp. 924-931. 



MOUNTAINS 



58i 



S. Weidman. Geology of North Central Wisconsin, Bull. 16, Wis. Geol. 
Survey, 1907, 697 pp. 

J. D. Whitney. The Coast Ranges and Sierra Nevada, Geol. Survey of Cali- 
fornia, Vol. 1, 1865, 498 pp. 

B. Willis. Round about Ashville, Nat. Geog. Mag., Vol. 1, 1889, pp. 291-300; 
The Northern Appalachians, Nat. Geog. Monographs, New York, 1896, 
pp. 169-202 ; Mechanics of Appalachian Structure, 13th Ann. Rept., U. S. 
. Geol. Survey, Part 2, 1893, pp. 211-281; Studies in Mountain Growth, 
Year Book 4, Carnegie Institution, Washington, 1906, pp. 192-203; 
Geological Structure of the Alps, Smithsonian Misc. Collections, No. 
2067, 191 2, 13 pp.; Lewis and Livingston Ranges, Bull. Geol. Soc. Amer., 
Vol. 13, 1902, pp. 305-352; Physiography and Deformation of the Wen- 
atchee-Chelan District, Cascade Range, Prof. Paper 19, U. S. Geol. 
Survey, 1903, pp. 41-97; Physiography of Northwestern China, Carnegie 
Institution, Publ. 54, 1907, pp. 203-264; Physiography of Southern 
Shen-si, ibid., pp. 319-340. 

A. W. G. Wilson. The Laurentian Peneplain, Journ. Geol., Vol. n, 1903, pp. 
615-669. 

H. M. Wilson. Topographic Forms of United States, Bull. Amer. Geog. Soc, 
Vol. S3, I 9 OI J PP- 3 OI ~304- 



Mt. Marcy, N.Y. 



TOPOGRAPHIC MAPS 

Adirondacks 
Lake Placid, N.Y. Newcomb, N.Y. 



Briceville, Tenn. 
Monterey, W.Va. 
Franklin, W.Va. 



Appalachians 

Pikeville, Tenn. 
Fort Payne, Ala. 
Maynardville, Tenn. 



Delaware Water Gap, Pa. 
Estillville, K>. 
Hazelton, Pa. 



Fish Springs, Utah 
Granite Range, Nev. 



Basin Ranges 

Alturas, Cal. 
Long Valley, Nev. 



Needles, Ariz. 



Coos Bay, Oreg. 



Coast Ranges 
Tamalpais, Cal. 



San Francisco. Cal. 



Delaware Water Gap, 
Becket, Mass. 
New Haven, Conn. 
Stonington, Conn. 



Mature Mountains 

Pa. West Point, N.Y. 
Lykens, Pa. 
Charlestown, R.I. 
Newcomb, N.Y. 



Springfield, Mass. 
Boothbay, Me. 
New London, Conn. 
Elizabethtown, N.Y. 



Antietam, Md. 
Estillville^ Ky. 
Hazelton, Pa. 



Mountain Ric 

Monterey, Va. 
Franklin, W.Va. 
Lykens, Pa. 



Fort Payne, Ala. 
Maynardville, Tenn. 



Mt. Mitchell, N.C. 



Never Glaciated Mountains 
Needles, Ariz. Cucamonga, Cal. 



5§2 



COLLEGE PHYSIOGRAPHY 



Mt. Washington, N.H. 
Hartford, Conn. 



New England Mountains 

Becket, Mass. 
New Haven, Conn. 



Monadnock, N.H. 
Charlestown, R.I. 



Old Mountains, Peneplains, and Monadnocks 



Monadnock, N.H. 
Atlanta, Ga. 



Boston Bay, Mass. 
Wausau, Wis. 



Farmville, Va. 
Marathon, Wis. 



Sawtooth, Idaho 
Marias Pass, Mont. 



Rocky Mountains 

Pike's Peak, Colo. 
Livingston, Mont. 



Chief Mountain, Mont. 
Huerfano Park, Colo. 



Sierraville, Cal. 



Sierra Nevada 
Yosemite Valley, Cal. 



Bishop, Cal. 



Alturas, Cal. 

Platte Canyon, Colo. 

Shasta, Cal. 



Young Mountains 

Pike's Peak, Colo. 
Huerfano Park, Colo. 
Sierraville, Cal. 



Telluride, Colo. 
San Francisco, Cal. 
Coos Bay, Oreg. 



CHAPTER XVI 
RELIEF FEATURES OF THE EARTH 

Relation to Previous Topics 

The forms of the land have in the previous chapters been considered 
topically and in some detail. It is intended in this chapter to study 
them in broad review, to consider them in their general relation one to 
another, and to inquire into the underlying causes for the earth relief. 

The Oblate Form 

The gravitational form of the sphere, by which the earth materials 
are arranged symmetrically around a centre, is disturbed by the 
centrifugal force resulting from rotation. This causes a slight flatten- 
ing at the poles and a slight bulging in the equatorial belt, producing 
the oblate spheroid form, in which the polar diameter is 7899 miles 
and the equatorial diameter 7926 miles, a total difference of about 
27 miles. Therefore from the pole to the equator there is a gradual 
increase in distance from the centre until the maximum of 13 J miles 
is reached. 

This bulging is plainly due to rotation, and if the axis of rotation 
should move, the position of the bulge would change; or, if the rate 
of rotation should increase or decrease, the amount of bulging would 
vary. This would happen because the main bulk of the earth is in a 
state of flowage, only the outer portion being rigid rock. Whether 
the earth's interior is molten, or merely plastic under the overlying 
load, adjustment would necessarily follow any change in rate or direc- 
tion of rotation. Adjustment has been reached in relation to the 
present rate of rotation and to the present position of the axis, giving 
the degree of oblateness that characterizes the earth. 

The oblate form is an important feature in the relief of the earth, 
being, in fact, the greatest known departure from the spherical form 
of the earth. It is so widely spread over the whole earth that the 
departure from the true sphere is not visible, and the land surface and 
ocean water alike conform to it. 

. Ocean Basins 

Ocean Basins and Epicontinental Seas. — Approximately three- 
fourths of the earth's surface lies below the mean sea level and is, 
therefore, covered by ocean water. The total area of the earth's 

583 



584 COLLEGE PHYSIOGRAPHY 

surface is about 196,940,000 square miles, while the area of the oceans 
is about 141,486,000 square miles. The greater part of this depressed 
area is in the form of a basin of irregular boundary, but the basin area 
is nearly 10,000,000 square miles less than the ocean area, for the 
ocean water overflows the edges of the continents throughout most of 
their extent. There the ocean water is shallow, in what have been 
called the epicontinental seas, but elsewhere the ocean basins are deep. 
The average depth of the ocean is from 12,000 to 13,000 feet, but, if 
the epicontinental seas were excluded, the average would be con- 
siderably greater. 

The Continental Slope. — The ocean basins, exclusive of the con- 
tinent fringe overflowed by the ocean water, are, on the average, 
steeply bounded by a slope, known as the continental slope. This 
sometimes rises above the sea, as along mountainous coasts like 
the Andes ; but much more commonly it rises beneath the sea and 
terminates in a submerged platform, known as the continental shelf, 
which varies in width from a fraction of a mile up to several hun- 
dred miles. It is, therefore, not visible and is not commonly repre- 
sented on maps. If there were no ocean waters to obscure the relief 
of the globe, there would be revealed a great area of depression of 
very irregular form, terminating in the steep continental slope. 
This borders the continental plateaus, only a portion of which now 
rise above sea level. 

The Ocean Bottom Plain. — On the whole the ocean basins form a 
vast plain of gentle -slopes, excepting at the margins, where the con- 
tinental slope rises more or less irregularly. Here and there local 
elevations occur, sometimes rising above the sea level. These eleva- 
tions are either (1) broad upward swells of the sea bottom, or (2) 
faulted and tilted blocks, or (3) volcanic cones. Frequently these are 
in combination. There are also areas of unusual depression, some- 
times broad, moderately sloping basins, sometimes troughs. Ocean 
depths are discussed later (Chap. XIX). 

Deposition the Prevailing Process. — These elevations and depres- 
sions, as well as the ocean basins themselves, are due to the operation 
of those forces which obtain their energy from conditions within the 
earth. The phenomena of denudation, by which the land surfaces 
are being diversified, are practically excluded from the area covered 
by the ocean waters, excepting at and near the shore line. Instead, 
the ocean bed becomes the seat of deposit from the denudation of the 
lands, either directly by the deposit of sediment, or indirectly by the 
deposit of organic remains, whose hard parts are mineral matter 
mainly derived from the land and distributed in the ocean waters 
in solution. This deposit tends to render the sea floor still more 
regular. In this respect the ocean basin topography presents a 
striking contrast to that of the denuded lands. Erosion on the con- 
tinents produces pleasing irregularity, while deposition on the floor of 
the sea causes monotonous simplicity in topography. 



RELIEF FEATURES OF THE EARTH 585 



Continental Plateaus 

Area and Height of Continents. — Reckoned from the ocean borders, 
the land areas occupy about one-fourth of the earth's surface, or some- 
what over 55,000,000 square miles; but if the submerged continent 
edge is included, the total area of the continental plateaus is some- 
thing like 10,000,000 square miles greater, and if the sea level should 
sink about 600 feet approximately, this amount would be added to the 
land area. While the ocean basins average 12,000 to 13,000 feet in 
depth, the continents rise only about 2300 feet above sea level on the 
average. The mean lithosphere level is, therefore, well below the 
level of the ocean, lying at a depth of about 7500 feet. That is to say, 
if all the continent platforms were planed off down to this level, they 
would fill the ocean basins up to within 7500 feet of the present sea 
level. The ocean would then spread over the entire earth with a depth 
of over 1 J miles, being 7500 feet plus the amount of water displaced 
by filling the ocean basins. 

The Maximum Relief. — The average departure of the earth's 
surface from mean lithosphere level is nearly 10,000 feet on the con- 
tinent side, and about 5000 feet on the ocean' basin side, and the 
larger portion of this departure is on the continental slope, which is 
really the most prominent diversity in the relief of the globe, though 
in the main hidden from view by the ocean water. Usually this slope, 
entirely beneath the sea, has a vertical relief of from 10,000 to 15,000 
feet ; but in some places where it is extended above the sea in lofty 
mountain chains, it is much higher. Thus along western South 
America it is approximately 40,000 feet from the ocean bottom to the 
highest Andean peak in a distance of about 75 miles. This is proba- 
bly the greatest relief feature on the earth within a limited area. 

The maximum relief of the land above sea level is 29,002 feet in 
Mount Everest in the Himalayas, and since the maximum depth of 
the sea is 32,114 feet, the total relief is 61,116 feet, or about nj 
miles, a little less than the difference between polar flattening and 
equatorial protuberance. This is about -g^o of the earth's radius, 
and, therefore, an exceedingly small amount as compared to the 
earth as a whole. 

Continents Rougher than Ocean Basins. — As in the ocean basins, 
the most characteristic topographic form of the land is the plain, 
though there is a smaller proportion of plains on the land than in the 
ocean, and on the whole the plains on the land are rougher than in the 
sea. There are also extensive plateaus, broad upward swells of parts 
of continent area, faulted and tilted blocks, and volcanoes, as in the 
ocean. But on the continents denudation is, and has been, active ; 
and deposition, while in progress locally, has no such widespread im- 
portance as in the sea. The lands, therefore, are greatly sculptured, 
uplifted portions of the earth are dissected into rugged form, and 
even lowlands present irregularities due to denudation. 



586 COLLEGE PHYSIOGRAPHY 

Comparisons and Contrasts with Ocean Basins. — In their main 
features, therefore, ocean basins and continent platforms present both 
resemblances and differences. They resemble each other in the 
presence of forms due to vulcanism and diastrophism ; but they differ 
(i) in the relative effects of denudation, and (2) in the fact that the 
one area is prevailingly elevated, the other depressed, the two areas 
being separated by the pronounced continental slope. The causes for 
the denudation and for the different effects in land and sea are readily 
understood, and have already been presented in detail in the pre- 
ceding chapters ; but the causes for the diastrophism and vulcanism 
by which the ocean basins, continental slope, continental plateau, 
mountain and plateau uplifts, and volcanic phenomena have been 
brought into existence are far less easy to interpret. These various 
phenomena are evidently allied in origin. 

Changes of Level 

Effect of Diastrophism, Vulcanism, and Other Movements. — 

Another phenomenon related in cause to those just mentioned is that 
of relative change of level of land and sea. This, as we have already 
seen, has in the past brought about great changes in the outline of the 
contact of land and sea, and in the level of parts of the land with refer- 
ence to sea level. Also, changes of this nature are still in progress. 
Some of these changes are surely related to mountain uplift, others to 
vulcanism, and probably still others are related to slower, more wide- 
spread, movements of the crust, either rising or sinking of the land, or 
changes in the level of the sea floor. If the ocean bottom should sub- 
side an average amount of a hundred feet, the water would be with- 
drawn from vast tracts of sea bottom in the epicontinental seas; 
if it should rise a like amount, vast tracts of low land on the continents 
would be flooded by rise of the ocean waters. Similar results would 
be brought about by elevation of the continent plateaus or by their 
depression. 

Effect of Withdrawal of Water. — While it is probable that most of 
the changes in relative level of land and sea are the outcome of some 
actual movements of the earth's crust, it is to be remembered that 
there may be actual changes in the amount of water. For example, 
water locked up in the earth's crust in the processes of weathering is 
removed from the sea; so is water locked up in snow and glaciers. 
On the other hand, water thrown out of volcanoes is added to the 
seas. If any cause were operating to change the rate of rotation, or 
the direction of the axis of rotation, this too would cause a partial 
redistribution of the liquid hydrosphere to conform to the changed 
conditions. 

Although there are three other possible causes for some of the phe- 
nomena included under change of level, it is generally believed that 
they are in the main the result of actual changes in crust level, either 



RELIEF FEATURES OF THE EARTH 



587 



operating directly to raise or lower the lithosphere, or indirectly by 
raising or lowering the level of the ocean as a result of changes in 
the submarine portion of the lithosphere surface. 

Distribution of Oceans and Continents 

Land and Water Hemispheres. — Speaking generally, the land 
areas of the globe are massed in the northern hemisphere, and the 
water areas in the southern. There is more than twice as much land 
in the former as in the latter. An even more striking division may be 



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^^ 






4-4Jj\ 


w 


1 


WK 




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Fig. 



379- 



Map of the North Polar basin, 
(de Martonne.) 



Depths in metres. 



made by dividing the earth into the so-called land and water hemi- 
spheres. The land hemisphere includes about six-sevenths of the land 
of the globe, but still more than half of its surface is water. In the 
water hemisphere only about one-fifth of the surface is land. 

The North Polar Basin. — Surrounding the North Pole is a deep polar 



588 COLLEGE PHYSIOGRAPHY 

basin, whose area and extent are not known, but in which deep water 
was found to exist by Nansen and later by Peary (Fig. 379) . This basin 
is fringed by a continental slope and then by a broad continental shelf, 
above which a number of islands rise. The continental shelf and islands 
almost cut off the polar basin from the rest of the oceanic areas, 
and even the Arctic Ocean itself, including the shallow parts which 
overflow the continental shelf, has but slight connection with the 
other oceans. It is almost cut off from the Pacific Ocean, being con- 
nected only by the shallow Bering Strait, only 50 miles wide. The 
connection with the Atlantic is more open, though if the sea bed were 
raised 600 feet, there would be only narrow straits here. The Arctic 
Ocean is, therefore, a nearly enclosed sea, really a bay-like prolonga- 
tion of the Atlantic Ocean, with a deep circumpolar basin. , 

The New World Continental Plateau. — From the continental 
slope which surrounds the north polar basin, there extend two land 
masses, the smaller forming the New World, the larger the Old World. 
The New World area, starting with maximum breadth in the polar 
portion, narrows southward toward the tropics, giving a triangular 
form to North America. Connected by the Isthmus of Panama, and 
partly connected by the Antilles, is another triangular land mass, also 
tapering to the south. This pair of connected, triangular land areas 
stretches through 135 of latitude, forming the largest north-south 
extent of land on the globe. It terminates in the islands at the south- 
ern tip of South America. 

The Old World Continental Plateau. — The Old World continental 
plateau has an entirely different form. It is, in the first place, broader 
where it commences in the circumpolar region, and it broadens greatly 
to form the huge Eurasian continent. Separated only by inland seas, 
the Mediterranean and Red, it extends into the African continent, 
which, broad in the north, tapers southward, giving the triangular 
form of Africa, which ends in a blunter point than South America 
and some 20 farther north. Elsewhere along the southern border, 
the Old World continental plateau terminates in peninsulas, like Arabia, 
India, and Indo-China. Toward the southeast it is fringed by island 
chains, and a maze of these partly connect it with the island continent 
of Australia. Although separated by straits, some of them very deep, 
Australia is essentially a part of the great Eurasian continental 
plateau. 

The South-pointing Continents. — There is, in fact, with slight 
breaks, continuous continental plateau from southern South America 
through Eurasia to the southern tip of Africa, and to the southern tip of 
Australia, and to the eastern tip of Asia. The only breaks in this 
continental plateau, seemingly greater than they are because the 
ocean water covers the continental shelf s, are (1) the polar basin, and 
its narrow outlets, (2) the Mediterranean and Red seas, and (3) the 
greatest break of all, the series of inter-island channels between south- 
ern Asia and Australia. It may practically be said, therefore, that the 



RELIEF FEATURES OF THE EARTH 



589 



continental plateau areas are massed around a basin in the north 
polar region, and that they extend thence southward, reaching farth- 
est along three tongues, two of which are notably triangular in form. 

The South Polar Continent. — With the oceanic areas the conditions 
are almost exactly opposite. There is around the South Pole a land 
area, called Antarctica, or sometimes the Antarctic Continent, instead 
of a circumpolar basin. It is completely separated from all the lands, 
being most nearly connected with South America. 

The North-pointing Ocean Basins. — Around the Antarctic land 
mass extends a continuous sheet of water, there being prevailingly 
deep ocean basins outside the continental shelf that fringes Antarctica. 
From the Antarctic Sea, the ocean basins extend northward in three 
great tongues, the smallest being the Indian Ocean, between Africa and 
the Australian prolongation of Eurasia. The largest is the Pacific, 
which extends northward to the point where North America and Eurasia 
nearly join. Intermediate in size is the Atlantic, an hour-glass-shaped 
body, narrowed in the equatorial region, and broadening again in the 
North Atlantic. It terminates northward in the roughly circular 
northern sea, the partly cut off Arctic Ocean. 

Islands within the Ocean Basins. — In all these oceans are islands 
and island groups, some small and single, more small and in chains, 
and some, like New Zealand, of large size. All are either (1) the crests 
of mountains rising above the sea, or (2) volcanic cones rising above 
sea level, or (3) coral islands built on volcanic cones or mountain peaks. 
These islands are present in the Atlantic Ocean and in the eastern 
Pacific; they are most numerous in the western Pacific and the 
Indian oceans. With the exception of those that fringe the conti- 
nental plateau, they are all to be classed as phenomena of the ocean 
basins, not of the continents; they are local elevations in the great 
terrestrial depressions. 

Continent Form 

Three of the continents, North America, South America, and Africa, 
are triangular in outline, with the broadest part in the north. Eurasia 
is far more irregular, and Australia is a large irregular island. 



Continent 


North-South 
Dimension 
in Miles 


East-West 
Dimension 
in Miles 


Area in 
Square Miles 


Africa 


4550 
4600 
5100 
2500 

5350 
1050 

3475 


495° 
3I50 
4000 
3400 
6000 
2360 


11,403,000 
7,598,000 
8,559,000 
3,796,000 

16,770,000 
2,974,000 
5,122,000 


South America 

North America 

Europe 

Asia 


Australia 

Antarctica ........ 



590 



COLLEGE PHYSIOGRAPHY 



Africa 

Regular Coast of Africa. — The coast lines of Africa are notably 
regular, with no pronounced peninsulas or extensive fringing island 
chains, and consequently with an absence of enclosed seas. Omitting 




Fig. 380. — Model of Africa. 



the Mediterranean and Red seas, which lie between Africa and 
Eurasia, the most notable indentation is the broadly open Gulf of 
Guinea. The largest projections are the Tunis Peninsula, where the 
Atlas Mountains extend into the Mediterranean, and the Somali 
Peninsula, which projects south of Arabia. Offshore there are some 



RELIEF FEATURES OF THE EARTH 



59i 



small island chains, like the Canary and Cape Verde islands, some 
individual small islands and island clusters, and a single large island, 
Madagascar, separated from the continent by the broad, deep Mozam- 
bique Channel (Fig. 380). 

A narrow continental shelf fringes the continent, varying in width 
up to 50 or 100 miles, and beyond this the continental slope descends 
to the deep sea basin. With the exception of the eastern Atlas 
Mountains there is no mountainous peninsula projecting into the 
sea, and there has been no general subsidence of any part of the 
coast, causing an irregular, drowned coast line. There is, therefore, 
a general scarcity of harbours, though here and there is one due to 
local causes. 

The African Plateaus. — The greater part of Africa is a plateau, de- 
scending rather abruptly along or near the coast to a narrow fringing 
coastal plain which merges into the continental shelf and which is 
delta land opposite the mouths of the river like the Zambezi, Nile, 
Niger, and Congo. In places the plateau edges are upturned, and 
there are mountainous areas back of the coast. Along the northern 
coast is the extensive chain of the Atlas Mountains, reaching a height 
of over 14,000 feet in the western half ; in South Africa are the almost 
insignificant Cape Mountains, and in eastern central Africa are ill- 
defined mountains with numerous volcanic cones. 

The highest of these are Ruwenzori (16,815 feet), Kenia (19,199 
feet), and Kilimanjaro (19,717 feet). In this region is Kirunga, an 
active volcano, 700 miles from the ocean. 

Rivers of Africa. — In descending from the interior upland the 
largest African streams are all interrupted by falls or rapids, the 
Zambezi by Victoria Falls, the Nile by the Cataracts, the Congo by the 
falls at Leopoldville, and the Niger by several rapids. These rapids 
and falls, and associated gorge sections, show clearly the topography of 
the drainage systems, and, therefore, testify to the fact that the 
present condition of the African plateau has not been of long duration. 
The interference with vegetation by the rapids and falls, together with 
the damp coastal lowlands of the tropical zone, and the desert areas 
in the southern and northern parts of the continent have seriously 
interfered with the exploration and occupation of Africa by white 
men. Largely for these reasons less is known about the physiography 
of Africa than any other continent. 



River 



Congo . 
Niger . 
Nile . 
Zambezi 



Length in 
Miles 



2,900 
2,600 
3,400 
1,500 



Area of Basin 
Square Miles 



1,200,000 
563,300 

1,273,000 
600,000 



Ocean 



Atlantic 
Atlantic 
Atlantic 
Indian 



592 



COLLEGE PHYSIOGRAPHY 



South America 



Simple Outline of Continent. — In outline South America is almost 
as simple and regular as Africa. There are no pronounced peninsulas, 
the coast is prevailingly regular, especially the west coast, and there 
are no prominent, fringing island groups. Consequently there are no 
large, enclosed seas or bays. The largest peninsula is in Colombia, 
where a spur of the Andes projects into the Caribbean Sea ; and 
another spur projects as the Isthmus of Panama. Aside from the 
Caribbean Sea, — enclosed between South America, southern North 
America, and the Antilles, — and the Gulf of Panama, — between the 
curving isthmus and the Columbian coast, — there are no other bays 
than small ones like the Gulf of Venezuela, the La Plata estuary, etc. 

Along the northern coast there are numerous small indentations, 
for this is a mountainous coast, in which crustal movements are still 
taking place ; indeed, there is essential continuity from the eastern- 
most spur of the Andes along the Venezuelan coast to the Antillean 
chain. There are also numerous small bays and harbours along the 
lower, eastern coast. In southern South America the coast becomes 
exceedingly irregular. On the low-lying eastern coast there are sev- 
eral broadly open bays ; on the mountainous western side is a system 
of fiords and fringing islands and channelways, like the coast of 
British Columbia and Alaska. In this intricate coast line glacial 
erosion has been a factor, and perhaps the chief one, in the develop- 
ment of the irregular coast line (Fig. 381). 

Although South America terminates in the curving point of Pata- 
gonia, which ends in the island of Tierra del Fuego, the continent 
platform really extends several hundred miles to the east here, and 
includes the Falkland Islands, which rise, not from the deep sea, but 
from the continental shelf. From this broad part the continental 
shelf extends along the east coast with a width of over 100 miles for 
most of the distance to the Antilles, excepting in that part of Brazil 
which projects farthest east. On the west coast there is also a broad 
continental shelf off the coast of southern Chile, from which the numer- 
ous islands rise, but north of this the Andean coast is fringed by only 
a very narrow coastal plain at best. An uplift of 600 feet would very 
materially alter and extend the eastern side of South America, but 
would notably alter the western side only in the southern portion. 

The Andes. — The main element of relief in South America is the 
Andean chain, which reaches its culminating height in Aconcagua, 
22,860 feet, though rising to heights of 18,000 to 20,000 feet in other 
volcanic peaks, such as Chimborazo, 20,498 feet; and Cotopaxi, 
19,613 feet. This vast mountain system extends from the southern 
tip of South America the whole length of the continent, consisting in 
the main of two parallel chains with an intermediate plateau, but with 
various minor ranges and interruptions. The mountain belt is nar- 
rowest and lowest in the south ; it is broadest in the central or Bolivian 



RELIEF FEATURES OF THE EARTH 



593 



portion, and in the north, where it frays out and spreads apart, as 
already stated, one prominent division extending into the Isthmus of 
Panama, one northeastward in the Antillean chain. On the western 







JHe^Bh? 9 w 








: 4§ 


■;■■■■ .'.'. '■ . ■■ 












):M,lm 


..-,- ;-.r.'-;. ;. - . 




?r l^l 








* 


4. 







Fig. 381. — Model of South America. 



face the descent to the narrow, fringing coastal plain is steep, and, 
excepting in the southern portion, the mountain face is remarkably 
regular, though in broad sweeping curves. The eastern face is less 
regular, but is also steep. As we have seen (p. 425), this great moun- 



2 



594 



COLLEGE PHYSIOGRAPHY 



tain system is still in process of growth ; earthquakes are frequent and 
violent, and volcanoes are active. 

Highlands of Guiana and Brazil. — There is a mountainous high- 
land in the north of South America, known as the Guiana or Vene- 
zuelan Highland, which is said to reach an elevation of 11,000 feet. 
A third highland occupies eastern Brazil and is known as the Brazilian 
Highland. It is a denuded mountain and plateau region of ancient 
origin and has a general elevation of from 2000 to 3500 feet, attaining 
an altitude of 9000 to 10,000 feet in the peak of Itatiaia in the south. 

South American Plains. — Between these three highland regions is 
an extensive area of plains, made of sediments worn from the border- 
ing mountains and mainly deposited in the ocean during a former, 
lower stand of the land. The plain is continuous from the pampas of 
southern South America, where it extends from the eastern base of 
the Andes to the sea, northward to the mouth of the Orinoco River. 
It narrows between the Brazilian Highlands and the Andes, but 
broadens farther north, and there, in the Amazon valley, extends from 
the Andes to the Atlantic. These plains, connecting the highlands 
from which their sediments were derived, give continuity to the land 
by binding the parts of the mountain skeleton together. 

Drainage of South America. — The drainage of South America 
is determined by the highland areas. Only short streams descend the 
Andes toward the west, though some of these in the south head on the 
eastern side of the mountains. Other streams flow eastward to join 
the Orinoco, Amazon, and Parana drainage, and a few flow straight to 
the ocean in Argentina, and northward to the Caribbean in Colombia 
and Venezuela. The Guiana Highland drains primarily northward 
into the Orinoco and southward into the Amazon, while the Brazilian 
Highland has a radiating system of streams, some flowing indepen- 
dently to the sea, but the majority entering the Parana or the Amazon. 

Besides the numerous smaller streams there are three great rivers 
flowing across the plains, the Parana southward, the Amazon and 
Orinoco eastward. With numerous tributaries fed from mountain 
regions, in a climate of abundant rainfall, these three rivers have large 
volume ; and since they flow over broad, level plains, not greatly ele- 
vated above sea level, they have primarily moderate slope, and are, there- 
fore, useful for navigation. Unfortunately they lie mainly in the tropi- 
cal zone, a region of unheal thfulness and sparse settlement, so that their 
usefulness is thereby diminished, the lower Parana which lies in the 
temperate zone being less influenced in this respect than the others. 



RELIEF FEATURES OF THE EARTH 595 



North America 

Western Coast. — In outline North America is far more irregular 
than either Africa or South America. Like them it has a general 
triangular shape, but it departs from this simple form very materially. 

In the northwest is the great projection of Alaska, bordered by a 
broad continental shelf which extends under Bering Sea to Asia and 
northward into the Arctic. Along this shelf the continental plateau is 
connected with that of Asia, and only a moderate uplift would unite 
the two continents by a broad lowland several hundred miles in width. 
From Alaska the narrow Alaska Peninsula extends southwestward and 
is continued by a narrow, submarine mountain ridge, upon which rise a 
number of volcanic islands with numerous active and extinct cones. 
These islands, the Aleutian, form the southern boundary of the shallow 
Bering Sea. 

Southeastward from Alaska down to the narrowest part of the con- 
tinent in the Isthmus of Panama, the coast is moderately regular on the 
whole, though with several broad curves. The regularity is broken 
in the north, where the sea occupies mountain valleys which have been 
profoundly deepened and broadened by glacial erosion. Here for a 
thousand miles a vessel may sail along the famous Inside Passage, 
shut out from the ocean by a maze of islands. The regularity is also 
broken by the long, narrow peninsula of Lower California, which 
encloses the deep, narrow Gulf of California. It is a southern spur 
of the Coast Ranges. 

This entire coast, from Panama to the Aleutian Islands, is, moun- 
tainous, and throughout much of the distance there has been recent 
uplift, and in places, as near Mount St. Elias and near San Francisco, 
crustal movements are still in progress. This coast resembles that of 
western South America in rising out of deep water for most of the 
distance, with only narrow strips of coastal plain and continental 
shelf here and there. 

Northern and Eastern Coast. — The northern coast of North 
America is low and irregular. Much of it is a worn-down mountain 
region of low relief, which, by subsidence, has been partly drowned, 
forming numerous straits and bays, of which the largest is the shallow 
Hudson Bay. The highest parts of this old land rise as islands and 
peninsulas, the largest of the islands being Baffin Land, and the 
largest peninsula Labrador. These islands and peninsulas rise above 
a broad continental shelf, which stretches northward to the deep polar 
basin. Greenland is essentially a part of this continental plateau, 
being really a part of it in the north, but in Baffin Bay being separated 
by a deep basin. If the land were uplifted, so as to raise this broad 
northern continental shelf above the sea, there would still be a long, 
deep bay between Baffin Land and Greenland, which would then be a 
peninsula projecting southward from the northern part of the con- 
tinent (Fig. 382). 



50 



COLLEGE PHYSIOGRAPHY 



The irregular, submerged coast of the old mountain land extends 
southward along the east coast to New York. Here also the coast is 




Fig. 382. — Model of North America. 



very irregular with numerous bays, peninsulas, and islands, the largest 
being the Gulf of St. Lawrence, the peninsula of Nova Scotia, and the 



RELIEF FEATURES OF THE EARTH 597 

island of Newfoundland. There is also a broad, fringing continental 
shelf, attaining greatest breadth southeast of Newfoundland, where 
lie the shallow Fishing Banks of Newfoundland. 

Southward from New York to Honduras there is a fringing coastal 
plain, attaining special breadth in Florida and Yucatan, two projecting 
peninsulas which rise as plateaus out of the sea, but with only a part of 
their size revealed because of the bordering continental shelf now cov- 
ered by shallow water. This coastal plain is low and only recently 
raised above the sea, though now somewhat irregular by a still more 
recent subsidence, giving rise to shallow bays at the stream mouths. 

Antillean Region. — Between southern United States and South 
America the conditions are more complex. The two continents are 
connected by the narrow irregular mountainous land of Central 
America. They are also nearly connected by the Antillean chain, a 
great mountain range rising from the deep ocean floor. Between this 
chain and the mainland of North and South America are the two 
partly enclosed seas, the Gulf of Mexico and the Caribbean Sea, 
shut in not only by the visible land, but even more so by a submerged 
mountain ridge, which is broken across by only two or three deep 
passages. 

The Antillean chain, which is continuous with the Andes of northern 
South America, sweeps in a great bend along the lesser Antilles and 
through Porto Rico and Haiti, where it splits into three parts. The 
northernmost, plateau-like, extends through the Bahamas and the 
shallow banks on which they lie, and, with only a narrow break, to 
the Florida plateau. The southernmost extends from the western 
point of Haiti through Jamaica and along a broad submarine ridge to 
the point of Honduras. The central branch extends through Cuba 
nearly to the Yucatan plateau ; but a submarine range also extends 
westward from southern Cuba. This branching mountain range, 
together with the peninsula of Yucatan, serves to so divide the great 
inland sea as to lead to its having the two separate names of Gulf of 
Mexico and Caribbean Sea. 

North American Cordillera. — North America, like South America, 
has its main mountain area in the west, but the mountain belt is far 
broader and more complex. The North American Cordillera is con- 
tinuous from southern Mexico to northern Alaska, and westward 
through the Aleutian Islands. The Rocky Mountains, varying in 
character and given different names, are traceable from northern 
Mexico to western Alaska. West of these mountains is a broad 
plateau area, mountainous in places, also traceable from southern 
Mexico to northern Alaska. The Pacific Mountains fringe the coast 
continuously from the southern part of Mexico to the Aleutian chain. 
Throughout this belt there has been uplift of recent date, and through- 
out it is a region of recent volcanic activity ; but only at the northern 
and southern ends is it at present a region of active volcanoes. The 
culminating point in this mountain belt is Mount McKinley in Alaska, 



59 8 COLLEGE PHYSIOGRAPHY 

20,464 feet; but the volcano Orizaba in Mexico is 18,314 feet; Pike's 
Peak in the Rockies is 14,111 feet; Mount Whitney in the Sierra 
Nevada is 14,502 feet ; and there are many other peaks above 10,000 
feet. 

In Central America there is a complex of mountains, apparently 
more related to the Antillean chain than to the Andes or the moun- 
tains of western North America. In this mountain section, as in the 
Antilles, evidence of recent growth is abundant, and active volcanic 
cones are numerous. The Central American Mountains attain eleva- 
tions of 12,000 to 13,000 feet. The Antillean Mountains rise from an 
ocean platform, 15,000 feet or more below sea level, and rise in Cuba to 
8600 feet in Pico del Turquino. These mountains, therefore, rise 
fully 25,000 feet above their base beneath the sea, and even more if 
reckoned from the Blake Deep near Porto Rico, which is 27,360 feet, 
while Mount Yunque in Porto Rico is 3609 feet, and Lowa Tina in 
Haiti, not far distant, is 10,300 feet, or 37,660 feet above the sea bot- 
tom of the Blake Deep. 

Appalachian and Laurentian Highlands. — Compared to the western 
mountains, the Appalachian system is both small and low. It extends 
northward from Alabama through New England, Nova Scotia, and 
Newfoundland, being ordinarily no more than 2000 or 3000 feet in 
elevation, though rising to a height of 67 n feet in Mount Mitchell in 
North Carolina, and 6279 feet in Mount Washington in New Hamp- 
shire. These mountains are old and much-denuded, and in this 
respect contrast also with the western mountains. 

The ancient Canadian mountains, or Laurentian Highlands, now 
worn to a plateau of low relief, attain considerable elevation in only 
one part, namely, in northern Labrador. Here are the highest moun- 
tains of eastern North America, but their exact elevation is unknown. 

Plains of North America. — Between the Appalachian and Rocky 
mountains and northward to the Laurentian Highlands are broad 
plains which also fringe the eastern Rocky Mountains northward 
to the Arctic. Coastal plains border the eastern and southern coasts, 
and a broad plateau lies between the Rocky and Pacific mountain 
chains. 

Drainage Features. — The chief drainage is determined by the four 
great features of the continent: (1) the eastern mountains, (2) the 
western mountains, (3) the Laurentian Highlands, and (4) the inter- 
vening plains. The St. Lawrence system lies essentially along the 
southern boundary of the Laurentian Highlands, though crossing 
spurs of it in the extreme west, and north of New York. It enters the 
sea by crossing the northern Appalachian system. Aside from the 
tributaries to the St. Lawrence, the Laurentian Highlands shed their 
waters mainly northward through a large number of lake-interrupted 
streams, none of which are of much importance because flowing 
through a country of sparse population into a region of even 
less value. 




Plate X 



t5 /JT^iN^rove-rr A d Newbury 



A \/Met>ffueny', V //(, ^^Weorgetowu 

EeML^Li: / / }?elham^ j \ ^aW r ? n ° e if ^0^ 

5\ \~"/'~7— V7""4"^ \^^/^vV ndover lMniwoova T ' 

V PonnnAn/ /- A Tyngsboro V /\l\ Tjlpswich 



Plum Island 



Part of the peneplain of southern New England, showing the harbours of Boston, Fall River, New Bed- 
ford, and adjacent cities, and the dense railway and street-car net of a long-settled industrial region 
in eastern United States. Contour intervals 50 and 100 meters. (From Boston Sheet, North K 19. 
International Map of the World on the scale of 1 : 1,000,000.) 



RELIEF FEATURES OF THE EARTH 



599 



The Appalachian Mountains shed water eastward by numerous 
short streams and westward to the Mississippi, which drains by far the 
greater portion of the interior plains. The Rockies also shed water 
to the Mississippi in their central portion ; in the south by smaller, 
shorter streams in Mexico directly to the sea, and farther north to the 
Rio Grande. In Canada water flows from the mountains eastward to 
the Mackenzie and to the Saskatchewan-Nelson. From the Rocky 
Mountain divide two large streams flow westward to the ocean, the 
Colorado and the Columbia, and numerous small streams flow into the 
Great Basin. In Mexico and Central America the streams are all 
short, and in Mexico there is a large area of interior drainage. Numer- 
ous streams from the mountains flow to the Pacific north of the Colum- 
bia, but the largest is the Yukon, which enters the Bering Sea arm of 
the Pacific. 

Thus the drainage of North America is complex, and many of the 
streams are of little or no use for navigation. The two principal 
exceptions are the St. Lawrence and Mississippi systems, which lie in 
part in the great area of central plains. 



River 


Length in 
Miles 


Area of Basin 
Square Miles 


Ocean 


Arkansas 

Colorado 

Columbia 

Mackenzie 

Missouri 

Missouri-Mississippi 

Saskatchewan-Nelson .... 

Ohio 

Rio Grande 

St. Lawrence 

Yukon 


2,170 
2,000 
1,400 
2,868 
3,000 
4,3oo 
3,840 
975 
1,800 
2,600 
2,300 


185,671 
225,049 
216,537 
677,400 

.527,155 
1,257,000 
486,500 
201,720 
240,000 
565,000 
330,000 


Atlantic 

Pacific 

Pacific 

Arctic 

Atlantic 

Atlantic 

Atlantic 

Atlantic 

Atlantic 

Atlantic 

Pacific 







Eurasia 



A Complex Mountainous Continent. — From every standpoint this 
is the most complex of the continents. Like the others it is roughly 
triangular in shape, but with the apex of the triangle in the west instead 
of in the south. It is the largest of the continents, the most mountain- 
ous, and the most irregular. The continents are so complex that it is 
very difficult to describe them in general terms. 

Perhaps the most striking feature is the great complex of mountains, 
extending with little break from Spain to China. These mountains 
vary in height, in direction, and in characteristics, but in the main 
they are young, and throughout much of the distance they are at pres- 
ent rising, judging from the frequency of earthquake shocks. 



6oo COLLEGE PHYSIOGRAPHY 



Europe 

Youthful Southern Mountains. — Spain is a somewhat broken 
plateau, bordered on one side by the Pyrenees, 11,168 feet, and their 
continuation, the Cantabrian Mountains, and, on the other, by the 
Sierra Nevada, 11,420 feet. East of these are the Alps, 15,781 feet 
with the Swiss plateau to the north, and the Jura Mountains, 5500 
feet, on its other border. On the western end the Alps curve sharply 
and extend to the Apennines, which pass southward through Italy, 
forming the backbone of this peninsula, then curve into Sicily, and by 
a submarine ridge to the Atlas Mountains of Tunis. The Alps dis- 
appear in Austria, but continuous mountain ranges extend southeast- 
ward through the Balkan Peninsula to the islands south of Greece, 
and into Asia Minor. Other mountains sweep in a great curve around 
the northern and eastern sides of the Hungarian Plain. These moun- 
tains are known by different names in their several parts, one portion 
being the Carpathians. Farther east are the Caucasus. 

These mountains are all young, many of them are still in process of 
uplift, and the depressions near them are in places still sinking. This 
is true especially of parts of the Mediterranean, a down-sunken part 
of the earth's crust developed in connection with the recent great 
mountain movements. This complex of mountains determines the 
main topographic features and outline of southern Europe. The 
peninsulas and islands are mountain areas, and, because of the irreg- 
ularity of direction of the mountains, these land forms are also irreg- 
ular. Valleys exist between the mountains and in the areas where 
the mountains curve, some filled with water like the Adriatic, others 
dry land like the Hungarian Plain and the Po valley, an extension of 
the Adriatic depression. 

Older Western Mountains. — In southern Ireland, Belgium, Ger- 
many, and Austria, north of the Alps, is an ancient mountain range 
worn to low relief, and the seat of recent volcanic activity. A very 
extensive mountain system of still more ancient date extends from 
France, through the British Isles, and to the northern end of the 
Scandinavian Peninsula. This has also been much reduced since its 
period of formation, but is still fairly rugged in Scotland and in 
Scandinavia, in the former reaching an elevation of 4406 feet, in the 
latter 8400 feet. 

European Plains. — Between the Scandinavian upland and the 
mountains of southern Europe is a great plain, broadening eastward 
where it extends into Asia. This plain is crossed by the low Ural 
Mountains, which extend 1500 miles, rising to a height of over 5000 
feet in the north and south, but very low in the middle. The moun- 
tain axis of the Urals is continued northward in the islands of Nova 
Zembla. 

Rivers. — Each peninsula of Europe has drainage of its own, but 
with this exception the drainage radiates mainly from two centres, 



RELIEF FEATURES OF THE EARTH 



60 1 



one in the lofty Alps, the other in the low plains of central Russia'. 
Some of the latter flow northward, others southward into the Black 
Sea, or into the Caspian as in the case of the Volga. The Alpine drain- 
age finds its way southward in the Po, eastward in the Danube, north- 
ward in the Rhine, and "southwestward in the Rhone. All these 
streams flow across extensive plains; and other well-known streams 
cross these plains, like the Elbe in Germany, and the streams of France, 
which radiate outward from the central highland. Because of their 
low grade, many of these streams are navigable. 



River 



Danube 
Dnieper 
Dwina 
Elbe . 
Po . . 
Rhine . 
Rhone . 
Seine . 
Thames 
Volga . 



Length in 
Miles 



1,770 
1,200 
1,000 

725 
400 
800 
500 
482 
228 
2,400 



Area of Basin 
Square Miles 



300,000 

242,000 

140,000 

55,000 

27,000 

75,000 

38,000 

30,300 

6,lOO 

503,300 



Ocean 



Atlantic 

Atlantic 

Arctic 

Atlantic 

Atlantic 

Atlantic 

Atlantic 

Atlantic 

Atlantic 

Caspian Sea 



Irregular Coast. — The coast line of Europe is one of extraordinary 
irregularity. Some of it is mountainous, especially in the south, and 
in the Scandinavian-Scottish mountain region ; but much of it is 
low-lying, notably where the plain extends to the sea along the English 
Channel, North Sea, and Baltic Sea. In the Mediterranean the 
irregularity is due mainly to movements associated with mountain 
growth. Locally, as in Greece and along the eastern Adriatic coast, 
subsidence has produced a typical drowned coast. The Black Sea 
occupies a deep basin between the mountains of Asia Minor and the 
Caucasus and their eastern continuation in the Crimean Peninsula. 
The Caspian, really in Asia, occupies a basin on the border between 
the Caucasus and other mountains of western Asia, on the one side, 
and the great Eurasian plain on the other. Owing to the arid climate, 
it does not rise to the point of overflow, and its surface lies 85 feet be- 
low sea level. 

The northern and western outline of Europe is determined mainly 
by subsidence. This is not true of the plateau peninsula of Spain nor 
of the Bay of Biscay, the latter occupying a deep basin between the 
Cantabrian Mountains and the coast of France, which here projects to 
form the peninsula of Brittany, the southernmost part of the Scandina- 
vian-Scottish mountain system. From this point northward, and 
thence along the Arctic coast, the coastal features are mainly the 
result of subsidence. The edge of the continental plateau, as deter- 



602 COLLEGE PHYSIOGRAPHY 

mined by the position of the continental slope, lies well outside of 
Brittany, Ireland, and Scandinavia. The British Isles rise from the 
continental shelf; the English Channel, North Sea, and Baltic are 
entirely on it ; and north of Scandinavia and Russia it extends in a 
broad submerged shelf northward to the deep polar basin. The 
subsidence has given rise to great irregularity in detail, as in the 
estuary of the small river Thames and to the larger features of outline 
as well. The higher parts of the old mountain land rise to form the 
British Isles and Scandinavia, while the lower parts are submerged 
between Brittany and England, and between Scotland and Scandina- 
via. The submergence has also permitted the ocean waters to spread 
over a broad tract of lowland plain in the southern part of the North 
Sea and in the Baltic Sea, the two being separated by a higher part of 
the plain where the Danish islands lie, and the peninsula of Jutland 
projects northward, almost uniting with the low, much-denuded moun- 
tain land of southern Sweden. The fiords of Norway are due wholly 
or in part to glacial erosion. 

The irregular coast line of Europe giving a multitude of harbours, 
and the enclosed seas penetrating far inland and offering both oppor- 
tunity for learning navigation and providing routes to a wide tract of 
country, have been factors of high importance in the development of 
Europe. The plains have given basis for agriculture, the navigable 
rivers have served as highways, and the mountains, of varied height 
and direction, have served as protection. The physiography gives 
ample basis for the high development of Europe ; it has had a power- 
ful influence in guiding its history; and it offers explanation of the 
large number of diverse nationalities which now hold possession of it. 

Relation to Adjacent Continents. — Europe cannot be separated 
from Asia by any natural dividing line ; it is really a west-extending 
peninsula of Eurasia. Although completely separated from Africa 
by the Mediterranean, it is almost connected at the Strait of Gi- 
braltar, is not far separated between Sicily and Tunis, and is only a 
little farther removed from Greece and Crete. The Mediterranean is 
really a huge basin in the great Eurasia-Africa continental plateau, 
similar on a larger scale to the Black Sea. 

Europe is much more definitely separated from North America. 
The deep polar basin lies between them, but they are less separated 
farther south. A long, broad submarine ridge extends from Scotland 
to Iceland with the Faroe Islands rising about it ; and Iceland is only 
slightly separated from Greenland, which, as we have seen, is really 
continuous with the North American continental plateau. Thus, if 
the continental slope is taken as the boundary of the continents instead 
of the present water margin, Europe and America are all but united. 
Even with the present distribution of water and land, it is but a series 
of short steps from Scotland to the Faroe Islands, to Iceland, to 
Greenland, and to Labrador. It was this shortest, easiest route that 
the Norsemen found in the first discovery of America. 



RELIEF FEATURES OF THE EARTH 603 

Asia 

Central and Southern Mountains. — The mountains of southern 
Europe do not terminate in the Balkan Peninsula, but are continued 
on through Asia Minor and Persia. On the northern side are the 
Caucasus, really in Europe, with one peak, Mount Elbruz, 18,200 feet 
high, but in Asia Minor and Persia the mountains are much lower and 
less definite ranges, with intermediate valleys and plateaus. Moun- 
tain growth is still in progress in this region, and earthquakes are fre- 
quent and destructive, and there are numerous volcanic cones, at least 
one of which, Ararat, is to be classed as active. These mountains 
extend in various directions, but their general trend is eastward. 

The great Arabian Peninsula is mainly, a plateau, separated from the 
mountainous country of Persia and Asia Minor by a broad depression. 
This lowland is occupied in the lower portion by the Persian Gulf and 
in the upper by the plain of the Tigris and Euphrates rivers. Arabia 
in form and characteristics seems less related to Asia than to Africa, 
with which it is connected by the Isthmus of Suez and nearly con- 
nected at the Straits of Bab-el-Mandeb. Between the two lies the 
long, narrow, and deep Red Sea basin, which with moderate changes 
in the southern end would become transformed to a lake. This is 
probably a rift valley, the bottom of which has sunk, much as has been 
the case in the long, narrow trough extending out of its northern end 
and in which the Dead Sea lies. 

East of Afghanistan and Bokhara comes a great complex of moun- 
tains. From a great mountain knot, the Pamir, called the " roof of 
the world," three main branches spread out, one the Tian Shan, or the 
Mountains of Heaven, extending eastward into Mongolia, another the 
Kuen Lun Mountains farther south, extending to China, and the two 
enclosing the East Turkestan Plateau or the Tarim Basin. Other 
mountains farther north border and extend into Mongolia. The 
third branch, the Himalayas, swing southeastward along northern 
India, and between these mountains and the Kuen Lun Mountains is 
the plateau of Tibet. This is the greatest mountain complex in the 
world and it includes the highest mountain peaks, — Everest in the 
Himalayas, 29,002 feet, and others approaching this elevation, — while 
between the spreading chains are the highest plateaus in the world — 
Tibet being 10,000-15,000 feet in elevation (Fig. 383). 

South of the mountains of Afghanistan and the Himalayas is a broad 
lowland, occupied by the Indus, Ganges, and Brahmaputra rivers. 
This lowland is, without doubt, a depression associated with the moun- 
tain uplift to the north. Within it movement is evidently still in 
progress, for it is the seat of frequent destructive earthquakes. It has 
been filled and raised above sea level by the deposit of sediment 
brought by the rivers from the Himalayas, which, because of their 
youth, are high, rugged, and the seat of rapid denudation. On a large 
scale, this is analogous to the condition in the Po valley at the southern 



604 



COLLEGE PHYSIOGRAPHY 



border of the Alps. South of the lowland is the triangular plateau of 
India, between which and Arabia lies the broad, deep Arabian Sea. 
Ceylon is essentially a part of India, being separated by shallow water ; 
and off the west coast of India is a north-south submarine mountain 
chain, reaching south of the equator, and with numerous small island 
peaks rising above sea level. 

In the east the mountains branch, their direction changes, and in 
place of east-west and northwest-southeast mountains there are 




Fig. 383. — Model of Eurasia. 



prevailing north-south and northeast-southwest mountains. A south- 
ward swing of the mountains east of the Himalayas gives rise to a 
series of north-south ranges in Indo-China, which, with India, encloses 
the broad, deep Gulf of Bengal. One branch of these mountains, 
extending through Burma, is continued to Sumatra by a submarine 
mountain ridge, on which a chain of small islands rises above sea level. 
Another branch extends down the Malay Peninsula, swinging east- 
ward toward Borneo. The eastern lobe of the peninsula of Indo- 



RELIEF FEATURES OF THE EARTH 605 

China is continued by a submarine plateau, which occupies all the space 
between Siam, Borneo, Java, and Sumatra. 

Eastern Mountains and Volcanoes. — These mountains swing east- 
ward through the East Indies and New Guinea, and beyond this, in the 
western Pacific, are severat submarine mountain ridges, from many of 
which volcanic cones rise. Throughout this island belt, mountain 
growth is in active progress, earthquakes are frequent, and active 
volcanoes numerous. The mountains also swing northeastward 
through Borneo, the Philippines, Formosa, the Japanese Islands, and 
Kamchatka. This is also a growing mountain range, with frequent 
earthquakes and active volcanoes. It rises from the ocean floor at a 
depth of 15,000 to 20,000 feet, and in places more, and extends more 
than 10,000 feet above sea level in many places. In Java, for instance, 
there are peaks 12,000 feet in height, in the Philippine Islands, 10,000 
feet, in Japan over 12,000 feet, and in New Guinea over 13,000 feet. 
Most of these lofty peaks are volcanic cones, like Fujiyama in Japan, 
which is 12,365 feet in elevation. 

This fringing-island-mountain-chain encloses a succession of more 
or less oval basins, occupied by epicontinental seas — the South China 
Sea, the East China Sea,, the Japan Sea, and the Okhotsk Sea, while 
still beyond lies the Bering Sea, also island-enclosed. The mainland 
coast is also mountainous in large part, and, in one place a mountainous 
peninsula projects. This Peninsula of Korea extends toward Japan, 
with which it is connected by a submarine plateau. It helps to enclose 
the Yellow Sea on the south and the Japan Sea on the north. The 
Japan Sea is enclosed on the south by the long island of Sakhalin, the 
crest of one fork of the Japanese mountain range, the other branch 
extending to the peninsula of Kamchatka by a submarine mountain 
ridge, from which the Kurile Islands rise. 

The extraordinary irregularity of the Asiatic coast is thus, in the 
main, determined by mountain and plateau blocks with intermediate 
depressions. Some of the mountains are on the mainland, some pro- 
ject from the mainland along peninsulas, and some fringe the coast as 
submarine chains with island crests. These fringing islands are most 
numerous and occupy the broadest area in the south, where the ranges 
curve in a great lobe. 

Siberian Plain. — Northern Asia is mainly a plain and a hilly region 
of low relief. This extends to the Arctic, where subsidence has 
given rise to an irregular coast, like that of Arctic Europe ; and here, 
also, there is a broad continental shelf, as in the case of all Arctic lands. 
Most of the Asiatic coast is fringed by a continental shelf, varying 
greatly in width from place to place, and becoming especially broad in 
some of the enclosed seas and in the great mountain bend between 
Borneo, Sumatra, and the Malay Peninsula. 

Features of the Asiatic Mountains. — The lobate form of some of 
the Asiatic mountains is a striking feature (Fig. 384.) Some of these 
lobes, like that just mentioned, are remarkable for their great size 



6o6 



COLLEGE PHYSIOGRAPHY 



and for the extent of the curvature. Others, like the Himalayas, 
merely bow out in moderate curves. Similar lobation of mountain 
chains is a common phenomenon ; for mountain chains rarely extend 
in straight lines, but have a curving outline, as in the Andes, the 
Pacific mountains of western North America, the Aleutian Islands, 
and many others. Also the recurving of mountains is a common 
phenomenon, as in the Alps, the Carpathians, and the northern 
Andes and West Indies. Still another noteworthy feature is the 

mountain knot like that 
of the Pamir, beyond 
which the mountains 
spread apart like a 
frayed rope, as the 
northern Andes and the 
Alaskan mountains do. 

The Asiatic mountains 
not only affect the con- 
tinent outline and the 
land topography, but 
they produce important 
effects on the climate, 
and have had a dominat- 
ing influence on the dis- 
tribution of people. 

Rivers of Asia. — The 
mountains also pro- 
foundly affect the drain- 
age. A part of the 
drainage is into interior basins, especially in the central part, from 
the Caspian and Aral seas, both without outlets, to Mongolia and 
Tibet. In this basin region there are extensive deserts, like the 
Desert of Gobi and the great Tarim Basin. Some of the lakes, such 
as Lob Nor in the Tarim Basin, have had notable fluctuations in level, 
as was the case with Lakes Bonneville and Lahontan in the Great 
Basin of western United States. The abandoned beaches of these 
ancient lake levels in central Asia are well preserved. Towns have 
been abandoned because of the climatic oscillations which caused 
these changes in the lakes and rivers. From the peripheries 
of this area of interior drainage, water is shed outward in all di- 
rections : the Ob, Yenisei, and Lena northward to the Arctic ; the 
Amur, Hoang-ho, and Yangste-kiang eastward to the fringing seas 
along the Pacific coast ; the Mekong, Salwen, Brahmaputra, Ganges, 
and Indus southward to the seas branching from the Indian Ocean. 
Omitting the minor streams and those of the islands and peninsulas, 
the drainage of Asia is of very simple plan : it is radial from a great 
central highland area of mountain and plateau, within which there is 
interior basin drainage. 




384. — The arcuate plan of the mountains in part 
of India. (St. Martin and Schrader.) 



RELIEF FEATURES OF THE EARTH 



607 



River 



Length in 
Miles 



Area of Basin 
Square Miles 



Ocean 



Amur . . 
Brahmaputra 
Ganges . . 
Hoang-ho 
Indus . . 
Irawadi . . 
Lena . . . 
Mekong . . 
Ob ... . 
Salwen . . 
Yangtse-kiang 
Yenisei . . 



2,800 
1,800 
1,500 
2,700 
1,800 
1,500 
2,800 
2,800 
3,200 
i,75o 
3,200 
3,000 



520,000 
425,000 
440,000 
570,000 
372,700 
158,000 
950,000 
280,000 
1,000,000 

548,000 
1,500,000 



Pacific 
Indian 
Indian 
Pacific 
Indian 
Indian 
Arctic 
Pacific 
Arctic 
Indian 
Pacific 
Arctic 



Australia and Oceania 

Relation to Eurasia. — Australia, greatest of the islands south of Asia, 
presents no great complexity of topography. It is surrounded by a 
broad continental shelf, beyond which there is a steep descent to the 
ocean floor, excepting in the north, where there is prevailing shallowness 
through the East Indian region to Asia. Australia may, therefore, be 
considered as essentially a part of the great Eurasian continental 
plateau. 

Regular Coast. — The coast line is moderately regular, though, 
along a large part of the coast, subsidence has given rise to a series of 
small bays and harbours. There is only one good-sized bay, the Gulf of 
Carpentaria in the north. Along the northeastern shore, for a thou- 
sand miles or so, is the Great Barrier Reef, the largest coral reef in the 
world, and between it and the coast is a shallow lagoon of variable 
breadth and depth. The only island of large size is Tasmania, rising 
from the continental shelf, and therefore essentially a part of the 
continent (Fig. 385). 

Plateau of Australia. — Most of the interior of Australia is a desert 
plain and low plateau, with low, short mountain ranges, 1000 to 2000 
feet high, as in the Great Basin of western United States. Like the 
Great Basin also a large part of Australia is a region of interior drain- 
age. Short streams flow to the sea from the border of the interior 
desert, and one, the Darling, the largest in Australia, rises, together 
with its tributary, the Murray, in the eastern mountains and flows 
to the sea across the desert. 



River 


Length in 

Miles 


Area op Basin 
Square Miles 


Ocean 


Cooper-Barcoo 

Darling 

Murray 


800 
I,IOO 
1,000 




Lake Eyre 




270,000 


Indian 



6o8 



COLLEGE PHYSIOGRAPHY 



Eastern Mountains. — The eastern mountains, the only notable 
mountain range in Australia, extend along the eastern coast, though 
separated from it by from 30 to 100 miles or more of low, hilly land and 
plains, fertile and well-watered. These mountains are neither lofty 
nor rugged, for they were raised at an earlier period and have long been 
denuded. They are not the seat either of vigorous earthquakes or 




Fig. 385. — Model of Australia. 



volcanic activity. They consist of a series of low, broken ranges, and 
the highest peak, Mount Kosciusko, rises only 7336 feet, and else- 
where the highest elevations commonly range between 4000 and 7000 
feet. 

New Zealand. — Twelve hundred miles east of southern Australia 
is the New Zealand mountain chain, severed by a submerged pass to 
form two large islands, each over 500 miles in length. This mountain 
range continues some distance on either end in a submarine ridge, and 
is possibly continuous toward the northeast with the ridge on which 
the Tonga and Friendly islands lie, and toward the northwest with the 
ridge from which Norfolk Island rises. Both the North and the South 



RELIEF FEATURES OF THE EARTH 609 

islands of New Zealand are mountainous, attaining an elevation of 
12,349 feet in Mount Cook in the South Island, where the mountains 
rival the Alps in height and in grandeur. The mountains in the North 
Island are lower, but here there are numerous volcanic cones, two of 
them being active. This mountain chain is one of recent uplift and is 
still growing, for vigorous earthquakes are experienced, and, in the 
North Island, volcanic activity has not died out. 

Islands of the Pacific. — The many islands in the Pacific east of 
Australia and Asia are mainly mountain crests or volcanic cones, rising 
from submarine mountain chains and often of considerable length. 
Others are isolated volcanic cones or groups of volcanoes, some reach- 
ing the surface only by the veneer of coral reef upon them. 

Antarctica 

Too little is known of this land mass to warrant more than the brief- 
est mention of it. One cannot yet be certain that there is continuous 
land there, for it is possible that there is an archipelago of large islands 
united by a great ice cap. Such evidence as we have points toward 
the conclusion that it is really a land of continental proportions, 
with much lofty mountain topography, including Mt. Fridtjof Nansen, 
15,000 feet. At the South Pole the ice plateau is about 10,700 feet 
high, and the mean altitude of the continent is approximately 6000 
feet. There are at least two volcanoes, one, the active cone Erebus, 
rising about 12,365 feet. The continent has no rivers, all the drainage 
being glacial, the Ross Barrier having retreated 20 to 30 miles since 
1839. The land is fringed by a broad, continental shelf, and there is 
geological reason for suspecting that, at some former period, Antarc- 
tica and South America may have been united. The coal deposits 
within 5 of the pole suggest vast changes of climate, such as would 
have taken place with a shifting of the earth's axis during the geo- 
logical past. 

References to Literature 

See the bibliographies of the chapters on Plains and Plateaus (p. 521), 
Mountains (p. 578), Shorelines (p. 384), The Ocean (p. 665). etc. The best 
treatments of the relief features of the earth and its several continents are in 
Mill's International Geography, Reclus' Earth and Its Inhabitants, Stanford's 
Compendium of Geography and Travel, and Suess' Face of the Earth. 

TOPOGRAPHIC MAPS 

See the map lists at the ends of the preceding chapters ; for the continents 
see, among others, the Sydow and Habenicht, Kiepert, Oxford, Johnston, 
Wagner and Debes, Kuhnert, Gaebler, Goode, Rand McNally, and other 
physical maps, or the large originals of Howell's models, used as illustrations 
in this chapter. 

For detailed maps of representative parts of each continent on the same 



6io 



COLLEGE PHYSIOGRAPHY 



scale see the International Map of the World on the scale of i 
of which the following sheets are typical : 



1,000,000 (PI. X), 



Boston, North K 19. 

Paris, North M 31. 

Lyon, North L 31. 

Scotland — The Highlands, North O 30. 

The Hebrides, North O 29. 

Rome, North K 33. 

Valencia, North J 30. 



Constantinople, North K 35. 

Budapest, North L 34. 

Buenos Aires, South I 21. 

Santiago de Chile, South I 18. 

I qui que, South F 19. 

Kenhardt, South Africa, South H 34. 

Tokio, North I 54. 



CHAPTER XVII 
THE EARTH'S INTERIOR 

Relation to Earth's Surface 

Since Physiography treats of the earth's surface, it has to some 
seemed outside its province to deal with the interior condition. But 
the surface phenomena are so profoundly influenced by this interior 
condition, and in such important cases directly caused by it, that one 
is constantly encountering questions whose solution must be sought in 
the nature of the earth's interior. This has been illustrated again 
and again in the preceding pages, and in two or three places it has been 
deemed necessary to give some consideration to the interior condition 
in explanation of phenomena of the surface. Now that the surface 
features of the earth have been described with some fulness, it seems 
well to look to their causes in the light of the facts that have been 
presented. 

Sufficient attention has already been paid to the causes of those 
land forms which result from the processes of denudation ; but there 
are a series of phenomena whose causes have not been thoroughly con- 
sidered. These are (i) the existence of the continent plateaus and 
ocean basins, (2) the changes in relative level of land and water, 
(3) mountain formation, (4) earthquakes, (5) vulcanism. These 
phenomena, though treated separately in the preceding chapters 
because of the difference in resulting land forms, are evidently closely 
related in cause. In the main they are surface expressions of conditions 
existing within the earth, even though the results at the surface are 
so widely different. 

In this inquiry as to the underlying cause or causes for the phenom- 
ena in question, we are confronted by the most serious difficulties (1) be- 
cause no direct observations of the interior have been possible, (2) be- 
cause such indirect observations as have been possible, such as the 
surface phenomena which we seek to explain, are capable of explana- 
tion on more than one hypothesis, and (3) because, so far, critical 
facts have not been discovered which will eliminate the multiplicity of 
hypotheses. There is perhaps a fourth difficulty, namely, that 
more than one cause may actually be in operation to produce the same 
surface phenomenon. This is certainly the case with earthquakes, 
which, as we have seen, are caused in numerous ways. 

In view of these difficulties it is not now possible to state either the 
exact condition of the earth's interior or the nature of the processes by 

6n 



6i2 COLLEGE PHYSIOGRAPHY 

which the phenomenon of vulcanism and diastrophism are brought 
about. The best that can be done is to put forward hypotheses; 
and in a book of this nature it will not be possible either to state all 
the hypotheses or to discuss them fully. All that can be attempted 
is to show approximately the state of our knowledge and to state some 
of the leading hypotheses. 

Evidence of Heat within the Earth 

Proof from Volcanoes and Igneous Rock. — There can be no ques- 
tion but that there is a great store of heat within the earth. Locally 
this is proved by the extrusion of molten rock from volcanoes. Over 
wider areas it is proved to have been the case in past ages by the exist- 
ence of former ash deposits, lava flows, dikes, laccolites, and batho- 
lites, many of them in regions where there is no present day vulcanism. 

Proof from Hot Springs and Deep Borings. — Hot springs, often 
in non-volcanic regions, give evidence of the presence of internal heat ; 
and still more widespread are the deep wells, borings, tunnels, and 
mines, which, no matter in what part of the earth they are made, 
invariably show an increase in temperature with increasing depth. 

Rate of Increase of Temperature with Depth. — This rate varies 
greatly, but is always sufficiently rapid to reach the melting point of 
rocks deep in the earth, if it continues. The rates of increase in tem- 
perature of i° range from 20 to 250 feet, perhaps averaging 50 feet. 
An increase of i° for every 60 or 70 feet amounts to a little less than 
ioo° for each mile of depth, 1000 for ten miles, and at depths of 20 to 
30 miles sufficient heat to melt rocks. The deepest boring, which is 
in the mines of South Africa, shows a temperature of 102 ° F. at 8000 
feet. 

Hypotheses as to Nature of Heated Interior. — It is not to be won- 
dered that, with this knowledge, it was early inferred that the earth's 
interior was not only hot but liquid with a solid crust. This was the 
most simple and natural conclusion to draw from the facts. The 
hypothesis of internal liquidity has long been abandoned, but it is still 
believed by many that the interior is so highly heated that, where the 
pressure is relieved, it flows as a liquid. A slight modification of this 
hypothesis is that there are areas in which the pressure is sufficiently 
low to permit melting, and that there exists a liquid substratum, either 
general or local, between the crust and the heated solid interior. A 
rival hypothesis to that of a heated interior is that the heat is purely 
local, being generated in the earth itself. 

Thus there are two diametrically opposed hypotheses, both neces- 
sarily admitting the existence of heat within the earth, but the one 
assuming it to be general and inherent in the earth from some primal 
state, the other assuming it to be localized and arising from conditions 
beneath the surface. These rival hypotheses will receive further con- 
sideration in later pages. 



THE EARTH'S INTERIOR 613 



Condition of Earth's Interior 

Evidence of Solidity. — As intimated in the preceding section, it is 
now quite generally agreed that the earth is essentially a solid, though 
admitting, of course, the presence of some liquid, as we must from the 
existence of volcanoes. By careful pendulum experiments it has been 
shown that the earth as a whole has a specific gravity of about 5.5, or 
five and a half times that of water. Since the specific gravity of the 
crust is between 2.4 and 3.3, it has been assumed that the interior con- 
sists of heavier elements than the crust, perhaps of iron and other 
metals. It is of course true, on the other hand, that the pressure of the 
interior will increase the density and consequently the specific gravity, 
though probably not to the extent required to account for the high 
specific gravity 5.5. 

The evidence of solidity of the interior is along several different lines, 
as follows : (1) If liquid, with a rigid crust, this crust must have been 
growing thicker during the geological ages, and there should, therefore, 
be evidence of decreasing vulcanism from early ages to the present. 
This is not the case, and it is doubtful if at any earlier period there 
was greater vulcanism than in the period immediately preceding the 
present. (2) If the earth consisted of a crust with a liquid interior, 
the tidal forces would distort it twice each day, with resulting buckling 
of the rigid crust. (3) It has "been shown that to produce the oceanic 
tides requires a solid sphere beneath the hydrosphere to a depth of not 
less than 2500 miles. (4) The astronomical phenomena of precession 
and nutation also demand an essentially solid globe, with the rigidity at 
least of glass. (5) The observed rate of travel of earthquake waves, 
after passing through the earth, detected by seismographs, is that of 
travel through a solid. (6) Finally, it has been urged that a solid 
crust could not develop on a liquid globe, for as soon as solidification 
took place, the greater specific gravity of the rock would cause it to 
settle into the liquid. 

These evidences opposing liquidity of the earth's interior are now 
accepted universally as proof that the earth is a solid body. 

Evidence of Plasticity. — Although solid, it does not follow that the 
rock of the earth's interior is incapable of flowage. It is a well-known 
fact that a rigid substance like ice is made to flow under pressure ; 
and steel will also flow under sufficient pressure. In the same way, it 
is inferred, the rocks of the earth will flow as a viscous or plastic solid, 
when subjected to differential pressures of sufficient amount. This 
does not mean melting, nor necessarily the presence of high degree of 
heat, but merely the plastic flow of a solid, as in ice or steel. 

The evidence of such flowage is of several kinds: (1) The rocks 
themselves, those formerly involved in intense mountain formation 
and now exposed by denudation, give evidence of having flowed with- 
out melting. (2) Gravity determinations point clearly to the flowage 
of rocks in the adjustment of differential loads and pressures, as is 



614 COLLEGE PHYSIOGRAPHY 

stated in the next section. (3) Adams has artificially imitated rock 
flowage in a series of careful experiments, thus demonstrating the 
rationality of the inference of rock flowage within the earth. 

From these evidences there seems no basis for questioning the con- 
clusion that the solid earth, whether hot or cold, is capable of deforma- 
tion by the flowage of its rocky materials under high pressures, such 
as exist deep within the earth. 

The solid interior of the earth appears to consist of a central core, 
occupying four-tenths of the radius, and an outer part of slightly 
different character. This has been deduced by Oldham from obser- 
vations of the different rates of propagation of earthquake waves. 

A different view of the earth's interior, however, is that it is partly 
gaseous. This has been proposed by Arrhenius, who holds that be- 
neath the solid outer crust is a molten zone, and beneath that is a 
gaseous centre. Just as the molten portion is capable of becoming 
solid if the pressure were relieved, so the gaseous centre could become 
molten if there were lower temperature. The heat is believed to be 
above the critical temperatures (p. 443) of all the earth materials, but 
the enormous pressure is thought to give this gaseous centre a density 
and rigidity which is quite in accordance with all we know of the 
earth's interior as a whole. This gaseous theory in no way interferes 
with the idea of plasticity in the zone of rock flowage. 

Evidence of Isostasy. — While the earth appears to be solid, it has 
long been known, from carefully conducted pendulum experiments, 
that there are notable differences in density in various parts. In 
general, the continents are regions of less density than normal for the 
earth, and the ocean basins are regions of greater density than normal. 
The reason for these differences is not known, but the fact is well 
established. Two very important deductions are drawn from the 
variations in density of earth material : (1) that water is drawn toward 
the areas of greater density, thus accounting in part for the distribu- 
tion of oceanic waters over the globe, (2) areas of low density become 
regions of relative elevation, and areas of high density of depression. 
With variation in density in a given locality, or with variations in 
mass, adjustment will follow (1) by transfer of surface water, (2) by 
underflowage of the plastic rock. 

The latter occurs as a result of an attempt to maintain isostatic 
adjustment. For example, Hayford finds the United States to be in a 
state of essential isostatic equilibrium, elevation being compensated 
for by decreased density. That is to say, a column from the crest of 
the Rocky Mountains downward, is no heavier than a column from 
the Mississippi valley downward, though some two miles higher. 
From his measurements he finds that the excess or defect does not 
exceed that equivalent to a stratum 250 feet thick at the density 2.67, 
the average density of the surface rocks. 

The theory of isostasy, first outlined by Dutton, is that, if one 
start with a surface in isostatic equilibrium and take away from it, as 



THE EARTH'S INTERIOR 615 

by denudation, or add to it by deposition, the isostatic equilibrium is 
disturbed, and that there will at once follow an adjustment to bring 
about isostatic adjustment to the changed conditions. This change 
will be in the nature of flowage, or as Hayford calls it " an undertow," 
from surrounding regions of higher density to those of lesser density. 
This will cause a settling of the surface in the regions of high density, 
and a rise in those of low density until equilibrium is again established. 
This flowage will occur in what he calls the zone of compensation, 
which he places at not over 87 miles below the surface, nor less than 
62 miles, with a probable mean depth of about 76 miles. 

Besides causing change of level by direct flowage, Hayford infers 
secondary effects as a result of chemical change, and of temperature 
change. He also believes that this phenomenon of isostasy will ac- 
count not merely for slow changes of level over broad areas, but also 
for the faulting and crumpling in mountain growth, due to the drag of 
the rigid crust by the " undertow." 

That there is a tendency toward isostatic equilibrium in the earth's 
surface layers, and that movements of the crust occur as a result of 
disturbance of this equilibrium is now quite generally admitted; 
and it seems that Hayford's careful geodetic studies demonstrate it. 
There are, however, very grave difficulties in the way of acceptance 
of isostasy as an explanation for the larger earth features, and for the 
greater earth movements. For instance, ocean basins, the greatest 
depressions of the earth, are not the seat of the heaviest deposits, as 
would be expected on the theory of isostasy. This theory also fails 
to account for the periods of excessive vulcanism or diastrophism ; for 
the long intervals of freedom from these with accompanying base- 
levelling ; for the rise of mountains in areas of heavy deposition where 
normally depression should continue ; and for other significant phe- 
nomena. 

It seems, therefore, that while we may accept isostasy as a potent 
agent of change on the earth's surface, it fails to account for all dia- 
strophic movements. It is one reason for the observed changes, but 
not the sole reason; nor does it seem probable that it is the most 
potent. 

Changes in Ocean Level. — There is general agreement that the 
level of the oceanic waters is subjected to changes of considerable 
amount in the course of long periods of time. The problem is a com- 
plex one and does not admit of definite mathematical statement be- 
cause of the varied factors involved and because of the lack of data for 
exact calculation; but of the broad conclusion that the ocean level 
fluctuates, there can be no doubt. 

Some of the causes for such fluctuation are as follows : (A) Causes 
for Rise in Sea Level: (1) The wearing down of the lands and the 
deposit of sediment in the ocean raises the level, and this cause may 
give rise to a very perceptible change during periods of long-continued 
denudation. (2) The lowering of land beneath the sea will displace 



616 COLLEGE PHYSIOGRAPHY 

ocean water and cause a rise of the level. (3) The addition of water 
to the oceans from volcanic and other deep-seated sources increases 
the volume of ocean water, and, therefore, has a tendency to raise its 
level. (4) An elevation of sea floor, or the building of volcanic cones 
there, displaces ocean water and causes a rise in sea level. (5) The 
melting and disappearance of glaciers adds water to the ocean level 
and causes it to rise. 

(B) Causes for Depression of Sea Level: (1) The withdrawal of 
ocean water in the processes of weathering is a cause for depression of 
sea level. (2) Waters locked up in glaciers is important in the same 
way. (3) Depression of sea level will follow a sinking of parts of the 
ocean bottom. 

(C) Causes for Both Rise and Fall of Sea Level: (1) Variation in 
rate of rotation or in position of the axis of rotation would necessarily 
be followed by a redistribution of the water, causing a rise in one place 
and a depression in another. (2) Changes in the centre of gravity 
will also result in redistribution of water, drawing it toward one part 
and away from the other. (3) The lateral attraction of land masses or 
ice masses will draw water toward them and away from other places. 

These several causes may operate at the same time, perhaps coun- 
terbalancing one another, or, if working together, producing a com- 
bined result, the product of the two. 

In these ways it is possible to account for some of the apparent 
changes of level of the land ; but, as we have seen, not for all, since 
there is undoubted proof that the crust of the earth itself is in motion. 
The phenomena of the ocean margin, therefore, as well as of the land 
itself, demand some movements of the lithosphere and call for an 
interpretation of the condition of the earth's interior. Indeed, among 
the causes given for changes in ocean level are diastrophic movements 
in the ocean bed ; and also it is to be noted that the extrusion of lava 
and of volcanic water from within the earth must be accompanied by 
compensating downward movements of the crust. 

In the present state of our knowledge it is impossible to assign to the 
several causes outlined above either numerical value or relative im- 
portance. Nor is it possible to state whether, in the total, the changes 
of level of ocean waters are more or less important than changes in 
level of the lands. It is, however, generally agreed that both causes 
are operative in modifying the earth's surface. At present the ocean 
waters rise higher, or the continents sink lower, than in a recent period, 
as indicated by the great extent of the continental shelf, and by the 
many drowned coast lines in both hemispheres. In earlier geological 
times the ocean waters rose even higher than now on the continents, 
though whether this was a general condition or merely local cannot be 
positively determined. 

Summary of Conclusions. — In the preceding pages the attempt 
has been made to keep, in the main, to a statement of points upon 
which there is a fairly uniform agreement and which seem to be pretty 



THE EARTH'S INTERIOR 617 

well established. In summary, these points are as follows: (1) 
The earth's interior is solid ; (2) it is, however, in a state of sufficient 
plasticity to admit of flowage and isostatic adjustment ; (3) there 
are changes in the level of the ocean as well as of the crust ; (4) there 
is heat within the earth. 

When we arrive at the latter point, we come to a great divergence 
of views and must enter upon a consideration of hypotheses upon 
which there is no general agreement. These hypotheses are of two 
main types : (1) those based upon a belief in general interior heat, 
(2) those assuming local areas of heat. We will next consider these 
hypotheses, beginning with that of general heat. 

Possible Sources of General Interior Heat 

Relation to Various Ideas of Earth Origin. — There are at present 
several hypotheses to account for the origin of the earth, each having 
its adherents, and each varying in relation to the source of general 
heat in the interior of the earth. The first and oldest of these is the 
Nebular Hypothesis ; among the other and more recent ones are the 
Meteoritic Hypothesis and the Planetesimal Hypothesis. 

The Nebular Hypothesis. — According to this hypothesis, which 
is largely the work of Laplace, the solar system was originally a highly 
heated gaseous mass or nebula slowly rotating, and occupying all 
the space of the solar system and extending even beyond it, that is, 
with a diameter in excess of 6,000,000,000 miles. As this gaseous mass 
lost heat by radiation, it contracted ; and, one by one, rings developed, 
in which, around some centre of greater density, the gaseous particles 
gathered, forming gaseous spheres which rotated around an axis, and 
revolved in the direction of the original rotation of the parent nebula. 
One by one the planetary spheres of the solar system developed, and 
the satellites developed in similar way from the individual planetary 
spheres. 

As cooling continued, the gases condensed to liquid and then, in 
most of the spheres, to solid state, growing smaller as they became 
cooler. The sun, the central part of the ancient nebula, and the larg- 
est body of the solar system, is still glowing hot. So small a body as 
the moon has cooled down to completely solid state, and both its 
ocean and its atmosphere have disappeared within its cold mass. 
Jupiter, the largest planet, is still so hot that its atmosphere includes 
the waters as well as the elements of the air. The earth is in a state 
intermediate between the moon and Jupiter, with a heated interior, 
a solid crust, and an atmosphere and hydrosphere resting upon the 
lithosphere. 

This is not the place for a discussion of the nebular hypothesis. 
It held sway for a long time and long seemed a rational explanation 
of the origin of the earth, having well-nigh universal acceptance. 
It is still held by many to be the most rational hypothesis yet put 



618 COLLEGE PHYSIOGRAPHY 

forward for the origin of the earth. Latterly, however, it has been 
subjected to criticism, and serious objections have been urged against 
it, while at the same time rival hypotheses have been put forward. 

The general interior heat is thought, under this hypothesis and 
the modification which supposed the earth to still retain a gaseous 
centre (p. 614), to be derived from the cooling of the gaseous nebula. 

The Meteoritic Hypothesis. — One of the recent rival hypotheses 
is the Meteoritic Hypothesis, which conceives the origin to have been 
the collision of particles of cosmical bodies swarming in space. These 
particles, which may be called meteors, collide with such force as 
to become vaporized by the heat ; and, as a mass grows by successive 
increments, it exerts a sufficient gravitative attraction to draw still 
more meteoritic matter to it. Thus the mass grows larger, and be- 
comes heated by collision. The members of the solar system are of 
this origin, and the heat of the earth's interior is thought to be still 
retained from this former state. 

The Planetesimal or Spiral Nebula Hypothesis. — This hypothesis 
assumes that the earth has been a cold mass and gradually became 
somewhat warmer. Instead of being a planet made by the collision 
of meteorites, as in the preceding hypothesis, it is thought to have 
been formed by the gathering-in of masses of nebulous matter or 
planetesimals, to a centre corresponding to one of the so-called " knots " 
on the spiral nebulae. The heat is thought to have been formed by 
internal compression and to have developed from the earth's centre 
outward, as the earth grew by the accretion of layer after layer of 
planetesimals. Moreover, during the earth's slow growth, this hy- 
pothesis, assumes that heat was carried by volcanic action from great 
depths to points near and at the earth's surface, a process thought to 
be still in operation at a diminished rate. 

The Contractional Hypothesis 

The Shrinking Interior of the Earth. — Assuming the earth's in- 
terior to be heated and surrounded by a solid cold crust, it follows that, 
as the heat is slowly conducted to the surface and radiated into space, 
the earth is slowly growing smaller. As the interior shrinks, the rigid 
outer crust settles upon it ; but, being itself already cooled, it does 
not sink equally. Consequently, to fit the shrinking interior it must 
become wrinkled. The comparison is often made to an apple, whose 
interior shrinks by loss of water, causing the more rigid skin to settle 
irregularly and wrinkle. Another comparison may be made to a ball 
around which is placed a leather or cloth cover larger than the ball. 
On pressing this down to fit the surface of the ball, it is necessarily 
wrinkled. 

This contractional hypothesis has long been before the scientific 
world, and it has many adherents. By many it is thought to be the 
main underlying cause for the phenomena of vulcanism and dias- 



THE EARTH'S INTERIOR 619 

trophism ; though isostasy is generally admitted as a supplementary 
cause for diastrophic phenomena. 

Subsidence and Lateral Thrust. — On the basis of the contractional 
hypothesis it is inferred that the surface of the earth is slowly subsid- 
ing ; but, in certain areas, subsidence is in progress in excess, as in 
the ocean bottoms. From these areas of subsidence lateral thrusts 
are applied, actually pushing up the crust in plateau-like areas, as 
would occur in fitting the cover to a ball which is too small for it. 
This lateral thrust may even cause local wrinkling, such as is found 
along mountain chains. Moreover, by the downward thrust of the 
sinking areas, the heated rock may be made to flow away from the 
areas of depression toward and under the areas of uprising. This 
would account for the great batholitic masses beneath mountains ; 
and, rising along the fissures opened in the rigid crust by the move- 
ments to which it is subjected, this heated rock may be squeezed 
out in fissure eruptions, or forced out of volcanic vents, partly by the 
pressure, partly by the expansive force of included gases. 

The contractional hypothesis is thus made to account for the major 
phenomena of vulcanism and diastrophism, though admitting of the 
operation of isostasy, and also of upward movements due to intrusion 
of lava and downward movements resulting from the extrusion of lava 
from beneath areas of the crust. By this hypothesis the ring of lofty 
mountains, numerous volcanoes, and frequent earthquakes are ex- 
plained as a result of lateral thrust from the subsidence in the great 
Pacific basin. The continental slope surrounding the continents is 
interpreted as the approximate boundary between areas of depression 
and areas of either (1) freedom from depression, or (2) of less depres- 
sion than in the ocean basins. The continental slope is on this theory 
either (1) the upthrow side of fault lines, or (2) the site of a sharp fold, 
or (3) sometimes one, sometimes the other. 

Objections to Contractional Hypothesis. — Although the contrac- 
tional hypothesis seems so natural a sequel to those hypotheses of 
earth origin which assume a heated condition, and although it so 
satisfactorily accounts for such a number of phenomena of diastro- 
phism and vulcanism, it cannot be considered established, nor is it 
universally considered satisfactory. There are a number of serious 
difficulties in the way of accepting it as an adequate hypothesis, 
among these being the following : (1) Even granting that the forces 
developed by contraction are concentrated along very narrow belts, 
the results produced in the recent elevation of mountain chains seems 
far too great for the cause proposed. (2) While upon the contrac- 
tional hypothesis a reason is assigned for the growth of mountains 
around the Pacific, there is no equally adequate reason for the recent 
rising of mountains along other belts, as, for example, the east-west 
mountain girdle. Though associated with areas of depression, these 
are not on the border of great areas of subsidence from which extensive 
lateral thrust is to be expected. (3) Even around the Pacific the 



620 COLLEGE PHYSIOGRAPHY 

mountain chains have a form indicating an origin from the land side 
rather than from the ocean. The great mountain loops, so typically 
shown in Asia, but also developed elsewhere, give the appearance of 
crustal movements toward the ocean, not away from it. It is difficult 
to explain these loops by any theory of thrust from the ocean. 
(4) The theory gives no explanation of (a) the development of moun- 
tains along one belt, and subsequent abandonment of that belt ; (b) of 
the diminution of volcanic activity such as has been in progress in 
the recent past ; (c) of the long periods of freedom from diastrophism 
and the development of peneplains. The earth history appears to 
have been one of intermittent activity, with periods of mountain 
growth and vulcanism, between which were periods of sufficient in- 
activity to permit of the widespread reduction of land surfaces. The 
present appears to represent a waning stage in a period of earth activ- 
ity, preceded by very great activity, in which peneplains were up- 
lifted, new mountains were formed, and old mountains revived, while 
great volcanic activity developed. Prior to this stage was a period 
of sufficient activity to permit of widespread peneplanation in Europe 
and America, and in at least portions of Asia and South America. 

Possible Sources of Local Earth Heat 

The Three Suggested Sources. — Hypotheses have been put for- 
ward to explain the phenomena of diastrophism and vulcanism on 
the assumption of local development of heat within the earth. Some 
of these have been intended solely to account for such localization of 
vulcanism as has been observed; others are advanced in explana- 
tion of volcanic and diastrophic phenomena in general. There are 
three known causes of phenomena which, whether the earth is assumed 
to be cold or hot in the interior, are capable of generating heat within 
the earth: (1) chemical change, (2) radioactivity, (3) mechanical 
movements. 

Chemical Change. — It has been pointed out that, if the earth con- 
sists of unoxidized metallic elements, with an oxidized crust, the per- 
colation of water down to the unoxidized portion would give rise to 
processes of oxidation which would generate heat. Granting the postu- 
lates, the result is certain ; but it is not so certain that sufficient heat 
would be generated to give rise to the phenomena of vulcanism. 
Furthermore, there is a limit to the downward percolation of water, 
which is confined to the zone of fracture. It might be assumed that . 
there are other chemical changes of unknown nature in the sub-crustal 
portions of the earth ; but this rests upon scant basis. 

Radioactivity. — Recently radioactivity has naturally been in- 
voked as a source of visible earth heat, and as an explanation of both 
diastrophic and volcanic phenomena. So little is known about this 
property, and so much less about the radium content of the earth, 
that this theory, which is necessarily of very recent origin, cannot have 
been very thoroughly considered or tested. 



THE EARTH'S INTERIOR 621 

Crustal Movements. — Movements in the earth's crust, or in the 
sub-crustal portions of the earth, are certainly capable of generating 
heat, and changes of pressure are also competent to produce heat. 
Thus it has been pointed out that in connection with isostatic adjust- 
ment, temperature changes of importance necessarily follow. And 
surely, in such movements as give rise to mountain folding, great 
development of heat results. The theory has been put forward that 
sufficient heat may be developed in such places to cause extensive 
melting of the rocks and perhaps to be one cause, if not the main one, 
for volcanic activity in areas of mountain growth. 

Objections to Hypotheses of Local Heat. — While it must cer- 
tainly be admitted that heat is produced by each of these causes, no 
one of them, on the basis of any of the processes mentioned above, 
is capable of satisfactorily accounting for the phenomena of diastro- 
phism and vulcanism observed on the earth. Even granting the maxi- 
mum efficiency, they still fail in some of the same important respects 
as the contractional hypothesis does: they do not explain (1) the 
peculiar mountain loops, (2) the localization of movements along 
belts, (3) the recent diminution of volcanic activity, (4) the intermit- 
tent activity with periods of relative inactivity. 

Hypothesis of Change of Earth's Axis of Rotation 

The Problem of Cause for such Change. — It has been suggested, 
always with extreme caution, that there may have been change in 
the axis of the earth. There is no known cause for a change in the 
earth's axis of rotation, and scientific men have naturally looked ask- 
ance at this hypothesis, because it makes appeal to an unknown cause. 
It is, therefore, with grave doubt, and with due caution, that it is 
brought forward here. If a cause for such a change were found, an 
hypothesis for the diastrophic -and volcanic phenomena of the earth 
could be formulated which would have a high degree of merit; by 
it, also, other puzzling phenomena would find explanation. There 
may be hope that such a cause will be found, now that it is known 
there is actually a variation of the earth's axis, though of small 
amount. 

In the absence of a known cause for change in the earth's axis, or 
even of a rational hypothesis for such a change, a tentative suggestion 
of its possibility is as far as one is warranted in going. It is interesting 
to note, however, how many phenomena could be accounted for by a 
change in the axis of the earth's rotation, and how readily it would 
solve some of the most puzzling phenomena of the earth's surface of 
the present and past times. 

Relation of Shifted Axis to Glaciation. — If we could assume a 
change in the axis of the earth's rotation, we would have an immediate 
and effective answer to the problem of the cause of continental glacia- 
tion in Europe and America during the Glacial Period. It would also 



622 COLLEGE PHYSIOGRAPHY 

account for the puzzling fact that the ice sheets were centred around 
the North Atlantic basin, and were absent from other far northern 
regions such as northern Alaska and Asia. Here would be explana- 
tion of the apparent diminution of glaciation toward the north. One 
of the most puzzling facts concerning former glaciation is the presence 
of great ice sheets in former geological periods in various parts of the 
world, and notably in South Africa, where an ice sheet developed in 
the tropical zone and moved toward the polar region. A change in 
the axis of the earth's rotation would satisfactorily explain this glacia- 
tion, the most difficult of all to account for by current theories of 
climatic change. 

Relation to Earth Movements. — Should the position of the earth's 
axis of rotation be changed, whether slowly or abruptly, there would 
follow, first an immediate change in distribution of water on the sur- 
face, with accompanying rise of the sea level in parts of the earth 
and lowering in others. More slowly, and lagging behind, would 
follow an adjustment of the lithosphere to the new axis of rotation 
and the development of the oblate spheroid form in accordance with 
the position of the new axis. During this adjustment there would 
be flow in the zone of flowage, and a dragging of the rigid outer crust, 
with accompanying changes of level, and local, linear areas of crum- 
pling and faulting. Heat would necessarily result from these move- 
ments, and, quite conceivably, heat enough to cause extensive melting 
of rocks along the areas of greatest disturbance. 

Relation to Volcanic and Diastrophic Activity. — If such changes 
could be assumed, a number of the most puzzling phenomena of dias- 
trophism and vulcanism would find explanation. For example, 
during the periods when no changes in the earth's axis occurred, there 
would be rest from volcanic and diastrophic activity, denudation would 
have full sway, the continents would be slowly worn down, and ex- 
tensive peneplanation would result. Such seems to have been the 
case in the early stages of the Tertiary Period, and it is noteworthy 
that warm temperate flora and fauna lived far within the Arctic, giv- 
ing rise to extensive coral beds, for example, in Spitzbergen, in latitude 
79 , where now is a land of snow and ice. 

If then the period of quiet is interrupted by a change in the axis, 
a warm temperate region may be transformed to a frigid region of 
continental glaciers, changes both of land and sea level will follow, 
and mountain folding may occur along favourable lines, as the crust 
is dragged forward on the undertow developed in the zone of flowage. 
Lofty mountains may rise, lava floods may issue from fissures in the 
crust, and volcanic mountains may be built up by the extrusion of 
lava, formed by the heat due to the crustal and sub-crustal movements. 
Both mountain formation and volcanic activity would slowly die out 
as adjustment was reached. 

Relation to Folding and Faulting. — The forward drag of the rigid 
crust would account for the great mountain loops, such as those of 



THE EARTH'S INTERIOR 623 

Asia, which seem to have moved outward from some point to the north. 
It would explain the great thrust faults, by which blocks of the crust 
have been dragged forward many miles and also the extensive com- 
pression of originally horizontal strata, so that they now occupy a 
much smaller horizontal space than formerly. In eastern United 
States it is estimated that there has been an apparent shortening of 
the arc of the earth's surface by fully 50 miles, and in other mountain 
regions similar apparent shortening has occurred. 

Relation to Volcanic Recurrence. — On the theory of a globe origi- 
nally heated and subjected to continued loss of heat during the 
millions of years of geological time, it is exceedingly difficult to explain 
the apparent fact that volcanic activity has not been diminishing 
progressively. It is also difficult to explain the apparently shallow 
source of the lava of volcanoes, though this may be due to the rise of 
batholitic masses into the crust. On the theory of change in the 
axis, both of these phenomena are readily explained, for the heat 
necessary for vulcanism is developed only at intervals. 

Summary of Results of Shifting Earth's Axis. — Could some ade- 
quate cause be found for a change in the position of the axis of earth's 
rotation, some of the obscure problems of earth form and condition 
would be more easily and satisfactorily explicable than under any 
theory at present before us. Present and recent diastrophism and 
vulcanism would be explained ; the location of areas of disturbance 
along different lines in different ages would be understandable ; 
changes of climate, including periods of glaciation, would not prove 
such puzzling phenomena ; and the limitations placed upon the length 
of geological time by physicists, who base their estimates upon the 
rate of cooling of a formerly heated globe, would lose their apparent 
force. Unfortunately, however, until some adequate cause for such 
changes appears, the hypothesis of change in the earth's axis can be 
put forward only in a tentative way. 

Age or the Earth 

Geological Time is of Great Duration. — Throughout the preceding 
chapters the phenomena of the earth's surface have been interpreted 
on the basis of the assumption that geological time has been of great 
duration. Indeed, it has become evident, from a study of the evolu- 
tion of the forms of the land, that these can be explained only on such 
an assumption. To deposit thousands of feet of sedimentary strata, 
to raise these into mountain folds, and to reduce the folded moun- 
tains to the condition of a peneplain, each requires long time periods ; 
and, since these processes have been repeated again and again, it is 
evident that there must have been a vast lapse of time during the 
geological past. From such evidences, as well as from others furnished 
by a study of geological history, the conclusion has become generally 
accepted that the period of geological time can be estimated only in 
millions of years. 



624 COLLEGE PHYSIOGRAPHY 

Naturally there has been a desire to reckon geological time more 
definitely than this, and many efforts have been made to that end. 
From such efforts there has been wide divergence of results, though 
all agree in the one conclusion that the age of the earth is very great. 

Estimates by Physicists. — The estimates that, seemingly, have 
the best basis, and that handle the problem with most mathematical 
exactness, are those of physicists who have followed three main lines 
of argument : (i) the rate of cooling of the earth to its present 
state, (2) the age of the sun's heat, (3) the effect of tidal retardation 
upon the rate of earth's rotation. From the first line of argument the 
conclusion has been reached that the earth cannot have required more 
than 20,000,000 years to have cooled down to its present state, as- 
suming a heated interior with cold crust. From the second it has 
been concluded that the sun cannot have supplied heat to the earth 
at the present rate for a period of more than 20,000,000 years. On the 
basis of the influence of tidal retardation, a similar age has been de- 
duced, and it has been agreed by physicists that the physical evidence 
" reduces the possible period which can be allowed to geologists to 
something less than 10 millions of years." 

There is a seeming mathematical exactness in these calculations 
which has perhaps led to placing rather more reliance upon them than 
is really warranted. In each case there are fundamental basal assump- 
tions which, if incorrect, destroy the value of the whole analysis. 
It is assumed, for instance, that the earth's interior is highly heated ; 
it is assumed that there is no renewal of supply to the sun's heat ; and 
it is assumed that a greater oblateness of the earth due to an earlier, 
more rapid rotation, would still be recognizable. No one of these 
assumptions is established, and there are reasons for doubting the 
correctness of some of them. 

Estimates by Geologists. — There seems really little reason for 
placing more reliance upon these figures obtained by physical analysis 
than upon the much more vague estimates of geologists. Consider- 
ing the great extent of sedimentation in past ages, the vast results 
of denudation, and the marvellous evolution of animal and plant life, 
revealed by the geological record, and assuming a past rate for these 
processes not greatly unlike that of the present, geologists have be- 
come profoundly impressed with the vast lapse of time demanded for 
them. Some have made rough estimates, admittedly inexact, and 
most of them have been far in excess of the physical estimates. A 
conservative geological estimate would be at least 60 to ibo million 
years; and, to some, many times this period seems demanded to 
account for the phenomena of earth history revealed by geological 
study. 

The physiographer, though interpreting the forms of the earth 
as they at present exist, must of necessity deal to some extent with 
this question of the lapse of past time, since the development of pres- 
ent land forms is an outcome of a long series of past changes. The 



THE EARTH'S INTERIOR 625 

study and solution of the problem is, however, within the province 
of the geologist and physicist rather than the physiographer. To 
him the point of prime importance is that there has been a vast lapse 
of time, during which the complex processes of denudation, diastro- 
phism, and vulcanism have been in operation. Whether this time 
period is 20,000,000 years or one hundred times that amount, must 
be left to the physicist and geologist to settle, but such evidence as 
the physiographer gathers points toward the larger rather than the 
smaller estimate. 

References to Literature 

F. D. Adams. Experimental Investigation into the Flow of Rocks, Amer. 
Journ. Sci., Vol. 179, 1910, pp. 465-487; Experimental Contribution to 
the Question of the Depth of the Zone of Flow in the Earth's Crust, Journ. 
Geol., Vol. 20, 1912, pp. 97-118. 

F. D. Adams and J. T. Nicolson. Experimental Investigation into the Flow 

of Marble, Phil. Trans. Roy. Soc, London, Vol. 195, 1901, pp. 363-401. 
S. Arrhenius. Worlds in the Making, New York, 1908, 230 pp. ; Die Feste 

Erdkruste und das Erdinnere, Lehrbuch der Kosmischen Physik, Leipzig, 

1903, pp. 278-347- 
R. S. Ball. The Earth's Beginning, London, 1901, 384 pp. 
J. Barrell. The Strength of the Earth's Crust, Journ. Geol., Vol. 22, 1914, 

pp. 28-48, 145-165, 209-236, 289-314. 
E. de Beaumont. Notice sur les Systemes des Montagnes, Paris, 1852, 1143 pp. 
H. T. de la Beche. A Geological Manual, Philadelphia, 1832, 535 pp. 

G. F. Becker. Relations of Radioactivity to Cosmogony and Geology, Bull. 

Geol. Soc. Amer., Vol. 19, 1908, pp. 113-146; The Age of the Earth, 
Smithsonian Misc. Collections, No. 1936, Washington, 1910, 28 pp. 

M. Bertrand. Deformation Tetraedrique de la Terre et Deplacement du 
Pole, Comptes Rendus Acad. Sci. Paris, Vol. 130, 1900, pp. 449-464. 

W. Bowie. Effect of Topography and Isostatic Compensation upon the 
Intensity of Gravity, Special Publication 12, U. S. Coast and Geodetic 
Survey, Washington, 1912; Amer. Journ. Sci., 4th series, Vol. 33, 1912, 
pp. 237-240; Isostasy and the Shape and Size of the Earth, Science, N. S., 
Vol. 39, 1914, pp. 697-707. 

T. C. Chamberlin. An Attempt to Test the Nebular Hypothesis by the Rela- 
tion of Masses and Momenta, Journ. Geol., Vol. 8, 1900, pp. 58-73; 
On a Possible Function of Disruptive Approach in the Formation of 
Meteorites, Comets and Nebulae, ibid., Vol. 9, 1901, pp. 369-392; 
The Bearing of Radioactivity on Geology, ibid., Vol. 19, 191 1, pp. 673- 
695; ibid., Trans. Illinois Acad. Sci., Vol. 23, 1912, pp. 57-75; Funda- 
mental Problems of Geology, Year Book 3, Carnegie Institution, Washing- 
ton, 1904, pp. 195-258; Diastrophism and the Formative Processes, 
Journ. Geol., Vol. 21, 1913, pp. 517-533, 577-587, 673-682; ibid., Vol. 22, 
1914, pp. 131-144, 268-274, 3I5-345- 

T. C. Chamberlin, F. R. Moulton, and Others. The Tidal and Other Problems, 
Publication 107, Carnegie Institution, 1909, 264 pp. 

T. C. Chamberlin and R. D. Salisbury. Geology, Vol. 2, 1906, pp. 3-81 ; 
ibid., Vol. 1, 1905, pp. 559-569. 

James Croll. Climate and Time, New York, 1875, 577 pp.; Climate and 
Cosmology, Edinburgh, 1885, 327 pp.; Nature, Vol. 18, 1878, pp. 267- 
268. 

W. O. Crosby. Origin and Relations of Continents and Ocean Basins, Proc. 
Bost. Soc. Nat. Hist., Vol. 22, 1884, pp. 443-485. 

J. D. Dana. Origin of Igneous Rocks of the Earth, Origin of Continents, Amer. 
2 s 



626 COLLEGE PHYSIOGRAPHY 

Journ. Sci., 2d series, Vol. 2, 1846, pp. 335-353; Earth Shaping, Moun- 
tain Making, and the Attendant Phenomena, Manual of Geology, 4th 
edition, 1896, pp. 345-39°; Length of Geological Time, ibid., pp. 1023- 
1026. 

N. H. Darton. Geothermal Data from Deep Artesian Wells in the Dakotas, 
Amer. Journ. Sci., Vol. 155, 1898, pp. 161-168. 

G. H. Darwin. Scientific Papers, Vol. 2, Tidal Friction and Cosmogony, 
Cambridge, 1908, 516 pp.; The Tides and Kindred Phenomena in the 
Solar System, Boston, 1898, London, 1911, 437 pp. 

W. M. Davis. The Bearing of Physiography upon Suess' Theories, Amer. 
Journ. Sci., Vol. 169, 1905, pp. 265-273. 

C. E. Dutton. A Criticism upon the Contractional Hypothesis, Amer. Journ. 
Sci., 3d series, Vol. 8, 1874, pp. 1 13-125; On Some of the Greater Prob- 
lems of Physical Geology (including isostasy), Bull. Phil. Soc. Washing- 
ton, Vol. n, 1889, pp. 51-64. 

B. K. Emerson. The Tetrahedral Earth and Zone of the Intercontinental 
Seas, Bull. Geol. Soc. Amer., Vol. 11, 1900, pp. 61-106. 

O. Fisher. Physics of the Earth's Crust, London, 1881, 299 pp. 

A. Geikie. Text-Book of Geology, 4th edition, London, 1903, pp. 13-83, 

351-397- 

G. K. Gilbert. The Strength of _ the Earth's Crust, Bull. Geol. Soc. Amer., 
Vol. 1, 1889, pp. 23-27; Continental Problems, ibid., Vol. 4, 1893, pp. 179- 
190; New Light on Isostasy, Journ. Geol., Vol. 3, 1895, pp. 331-334; 
Earth, Universal (Johnson's) Encyclopedia, Vol. 2, 1893, pp. 886-892; 
The Moon's Face, Bull. Phil. Soc. Washington, Vol. 12, 1893, pp. 241- 
292; Rhythms and Geologic Time, Science, N. S., Vol. 11, 1900, pp. 1001- 
1012; Interpretation of Anomalies of Gravity, Prof. Paper 85-C, U. S. 
Geol. Survey, 1913, pp. 29-37. 

W. L. Green. Vestiges of a Molten Globe, London, 1875, an d Honolulu, 1887. 

J. W. Gregory. The Plan of the Earth and Its Causes, Geog. Journ., Vol. 13, 
1899, pp. 225-251; Mill's International Geography, New York, 1907, 
pp. 36-45 ; The Making of the Earth, New York, 191 2, 256 pp. 

E. Haug. Les Theories Orogenique, Traite de Geologie, Paris, 1907, pp. 511- 
536. 

J. F. Hayford. The Figure of the Earth and Isostasy from Measurements in 
the United States, U. S. Coast and Geodetic Survey, Washington, 1909; 
Supplementary Investigation in 1909 of the Figure of the Earth and Isos- 
tasy, ibid., 1910; The Effect of Topography and Isostatic Compensation 
upon the Intensity of Gravity, International Geodetic Association, 16th 
Report, Vol. 1, pp. 365-389; ibid, (with W. Bowie), Special Publication 
10, U. S. Coast and Geodetic Survey, 1912; Relations of Isostasy to 
Geodesy, Geophysics, and Geology, Science, Vol. 33, 1911, pp. 199-208; 
Isostasy, a Rejoinder to the Article by Harmon Lewis, Journ. Geol., Vol. 
20, 1912, pp. 562-578. 

W. H. Hobbs. Mechanics of Formation of Arcuate Mountains, Journ. Geol., 
Vol. 22, 1914, pp. 71-90, 166-188, 193-208. 

A. Holmes. The Age of the Earth, London, 1913, 196 pp. 

J. Joly. Uranium and Geology, Nature, Vol. 78, 1908, pp. 456-466; Radio- 
activity and Geology, 1909, p. 211 ; An Estimate of the Geological Age of 
the Earth, Trans. Roy. Soc. Dublin, series 2, Vol. 7, 1899, pp. 23-65; 
Rept. Brit. Assoc. Adv. Sci., 1900, pp. 369-379. 

Immanuel Kant. Allgemeine Naturgeschichte und Theorie des Himmels, Konigs- 
berg, 1755. 

Clarence King. The Age of the Earth, Amer. Journ. Sci., 3d series, Vol. 45, 
1893, pp. 1-20; Ann. Rept. Smithsonian Institution, 1892-1893, pp. 335- 
352. 

P. S. Laplace. Systeme du Monde, Paris, 1796; edition in English, 2 vols., 
Dublin, 1830, — Considerations on the System of the World, Vol. 2, pp. 324- 
342. 



THE EARTH'S INTERIOR 627 

A. de Lapparent. De la Mesure du Temps Paries Phenomenes de Sedimen- 
tation, Bull. Geol. Soc. France, Vol. 18, 1890, pp. 351-355 ; La Destinee de 
la Terre Ferme et Duree des Temps Geologique, Brussels, 1891, 38 pp.; 
Sur la Symetrie Tetraedrique du Globe Terrestre, Comptes Rendus Acad. 
Sci. Paris, Vol. 130, 1900, pp. 614-619. 

J. Le Conte. Earth-Crust Movements and their Cause, Bull. Geol. Soc. Amer., 
Vol. 8, 1897, pp. 1 13-126; Igneous Agencies, Elements of Geology, New 
York, 1885, pp. 76-132; General Form and Structure of the Earth, ibid., 
pp. 164-170. 

H. Lewis. The Theory of Isostasy, Journ. Geol., Vol. 19, 191 1, pp. 603-626. 

J. N. Lockyer. The Meteoritic Hypothesis, London, 1890, 560 pp. ; Chemis- 
try of the Sun, New York, 1887. 

A. E. H. Love. The Gravitational Stability of the Earth, Phil. Trans. Roy. 
Soc, Vol. 207, 1908, pp. 1 71-241 ; Dynamical Theory of the Shape of the 
Earth, Nature, Vol. 76, 1907, pp. 327-332. 

Sir Charles Lyell. Comparative Duration of the Glacial and the Antecedent 
Tertiary, Secondary, and Primary Epochs, Principles of Geology, nth 
edition, 1873, Vol. 1, pp. 300-304; Causes of Earthquakes and Volcanoes, 
ibid., Vol. 2, pp. 198-213. 

W J McGee. The Gulf of Mexico as a Measure of Isostasy, Amer. Journ. 
Sci., Vol. 44, 1892, pp. 177-192. 

A. A. Michelson. Preliminary Results of Measurements of the Rigidity of the 
Earth, Journ. Geol., Vol. 22, 1914, pp. 97-130. See also H. G. Gale, 
Science, N. S., Vol. 39, 1914, pp. 927-933. 

F. R. Moulton. The Shape of the Earth, Journ. Geog., Vol. 2, 1903, pp. 481- 
486, 521-527; The Motions of the Earth, ibid., Vol. 3, 1904, pp. 145-150, 
213-222; An Attempt to Test the Nebular Hypothesis by an Appeal to 
the Laws of Dynamics, Astrophysical Journ., Vol. n, 1900, pp. 103-130; 
The Spiral Nebula Hypothesis, Introduction to Astronomy, New York, 
1913, pp. 463-487. 

P. G. Nutting. Isostasy, Oceanic Precipitation, and the Formation of Mountain 
Systems, Science, N. S., Vol. 34, 1911, pp. 453-454. 

R. D. Oldham. Constitution of the Interior of the Earth, Quart. Journ. Geol. 
Soc, Vol. 62, 1906, pp. 456-473. 

F. L. Ransome. The Great Valley of California, Bull. Dept. Geol. Univ., 
California, Vol. 1, 1896, pp. 371-428. 

T. M. Reade. Measure of Geological Time, Geol. Mag., Vol. 10, 1893, pp. 
99-100; Chemical Denudation in Relation to Geological Time, London, 

1879- 
W. B. Scott. The Internal Constitution of the Earth, Introduction to Geology, 

2d edition, New York, 1907, pp. 90-96; The Causes of Folding and. Dis- 
location, ibid., pp. 358-368. 
E. H. L. Schwartz. Causal Geology, London, 1910, 248 pp. 
T. J. J. See. The Cause of Earthquakes, Mountain Formation, and Kindred 

Phenomena, Proc Amer. Phil. Soc, Vol. 45, 1906, pp. 274-414; On the 

Temperature, Secular Cooling, and Contraction of the Earth, ibid., Vol. 

46, 1907, pp. 191-299; The New Theory of Earthquakes and Mountain 

Formation, ibid., pp. 369-415. 
N. S. Shaler. A Comparison of the Features of the Earth and the Moon, 

Smithsonian Contributions, Vol. 34, 1903, 79 pp. 
W. J. Sollas. The Age of the Earth and Other Geological Studies, London, 

1905, 328 pp. 
J. W. Spencer. Relation between Terrestrial Gravity and Observed Earth 

Movements of Eastern America, Amer. Journ. Sci., 4th series, Vol. 35, 

1913, pp. 561-573- 
R. J. Strutt. Radio-Active Changes in the Earth, Nature, Vol. 79, 1908, pp. 

206-208. 

E. Suess. Das Antlitz der Erde, 4 vols., in German, French, and English. 

F. B. Taylor. Bearing of the Tertiary Mountain Belt on the Origin of the 

Earth's Plan, Bull. Geol. Soc. Amer., Vol. 21, 1910, pp. 179-226. 



628 COLLEGE PHYSIOGRAPHY 

W. Thomson (Lord Kelvin). On the Secular Cooling of the Earth, Trans. 
Roy. Soc. Edinburgh, Vol. 23, 1862; On the Age of the Sun's Heat, 
Macmillan's Magazine, 1862, — see Thomson and Tait's Treatise on 
Natural Philosophy, London, 1883, Cambridge, 1890, Part 2, pp. 468- 
494; The Internal Condition of the Earth as to Temperature, Fluidity, 
and Rigidity, Popular Lectures and Addresses, Vol. 2, pp. 299-318; The 
Age of the Earth as an Abode Fitted for Life, Science, N. S., Vol. 9, 1899, 
pp. 665-674, 704-711. 

S. D. Townley. The Shifting of the Earth's Axis, Pop. Sci. Monthly, Vol. 

75, 1909, PP- 4I7-434- 

Warren Upham. Estimates of Geologic Time, Amer. Journ. Sci., 3d series, 
Vol. 45, 1893, pp. 209-220. 

C. R. Van Hise. Estimates and Causes of Crustal Shortening, Journ. Geol., 
Vol. 6, 1898, pp. 10-64; Deformation of Rocks, ibid., Vol. 4, 1896, pp. 
i95- 2I 3, 3 I2 ~353> 449~483, 593-629; Vol. 5, 1897, pp. 178-193; Meta- 
morphism of Rocks and Rock Flo wage, Amer. Journ. Sci., Vol. 156, 1898, 
PP- 75-9 1 ; A Treatise on Metamorphism, Monograph 47, U. S. Geol. 
Survey, 1904, 1286 pp. 

C. D. Walcott. Geologic Time as Indicated by the Sedimentary Rocks of 
North America, Journ. Geol., Vol. 1, 1893, pp. 639-676. 

T. L. Watson. Underground Temperatures, Science, N. S., Vol. ^$, 191 1, 
pp. 828-831; ibid., Vol. 34, 1911, pp. 125-126. 

B. Willis. A Theory of Continental Structure Applied to North America, 
Bull. Geol. Soc. Amer., Vol. 18, 1907, pp. 389-412 ; What is Terra Firma? 
— A Review of Current Research in Isostasy, Ann. Rept. Smithsonian 
Institution, Washington, 191 1, pp. 391-406. 

Alexander Winchell. Comparative Geology, Chicago, 1883, 642 pp. 

R. S. Woodward. The Mathematical Theories of the Earth, Smithsonian 
Rept. for 1890, pp. 183-200; The Century's Progress in Applied Math- 
ematics, Bull. Amer. Math. Soc, Vol. 6, 1900, pp. 147-148. 



CHAPTER XVIII 

TERRESTRIAL MAGNETISM 

Magnetism of the Earth 

The Compass. — A familiar instrument is the compass, whose 
needle we think of as always pointing north. Any magnetized bar 
or needle of magnetized steel so suspended that it will swing freely 
in a horizontal plane is a compass; in different parts of the world 
it points in quite different directions. At some places in the world 
the compass needle does set itself exactly north and south; that is, 
in the direction of the true or geographical poles. When Admiral 
Markham was travelling due north toward the North Pole in 1876, 
he was steering east- southeast by his compass, which pointed toward 
the magnetic north pole. At most points, however, the north-seek- 
ing end of the compass needle points either east or west of true north. 

Isogonic Maps. — Figure 386 is a map of United States which indi- 
cates the directions in which the compass pointed in different parts 
of the country in the year 19 10. 

The heavy line on the map marked o° goes through those places 
at which the compass needle points true. Along this line the compass 
would show no variation from due north ; it is called the agonic line. 
There are places of no compass variation or declination from Lake 
Superior to South Carolina, points like Fort Wayne, Indiana, and 
Savannah, Ga. Because of what is known as secular change, the 
compass points due north at those places only at certain times, in 
this case the year 19 10. 

East of this agonic line in United States the needle points west of 
true north, and the compass is said to have west declination, while to 
the west the variation is spoken of as east declination. 

At Ithaca, N. Y., the compass needle pointed 8° west of north; 
at Madison, Wis., it pointed 4J east of north; and at Seattle, 
Wash., 23 \° east of true north. This can be determined by 
studying the lines on the map (Fig. 386), the variation of the com- 
pass at Madison, for example, being nearly the same as that at New 
Orleans to the south, but very different from that at Boston to the 
east and Salt Lake City to the west. These lines go through places 
that have equal compass variation and are called isogonic lines, and 
the map is called an isogonic map. 

Figure 387 is an isogonic map of the world, showing the several 

629 



COLLEGE PHYSIOGRAPHY 




TERRESTRIAL MAGNETISM 631 

agonic lines, marked no variation, and the convergence of all these 
magnetic meridians toward the polar regions. 

Magnetic Poles. — The place toward which the compass needle 
points is called a magnetic pole, the north magnetic pole being nearly 
1400 miles from the true or geographical north pole. It is located in 
the Boothia Peninsula west of Hudson Bay in Canada near latitude 
70 5' north and longitude 96 46' west. Its position was first deter- 
mined by Sir James Clark Ross in 1831. The south magnetic pole, 
according to the recent Antarctic expeditions, is in the continent of 
Antarctica south of Australia near 71 30' south latitude and 153 
east longitude. The two magnetic poles are not antipodal as the 
geographical poles are, and a line passing through the former would 
miss the centre of the earth by nearly 750 miles, or about J of the earth's 
radius. 

Magnetic Intensity. — The magnetic force which acts on a hori- 
zontal compass needle diminishes with approach to the magnetic 
poles, so that near these poles the compass is practically useless for 
determining directions. In consequence, also, the effect on the com- 
pass needle of irregular disturbances, called magnetic storms, is greater 
in the polar than in the equatorial regions. With the time varia- 
tions in magnetic intensity there seem to be difficulties in the use 
of wireless telegraphy and in sending messages over telegraph 
wires. 

Changes in Magnetic Declination, or Compass Direction. — The 
direction assumed by a compass needle at any one place changes with 
the lapse of time, as is proved by repeated observations in many 
parts of the world. At London the compass pointed n° east of north 
in 1580, due north about 1658, 24!° west of north in about 181 2, 
and only 15J west of north in 191 2. It is not possible, at present, to 
predict for more than about 5 years the amount of increase or decrease 
in magnetic declination for a given place with sufficient accuracy 
for the purposes of the mariner and the surveyor. The compass direc- 
tion in United States ranged from 20 west to 24 east of true north 
in 1 9 10, but in 1800 it only ranged from 14 west to 19 east of true 
north. At present the agonic line in United States seems to be shifting 
slowly westward. It was west of Richmond, Va., from 1750 to 
about 1772, and east of Richmond from 1772 to 1838, since which it 
has slowly travelled westward. 

The Dip Needle. — Every one who uses a compass has noted that 
one arm of the needle is weighted. This is to make the needle re- 
main in a horizontal position, for otherwise one end would be found 
to " dip " or point downward. The earth acts as a great magnet, 
having lines of force which extend parallel to the surface in the equa- 
torial region and at an increasing angle with the surface as the poles 
are approached (Fig. 388). To these lines of force the compass needle 
is parallel, so that the inclination amounts, on the average, to about 
75 in northern United States and increases toward the magnetic 



632 



COLLEGE PHYSIOGRAPHY 




TERRESTRIAL MAGNETISM 



633 



N.Mag. 




Fig. 388. — The earth as a magnet, 
with lines of force nearly parallel 
to the magnetic equator and in- 
creasing in inclination as the 
magnetic poles are approached. 
(Richardson.) 



nortK pole. There the end of the needle points straight down into 
the ground. The amount of inclination depends upon the distance 
from the magnetic equator, which is situated not far from the geo- 
graphical equator. The dip is zero at the magnetic equator. To the 
north the dipping end of the needle is the north-pointing end, whereas 
south of this line, the south-pointing 
end of the needle dips. Always the end 
of the needle opposite to the dipping end 
is weighted. Another style of instru- 
ment, a dipping compass, or, more cor- 
rectly, a dip circle, is used to measure 
the amount of inclination from the 
horizontal. This is shown by a needle 
so mounted as to swing freely on a hori- 
zontal axis. 

Local Magnetism. — Not only is there 
a field of magnetism of the earth as a 
whole, but there are parts of it which 
are locally magnetic. This is because 
of the presence of rocks which attract 
the needle ; for example, certain iron 
ores such as magnetite. In such places 
the compass may be affected more largely 
by the local than the general magnet- 
ism, but the behaviour of the compass needle will be a balance be- 
tween the influence of both. Near Juneau, Alaska, such a body of 
magnetic iron ore disturbs the ordinary compass needle so that it is 
weak and points this way and that, while the dip needle points straight 
down, as if it were directly over the north or south magnetic pole. 
Near such ore bodies, or other igneous rocks which happen to be mag- 
netic, ships are likely to be wrecked, if corrections for local magnetism 
are not made. Lines of local magnetic attraction, therefore, tend to 
interfere with the observations of general terrestrial magnetism. By 
the use of the miner's dip needle or the dipping compass, iron ores 
are often discovered in regions where the deposit lies concealed deep 
beneath the surface. 

Magnetic Survey of the Globe. — In recent years a very detailed 
study of the terrestrial magnetism of the whole earth has been under- 
taken under the direction of L. A. Bauer, in charge of the Depart- 
ment of Terrestrial Magnetism of the Carnegie Institution of Wash- 
ington. In connection with this work, careful observations are being 
made on land and sea, in the most remote parts of the world as well 
as in civilized lands. For the work on the oceans a special ship had 
to be built, and, because of the effect of iron and steel upon the com- 
pass needle, practically all the metal parts of this ship were made of 
non-magnetic substances like bronze, gunmetal, and copper, the hull 
being wooden. 



634 



COLLEGE PHYSIOGRAPHY 



Upon this ship some very interesting and valuable observations 
have been made. Errors of considerable magnitude — often of a 
persistent nature for long stretches — have been found to exist in 
some parts of the magnetic charts used by mariners. In certain areas 
in the Indian Ocean, for example, errors in the charted compass direc- 
tions amounting to 6° were found. The errors in the charted compass 
directions over the greater portion of the Atlantic Ocean amount to 
about 2°. A vessel sailing 2000 miles from San Francisco to Honolulu 
might be 35 miles too far north at the end of the voyage, if depending 
upon the older charts and not seeing the sun or stars. 

Cause of Terrestrial Magnetism. — The patient gathering of data 
by the Department of Terrestrial Magnetism of the Carnegie Institu- 
tion of Washington, by the United States Coast and Geodetic Survey, 
and similar bureaus in European countries, will result in vast additions 
to our knowledge concerning magnetism of the earth, its rate of varia- 
tion, and, perhaps eventually, its cause, which as yet is unknown. 
It is some magnetic condition which we most commonly think of as 
being deep within the earth. It may be due to rotation. It may 
be connected with the heated interior. Its ultimate cause may 
even be outside the earth. At all events, the earth acts as a great 
magnet. 

Possible Relation of Magnetism to the Aurora. — The Aurora 
Borealis, or Northern Lights, and the similar phenomenon of the 



82/5* We 1888 ISSo 35i «5* 1898 rfoo ISoz 




Fig. 389. — Coincidence of magnetic storms and sun spots. (Moulton.) 



southern hemisphere, the Aurora Australis, are thought to be in some 
way related to terrestrial magnetism. The strange light in the north- 
ern sky, the brilliant colours, and the rapid shifting of bright streamers, 






TERRESTRIAL MAGNETISM 635 

which dart from horizon to zenith, are sometimes seen in winter in 
northern United States and are very commonly observed by polar 
explorers. Because the aurora is seen with greatest intensity near the 
magnetic poles, and because similar colours have been artificially pro- 
duced by the discharge of an electric spark in a test-tube from which the 
air had been partially exhausted, it is thought possible that the aurora 
may be due to faint electrical discharges in the higher layers of the air, 
with some unascertained relationship to terrestrial magnetism. It is 
said that auroras and magnetic storms are most frequent every 
eleven years, and that they coincide with greatest frequency of 
sun spots (Fig. 389). 

Importance or Terrestrial Magnetism to Man 

Relation to Navigation. — Until the compass was invented, it was 
never possible for men to venture out of sight of land in ships, with 
confidence of being able to return. The compass is said to have been 
discovered by the Chinese as early as 1100 B.C., but it was not intro- 
duced in Europe until the twelfth century a.d. Upon the use of this 
instrument depends all of our commerce upon the seas. Columbus is 
thought by some to have been the first to note that the compass does 
not point to the true north and that the amount of divergence varies 
from place to place. 

Unless we know the amount and rate of change of the compass direc- 
tion we cannot safely use the compass in navigation. Along the coast 
of southeastern Alaska, for example, the compass pointed 30^° east 
of true north in 19 10. In sailing from Alaska to Seattle or San 
Francisco, a ship might be some distance west of California toward the 
Hawaiian Islands if the variation of the compass were not known and 
corrected. 

Relation to Exploration, Surveying, and Map Making. — The use of 
the small pocket compass as a guide in going through a strange country 
or in the woods, or in travelling on a cloudy day or at night when the 
sun is invisible, is of great importance to man. 

The use of the compass by surveyors is at the basis of all our laying 
out of lands and the making of maps. It is for this reason that we 
must know exactly how much east or west of north the compass 
points. The determination of directions, the so-called points of the 
compass, are not always conveniently made by the true north shadow 
of a post or tree at noon or the observation of Polaris, the North Star, 
at night. Even if maps were made upon the basis of the present 
compass declination, with corrections for local magnetism, due to 
substances in the earth, or for the presence of iron in buildings, we 
should still need the precise determination of the yearly amount of 
change in our isogonic lines, as in a case where a boundary line might 
be in dispute or any other artificial line established by man with the 
use of the surveyor's compass. 



636 COLLEGE PHYSIOGRAPHY 



REFERENCES TO LITERATURE 

L. A. Bauer. United States Magnetic Declination Tables and Principal Facts 
Relating to the Earth's Magnetism, Washington, 1902 ; United States 
Magnetic Tables and Magnetic Charts for 1905, Department of Commerce 
and Labor, Coast and Geodetic Survey, Washington, 1908; Terrestrial 
Magnetism, A Consistent Theory of the Origin of the Earth's Magnetic 
Field, Journ. Wash. Acad. Sci., Vol. 3, 1913, pp. 1-7; The Magnetic 
Survey Yacht " Carnegie " and her Work, Terrestrial Magnetism and 
Atmospheric Electricity, Vol. 14, 1909, pp. 57-66. 

Charles Chree. Terrestrial Magnetism, Encyclopaedia Britannica, nth edi- 
tion, Vol. 17, 191 1, pp. 353-385- 

A. Nippoldt. Erdmagnetismus, Erdstrom, und Polarlicht, Leipzig, 191 2, 143 pp. 

Terrestrial Magnetism and Atmospheric Electricity, an International Quar- 
terly Journal, the Johns Hopkins Press, Baltimore, Md. 



PART II. THE HYDROSPHERE 

CHAPTER XIX 

THE OCEAN 

Oceanography as a Science 

The Content of Oceanography. — The scientific study of the oceans 
is known as Oceanography. This comprehensive science considers 
(i) the distribution and depth of oceanic waters, (2) the composition 
of the water, (3) its colour, (4) its temperature, (5) its movements, 
(6) the relation of organic life to its environment, (7) the topography 
and other conditions on the ocean bottom. 

Former Beliefs of Deep Sea Conditions. — Oceanographic study 
received a great impetus when it was found that the ocean bottom was 
inhabited by life and that there was a great world, hitherto unknown, 
inviting exploration. Before that time it was supposed that the deep 
sea was a vast desert, incapable of supporting life because of its utter 
darkness and the enormous pressures that the column of water exerted 
in great depths. Reports that animals were drawn to the surface 
from great depths were received with incredulity ; but when oceanic 
cables were laid, and, upon being drawn to the surface, were found to 
have animals fastened upon them, there could be no escape from the 
conclusion that the ocean bottom was inhabited. 

Explorations by the Challenger and Other Ships. — Expeditions 
were fitted out to explore this new world, and a wealth of scientific 
information was gathered, not only regarding the deep sea, but upon 
other phases of oceanography as well. This material, added to that 
which was previously known regarding the oceans, and supplemented 
by other later investigations, have given us a fairly full knowledge 
regarding the general features and conditions in the hydrosphere. 

Among these expeditions the one most noted was that of the Chal- 
lenger, a ship sent out by the British government between 1872 and 
1876, which made comprehensive explorations during a journey around 
the world. The extensive series of reports of this expedition, including 
the writings, here and elsewhere, of Sir John Murray, are to this day 
the most valuable source of knowledge concerning the oceans. 

There have been many other, less pretentious expeditions which 
have, however, contributed greatly to the science of oceanography. 
Among these are several other British expeditions, including Antarctic 
expeditions, the German expeditions in the Gazelle in 1 874-1876, the 

637 



638 COLLEGE PHYSIOGRAPHY 

series of Norwegian expeditions in the North Atlantic in the Michael 
Sars, and the long-continued work of the Prince of Monaco in the 
Hirondelle and other yachts. 

Knowledge concerning the conditions in the Arctic was obtained 
by Nansen in the Fram, and Peary has added some data during his 
expeditions. 

The United States has carried on extensive oceanographic work, 
especially near the American coast, the work of Alexander Agassiz 
and many others in the Blake, the Fish Hawk, and Albatross being 
the most valuable. Maury's early work on " The Physical Geography 
of the Sea " is a classic. Much data has also been gathered in con- 
nection with the laying of oceanic cables. 

Methods of Oceanographic Study 

Most Detailed Work near Coasts. — Naturally the most thorough 
oceanographic work has been carried on along the coast lines of the 
leading nations, where the shores are accurately charted, the depths 
of the water determined in great detail, characteristics of the tide 
thoroughly worked out, and the distribution of life understood. In the 
open ocean and along the more remote coasts less is known. The study 
of the deeper parts of the ocean is far more difficult than the study of 
the coast, and, for this work, especially constructed apparatus is needed. 

Sounding. — One of the principal instruments employed is the 
sounding machine, by which the depth is determined. Attached to 
the sounding line are thermometers which automatically record the 
temperature at the bottom ; and others, attached at various intervals, 
record the temperature at different depths. Samples of water are 
also brought up in metal tubes, which become automatically closed 
when the apparatus is drawn up ; and samples of the ocean bottom 
deposits are obtained by means of soap or other sticky substance on 
the end of the sounding apparatus. Even photographic exposure is 
made in order to determine the amount of light which penetrates to 
different depths. In the great depths of the ocean a single sounding 
may require an hour or two, but by it much information is obtained 
concerning conditions from the surface to the bottom of the sea. 

Dredging. — Following this, a dredge may be lowered to the bottom 
and dragged over it, in order to secure animals and samples of the de- 
posits on the sea floor. There are various forms of dredge, one of 
them being the deep sea trawl, which consists of a long, rectangular 
iron frame with a bag net attached. Dragged over the ocean bottom, 
the frame scoops up the loose deposits, and this, together with animal 
life, passes into the open mouth of the net, in which it may be drawn 
to the surface. 

Tow nets are dragged over the surface to capture the forms of life 
there ; and others are dragged at various depths to determine the 
nature of life between the surface and the sea bottom. The tempera- 



THE OCEAN 



639 



ture of the surface water, its composition and specific gravity, and its 
movements are also studied. 

Altogether, therefore, a vast .amount of knowledge has been obtained 
with regard to the oceans, not alone along the coasts and at the surface, 
but also at the bottom and in intermediate depths. 

Extent of the Ocean 

Nearly three-fourths of the earth's surface is covered by the ocean 
waters, with an average depth of 12,000 to 15,000 feet (Fig. 390). It fills 
the great ocean basins and overflows the continent edges over an area 
of about 10,000,000 square miles. There is so much water that, as 
already stated, if the earth were planed to perfectly regular form, it 

4500 
4000 
3500 
3000 
2500 
2000 
1500 
1000 
500 

/500 
1000 
1500 
2000 
2500 
3000 
3500 
4000 
,4500 

"~10^ 20.V 30^- 40^- 50^- 60^ 70# 80^" 90# 100^ 

Fig. 390. — Diagram showing proportions of the ocean at various depths. (Murray. ) 

would be covered by a universal ocean nearly two miles in depth. 
Yet, compared with the lithosphere as a whole, it is a mere surface film. 
The main facts about the distribution of ocean water have already 
been stated. From a broad belt in the southern hemisphere it extends 
northward between the continents, with which it is in contact along an 
exceedingly irregular line with many partly enclosed branches. Along 
this contact zone there is great activity of wave and tidal work giving 
rise to complicated shoreline phenomena, already studied (Chap. XI). 
The oceans cover over 139 million square miles. 



- 






- 


Area 


Percentage 


to 100 fms. 15 mill.sq. mis. 8 ^ 


ji 


100-500 " 26 " " -' 


13 r ^ ' * 
5 V Land 29$ / 


- as 


500--1000 " 10 " " ' 


.N 


1000-2000 " 4 " " > 
over 2000 " 2 >' » ' 


' ') J 




67 " "-' 








, SEA LE Y EL , , _— T-cS^NT 


. 




CONTINENTAL £DGE-^^C C oNTINENTAL SHELF 








-a 


OCEAN 


-^ 


-M 


^Z^^' 


^-~~~~^ Area Percentage 




to 100 fms . 10 mill.sq. mis. 5 >^ 


fey 


« — ■ — ' 


100-500 " r " " " 3 I 






500-1000 " 5 2 [ Water 


- / 




1000-2000 " 27 " " " 15 [ life 


/ 


' ..!. , f 


2000-3000 " 81 " " " 41 1 
OTer3000 " 10 " •• " bJ 

\ i 1 H0 r " r 



Ocean 


Atlantic l 


Pacific 2 


Indian 


Area in Square Miles 


41,321,000 


68,634,000 


29,340,000 



1 Including Arctic Ocean, Mediterranean Sea, etc. 

2 Antarctic or Southern Ocean divided between the Atlantic, Indian, and Pacific. 



6 4 o COLLEGE PHYSIOGRAPHY 



The Ocean Surface 

The surface of the oceans consists of saline water, varying consid- 
erably in composition and density, in temperature, and in colour, 
and disturbed by waves, tides, and currents. 

Curvature of Sea Level. — This surface, so level as compared to the 
lands that it is commonly called the sea level, is really a curved surface 
(Fig. 392) conforming to the oblate spheroid form of the earth. It 
departs somewhat from this perfect form because, in addition to the 
main attraction of gravity which holds the water in place, there is a 
lateral attraction from the lands which border the oceans. 

Distortion near Mountains. — The extent to which the ocean is 
distorted from the spheroidal form depends upon (1) the mass of land, 
(2) the density; and calculations of the amount of distortion vary 
because of the uncertainty of these factors, especially the second. 

It has been estimated that the lateral attraction exerted by the 
Himalayas causes the surface of the ocean to be 300 feet higher at the 
head of the Bay of Bengal than at the southern end of the Indian 
peninsula, meaning, of course, that the water is 300 feet farther from 
the earth's centre in the former than in the latter place. In the same 
way the Atlantic water along the coast of North and South America 
must be nearer the earth's centre than that on the Pacific coast where 
lofty mountains rise. These variations in the sea level are of course 
liable to change, as mountains rise higher or are worn lower by denu- 
dation. This is doubtless one cause for changes of relative level of 
land and sea during past geological ages. 

Topography of the Ocean Bottom 

The general topographic features of the ocean floor (Fig. 391) have 
already been stated in the study of the lithosphere. In general, these 
are (1) a continental shelf of varying width fringing the continents, 
(2) a continental slope descending to the ocean basins, (3) broad 
expanses of plains smoothed by deposit, (4) linear elevations, some 
ridge-like, others broad swells, others plateau-like, (5) cliffs due to 
faulting, (6) volcanic cones, (7) depressions, usually linear, known as 
deeps. The nature of these features may be more fully understood by 
a somewhat detailed description of one ocean, the Atlantic. 

The Atlantic Ocean 

The Continental Shelf. — Extending eastward to a varying distance 
from the coast line of United States, is a fairly level, submerged plain, 
broadest off Newfoundland, and sloping seaward at an average rate of 
1 or 2 feet per mile. This continental slope is somewhat diversified by 
elevations and depressions, some of the former rising up to or nearly 



THE OCEAN 



641 



to the surface near the coast, forming banks, shoals, or islands. The 
depressions are linear valleys. On the opposite side of the Atlantic, 
on the European coast, there is a corresponding continental shelf with 
similar characteristics. 

It is inferred that this extension of the continental plateau is really 
a former land surface, worn to low relief and now submerged. This 



to 6,0^0 feet l6 °" We3t 

to '2.000 feet 
112,000 to 14,000 feet 
Over 18,000 feet {"Deeps") 
Over 24,000 # 30,000 feet 



wSlira& 4'0- EaBt 6V hong. 12<^lI§If 




Fig. 391. — Map showing depths of the ocean. (Murray.) 

inference finds support, (i) from the resemblance of the submerged 
topography to that of the neighbouring land, (2) from the evidence 
that the land has been recently lowered in its relation to the sea, 
(3) from the presence, on the continental shelf, of valleys, which are 
apparently continuous with existing and valleys. Thus, off the 
mouth of the Hudson River, there is a valley extending clear to the 




Fig. 392. — Cross-section of the north Atlantic, (de Martonne.) 



edge of the continental shelf, where it forms a canyon some 2400 feet 
deep. Other similar, though less pronounced, valleys are found on the 
continental shelf off the mouths of the Delaware and Susquehanna 
rivers ; and a pronounced valley has been traced off the mouth of the 
St. Lawrence. Similar submerged valleys cross the continental shelf 
on the European coast, notably in the North Sea. 



642 



COLLEGE PHYSIOGRAPHY 



The Continental Slope. — On each side of the Atlantic the slope 
increases decidedly on the outer edge of the continental shelf, and there 
is a descent to the ocean basins (Fig. 393). This continental slope is 
not to be thought of as precipitous, though it may be locally. It is 
usually not even a steep slope, for the descent of a mile or two ver- 




FlG. 393. — Topographic map of the steep continental slope in the Atlantic east of Massa- 
chusetts with the relatively smooth continental shelf to the northwest. Depths in 
metres and in fathoms, i.e., 200 (109), meaning 200 metres — 109 fathoms = 654 feet. 
(From Boston Sheet, North K 19, International Map of the World on the scale of 
1 : 1,000,000.) 

tically may be distributed through a space of 50 to 100 miles. Yet, 
as the slopes on the ocean floor go, this continental slope is unusual, 
and it is striking in character because it completely encloses the 
oceanic depression. 

The Ocean Bottom Plain and Mid-Atlantic Ridge. — At the base 
of the continental slope the grade flattens again, and there stretches out 
a vast plain which extends throughout most of the Atlantic Ocean. 
About midway across the ocean the bottom rises in a broad swell, or 



THE OCEAN 



643 



plateau, which extends the length of the Atlantic, winding roughly 
parallel to the enclosing continents. Its elevation varies, being cov- 
ered usually by several thousand feet of water. It is sometimes called 
the Mid-Atlantic Ridge, but is really a series of three plateaus, varying 




Fig. 394. — Map showing depths in Atlantic Ocean. The Mid-Atlantic Ridge is made 
up of the Dolphin, Challenger, and Connecting Plateaus. The continental shelves 
are stippled. (Challenger Reports.) 



in breadth and elevation, forming an elevation not far from the mid- 
Atlantic. Both to the east and west of this linear series of plateaus 
the water is deep, being usually 15,000 to 18,000 feet, and in sev- 



644 COLLEGE PHYSIOGRAPHY 

eral areas descending in the so-called deeps to 20,000 feet or more 
(Fig. 394). 

Oceanic Volcanoes. — Some volcanoes rise from the mid-Atlantic 
plateau. Iceland lies near its northern end ; the Azores form a chain 
extending part way across it; and farther south are the volcanic 
islands of St. Paul, Ascension, and Tristan da Cunha. Other vol- 
canoes rise from the deeper waters on either side of the plateau, such 
as the Cape Verde Islands and St. Helena on the eastern side and 
the Bermuda Islands on the west. 

Relationship of Mediterranean Seas. — Both the Mediterranean 
Sea and the Caribbean- Gulf of Mexico enclosed sea really occupy basins 
within the continental area, the former more so than the latter. The 
West Indian mountain chain rises as a barrier between the American 
mediterranean and the ocean basin, and on the outer base of this bar- 
rier, close by Porto Rico,. is the deepest known point in the Atlantic 
Ocean, 27,972 feet, in a linear trough known as the Nares Deep. In the 
West Indian mountain area and in the Mediterranean there are some 
very steep submarine slopes, some of them being lofty precipices. 
The Mediterranean has a maximum depth of 14,400 feet, the Gulf of 
Mexico of 12,480 feet, and the Caribbean of 20,568 feet. Since each 
of these seas is separated from the ocean basin by an elevation, they 
are really separate basins, though continuous with the ocean at and 
near the surface. The lowest point at the entrance to the Mediter- 
ranean is 1 500 feet, the lowest point in the rim of the American medi- 
terranean being 5400 feet. 

Other Irregularities. — With the exception of a few broad indenta- 
tions, such as Davis Strait, the Bay of Biscay, and the Gulf of Guinea, 
the other irregularities of the Atlantic border are all located on the 
continental shelf. The Arctic Ocean resembles the mediterranean 
seas in that it is a basin surrounded either by land or by relatively 
shallow water. Much of its area is on the continental shelf of North 
America and Eurasia, but in the polar portions depths as great as 
14,400 feet have been found by Nansen and Peary (Fig. 379). 

Other Oceans 

In each of the other oceans the topography of the ocean bottom is 
similar in general features to that of the Atlantic. That is to say, 
there is a fringing continental shelf terminated by a continental 
slope along each of the continents, but varying greatly in width. 
Beyond this is the great ocean basin with level floor in the main, but 
diversified by both elevations and depressions similar to those de- 
scribed in the Atlantic. In none of the other oceans is there a medial 
plateau, as in the Atlantic, but there are numerous plateau areas, often 
with volcanic peaks rising from the crest. The topography of the 
western Pacific is particularly diversified by plateau uplifts, linear 
mountain chains, volcanic cones, and deeps (Fig. 391). 



THE OCEAN 645 

The general topography of the ocean bottoms is indicated on the 
accompanying maps, and will not be described in detail. The Pacific 
is the deepest of the oceans, having an average depth of 2f miles as 
compared with the average depth of the oceans as a whole, which is 
about 2 J miles. The Atlantic is slightly deeper than the average 
(about 2 J miles), and the Indian Ocean is about the average depth. 
Not only is the Pacific deeper on the average, but it includes the 
greatest known oceanic depths — the Planet Deep, 32,114 feet, near 
the Philippine Islands, 31,614 feet near Guam, 30,930 feet near New 
Zealand, 27,930 feet near the Kurile Islands, all in linear deeps close 
by pronounced uplifts. All together there are known to be 57 deeps 
in over 3 miles of water, 11 in over 4 miles, and at least 5 in which 
the water is over 5 miles deep. 

Origin of the Topographic Forms 

The Three Processes. — We cannot study the topography of the 
ocean bottom as we can that of the land, and hence the details of topo- 
graphic form are not so well known, nor can we bring so many facts to 
bear upon the interpretation of this form. In general, the topography 
of the ocean floor is clearly the result of either (a) diastrophism, 
(b) vulcanism, (c) deposition, or a combination of these. The only 
known exception is on the continental shelf near the continents, where 
there are erosional forms, now submerged. None of the oceanic agen- 
cies have erosional power, excepting in shallow, water and along con- 
tinental margins. 

Vulcanism in the Ocean. — Forms due to vulcanism abound in the 
ocean. Many of the volcanic cones rise above sea level, both along 
the continent borders and in the open oceans ; but many others are 
known which rise only part way to the surface. Whether there are 
submarine lava plateaus and other forms of volcanic deposit, is not 
known. 

Submarine Diastrophism. — Diastrophism has played a far more 
important role in the development of the ocean bottom topography 
than vulcanism. The great ocean basins are themselves depressed 
areas, and the continental slope is apparently in the main either a line 
of faulting or of warping, probably in some places faulting, in others 
warping. The broad ocean bottom plateaus are evidently unwarped 
portions of the sea floor, the submarine mountain ranges are more 
sharply folded and faulted zones, and the deeps are areas of exceptional 
subsidence. Movements such as have produced these features are 
evidently still in progress on the ocean bottom. 

Marine Deposition. — Deposition is in progress all over the sea 
floor, but in the deeper ocean, far from land, it is evidently very slow 
and is hardly a factor of prime importance in determining the general 
levelness. Near the continents, especially on the continental shelves, 
on the other hand, the waste of the land is strewn over the sea floor 



646 



COLLEGE PHYSIOGRAPHY 



in an extensive sheet. Much of the levelness of the continental shelf 
is, without doubt, due to this deposit ; and it is possible that the shelf 
itself is in part built by deposit from the waste of the land. Even the 
continental shelf may in places represent the outward advance of the 
deposit borne to the sea from the waste of the land. 

Deposits on the Ocean Floor 

The Three Types of Deposits. — In sounding, small samples of the 
ocean bottom deposits are commonly obtained ; and in dredging larger 
quantities are brought to the surface. There is, therefore, fairly 
extensive knowledge of the materials covering the ocean bed. The 
nature of these materials varies with the distance from the land and 
with depth. These differences may best be understood by a descrip- 
tion of three zones : (i) the continent borders, the seat of land-derived 
deposits; (2) the ocean basins, down to depths of 12,000 to 15,000 




Fig. 395. — Distribution of deep sea deposits. (Murray.) 



feet, over which oozes are deposited ; (3) the deeper parts of the 
ocean basins, below 12,000 to 15,000 feet, over which red clay occurs 
(Fig. 395). 

Land-derived Deposits. — As might be expected from their origin, 
the most notable characteristic of the deposits along the continental 
borders is their variability. These littoral deposits vary in texture, 
from gravels and sands near the shore to exceedingly fine muds off- 
shore. They also vary in composition (1) according to the abundance 
of included organic remains from organic life, (2) according to the 
nature of material supplied from the land. Thus, in the first direction 
the sediments may vary from almost pure organic matter to clastic 



THE OCEAN 



647 



fragments nearly free from organic remains. In the second direction 
there is great variation, depending upon the rocks along the shore, and 
the nature of the sediment brought to the sea by rivers. Thus the 
littoral deposits may be calcareous where derived from limestone 
regions, or they may be made up of detritus of granitic, or volcanic, or 
any other of the many rocks of the land. Soundings in the zone of 
littoral deposits thus reveal great diversity both in texture and in 
composition (PL VI). 

Ocean Bottom Oozes. — This contrasts very strikingly with the 
comparative uniformity of conditions in the area of the ocean bottom 
covered by the deep sea oozes. Here, over an area equal to more than 
a third of the ocean bottom, 
over 50 million square 
miles, the ocean floor is 
covered by an exceedingly 
fine-grained, calcareous 

ooze, composed mainly of 
the remains of organisms 
that have lived in the waters 
of the ocean, and, upon 
death, have fallen to the sea 
bottom. Mixed with this 
organic matter are (a) re- 
mains of ocean bottom 
animals, (b) volcanic ma- 
terial, especially bits of 
pumice that have floated on 
the ocean and fallen to the 
bottom on becoming water- 
soaked, (c) minute quanti- 
ties of fine-grained rock 
fragments from the land, 

(d) some chemical deposits, 

(e) particles of iron derived 
from the fall of meteorites. 

In the main the deposit 
is composed of the remains 
of minute and even microscopic organisms that live in vast numbers in 
the waters at the surface and at intermediate depths. Some of these 
are perfect in form, but many are comminuted by the action of ocean 
bottom animals through whose digestive tracts the ooze has passed. In 
the deeper waters the organic remains have suffered also from solution 
in the deep sea water. There is great variety in the organic remains, 
but over large areas the predominant forms are Foraminifera, partic- 
ularly various species of Globigerina (Figs. 396, 397). This has given 
rise to the name globigerina ooze applied to these calcareous deposits of 
the deep sea. It is estimated that the deposit of globigerina ooze 




Fig. 396. — Globigerina from the surface of the 
ocean, much enlarged. (Challenger Reports.) 



64S 



COLLEGE PHYSIOGRAPHY 



covers an area of 47,752,000 square miles at a mean depth of about 
12,000 feet. In colour the ooze is commonly pale gray, though at 
times coloured red by iron oxides or brown by manganese. 

In parts of the ocean there are oozes in which other organisms are 
dominant, or form so large a proportion as to give rise to other names. 
These are, for instance, pteropod oozes and silicious oozes, especially 

radiolarian and diato- 
maceous oozes. In all 
cases the origin is the 
same, the difference 
being in the percentage 
of certain types of or- 
ganisms. Even in an 
ooze bearing a certain 
name, as globigerina, 
there are remains of 
other organisms, such 
as pteropod, radiolaria, 
etc. 

Red Clay. — Over an 
area even greater than 
that occupied by globi- 
gerina ooze, estimated 
to cover some 55,000,000 square miles, or nearly that of the area 
of the lands, is a peculiar clay, usually red in colour because of iron 
stain, though sometimes chocolate because of manganese stain. This 
red day deposit, which occupies the deeper parts of the ocean, at 
depths below 12,000 to 15,000 feet, is the most extensive deposit on 
the earth, as well as one of the most slowly forming. 

In it there are remains of organisms which have lived in the waters 
above, but only these which are sufficiently insoluble to have resisted 




Fig. 397. — Globigerina ooze from the bottom of the sea. 
(Challenger Reports.) 



n*t 



Sea l eve f 




Fig. 398. — The relations of distribution of ooze and red clay to depth, shown on the left, 
and the decreasing proportion of lime in the latter, on the right. (Murray.) 

the solvent action of the deep sea water, which is charged with carbon 
dioxide. Calcareous remains and other soluble substances are dis- 
solved in passage through the deeper waters, and only their insoluble 
residue passes on to the bottom (Fig. 398), making minute contributions 



THE OCEAN 649 

to the sediment. Grains of volcanic materials and bits of pumice also 
constitute a part of this peculiar sediment. How slowly the red clay 
is accumulating is indicated by the fact that the silicious ear bones of 
whales and teeth of sharks are frequently drawn up in dredgings in the 
red clay area far from land. Since few such animals would fall to the 
bottom in any one locality in a brief interval of time, the fact that 
small parts of such animals are brought up in the dredges is clear proof 
of the great slowness of accumulation of the red clay. Even more 
striking is the fact that particles of metallic iron are also found in 
dredging in these great depths. These iron particles are evidently 
portions of meteorites which have fallen into the sea. One would 
need to make a careful and long extended search on the land to find 
even one of these fragments ; but the deep sea dredgings, located by 
chance, have frequently brought them to the surface. Since it cannot 
be inferred that such material falls more abundantly in these areas 
than on the land, we must conclude that their abundance is the result 
of concentration through centuries in an area of such slight deposit 
that they are not deeply buried. 

The iron and manganese which discolour the deep sea clay are derived 
from the insoluble residue of the marine organisms, from the volcanic 
minerals, and from the cosmic iron particles. Oxidized in the impure 
waters of the deep sea, the iron assumes the strong red colour which 
gives the name to the red clay. 

Absence of Deep Sea Sediments on Land. — It is a noteworthy 
fact that, though the red clay covers an area greater than that of the 
lands, it is not recognized among the sediments of the continents. 
Even oozes, covering nearly as great an area, are not conspicuous 
among the sedimentary strata of the land, the chalk deposits - — a 
variety of limestone — found in a few places being the sole exceptions. 

From these facts one is forced to the conclusion that, although por- 
tions of the continents have again and again been lowered beneath the 
sea, now here, now there, such depressions have rarely extended far 
enough to introduce real deep sea conditions ; and never, so far as we 
know, have any parts of the existing continents been lowered to the 
depth where red clay was deposited. Whether this is an argument for 
relative permanence of ocean basin and continent plateau position, or 
whether it is merely an indication that in earlier ages deep sea condi- 
tions did not exist, cannot at present be stated. It is of course also 
possible that parts of continent areas have actually been depressed 
to deep sea conditions and have then remained there. 

Conditions on the Ocean Bottom 

Uniformity and Monotony. — There is no part of the earth's surface 
where there is such uniformity of conditions and such monotony as on 
the deep sea bottom. No sunlight penetrates to these great depths, 
arid a condition of perpetual darkness prevails there, excepting as 



650 COLLEGE PHYSIOGRAPHY 

relieved by the phosphorescent light emitted by deep sea animals. 
The differences between day and night and summer and winter, there- 
fore, produce no effects on the ocean bottom. 

Without sunlight there can be no plant life ; but animal life exists 
in considerable variety, depending for its food supply upon that which 
rains down from above, as organisms in the upper levels of the ocean 
die and fall to the bottom. 

The Great Pressure. — Since the pressure increases from i^ tons 
to the square inch in 6000 feet to 6f tons per square inch in the 
deepest water, it is clear that the deep sea animals live under an enor- 
mous pressure. The superincumbent water has no noticeable effects, 
since the pressure, being equal in all directions, is counterbalanced in 
all parts of their bodies. Only when the deep sea forms are raised to 
the lighter pressures of the atmosphere are the effects of these deep sea 
pressure conditions noticed. Then, with pressure removed from out- 
side, expansion of gases within causes their air bladders to protrude 
from the mouth, their eyes to project from their sockets, and their skins 
to crack open. 

Coldness. — Besides darkness and great pressure, the ocean bottom 
is perpetually cold. The temperature of the ocean water in general 
decreases with depth, and, throughout most of the deep ocean basins, 
is within four degrees of the freezing point, and in places even below the 
freezing point of fresh water. Such uniformly low temperatures 
naturally reduce the vitality and diminish the variety and abundance 
of deep sea life, especially in the deeper, colder portions. 

Slight Movement. — In the deep sea there are no rapid movements 
of the water, such as one finds on the surface and along the ocean 
margins, but there is a very slow current, or drift, by which the low 
temperatures are imported. This drift is also the source of the 
oxygen which the deep sea life must have, but in the deeper waters, 
far from land, it is not sufficient to bear away the carbon dioxide. 
In such places the bottom water is so charged with these gases that it 
performs the solvent work already referred to in describing the red 
clay deposits. 

Uniform high pressure and cold characterize the bottom of the great 
ocean basins. In addition to the lack of variation with the seasons, 
or with day and night, there is no diversity of conditions due to move- 
ments of the oceanic waters. Far and wide is a broad expanse of 
ooze or clay, and the only change is that which is caused by the slow 
rain of organic remains from above, — the source of the food supply, 
— and the slow drift of the cold waters, — the source of the oxygen 
supply, of the deep sea animals. 

Composition of the Ocean Water 

Amount of Mineral Matter in the Sea. — The waters of the ocean 
differ from those of the land in being salt. But, besides common salt, 



THE OCEAN 651 

there are a great number of substances dissolved in the ocean water. 
Altogether there are about 3 J parts of dissolved mineral matter to every 
100 parts of ocean water. In other words, there are 3I tons of dis- 
solved mineral in every hundred tons of water. Altogether the oceans 
contain about 5§ million cubic yards of dissolved material ; \\ 
million cubic yards of this is common salt. If all this material 
could be removed and deposited in a uniform layer, it would form a 
layer 175 feet thick over the entire ocean bottom. There is fully \ 
as much mineral substance in solution in the oceans as exists in the 
lands above sea level. It is this dissolved mineral that causes ocean 
water to be heavier than fresh water. Taking the specific gravity of 
fresh water as 1, that of sea water is, on the average, 1.026. 

Substances Present. — Among mineral substances dissolved in the 
ocean, common salt, or sodium chloride, is by far the most abundant, 
constituting 77.758 per cent, or more than two- thirds of the whole. 
Then follow magnesium chloride (10.878 %), sulphate of magnesium 
(4.737 %), sulphate of lime (3.6 '%), sulphate of potash (2.465 %), 
carbonate of lime (0.345 %), and bromide of magnesium (0.217 %). 
A complete analysis, while doubtless revealing the same order of im- 
portance, would give slightly different percentages, for a great 
variety of substances are in solution in the ocean in even more minute 
quantities. No less than 32 different elements have been detected in 
sea water, and there is little doubt that all the elements in the rocks of 
the earth's crust exist in some combination or combinations in sea 
water. Among the elements known to exist in solution in the ocean 
water are gold, silver, copper, zinc, lead, cobalt, nickel, manganese, 
aluminum, iron, and silicon. 

Source of Salts in the Sea. — The source of these salts is not far to 
seek. While some are doubtless supplied during submarine volcanic 
eruption, certainly a large proportion of them reach the sea in solution 
in the fresh water of the lands. Through the complex processes of rock 
disintegration and the chemical changes caused by underground water, 
a mineral load is supplied to running water which finds its way to the 
sea. That this source is capable of causing salinity, and the concen- 
tration of other mineral substances in water where there is concentra- 
tion through evaporation,' has already been pointed out (p. 324) in 
connection with the development of salt lakes. 

The present chemical impurity of the ocean water could readily be 
accounted for on the simple assumption that through the geological 
ages the waste of the land has supplied this mineral load, the water 
evaporating as fresh water vapour from the surface of the sea, passing 
over the land and bearing a load of dissolved mineral matter, and later 
passing through a similar cycle on again being exposed to evaporation. 

Indeed, on the assumption of an original fresh water body, growing 
progressively Salter, an attempt has been made to estimate the age of 
the ocean, with the result of about 370,000,000 years. Such a cal- 
culation can have no certain value; because (1) it is not certain that the 



652 COLLEGE PHYSIOGRAPHY 

original ocean was fresh ; (2) the volume of the ocean may have varied 
widely during geological time; (3) the possible supply of sodium chlo- 
ride from other sources than rivers of the land is of unknown quantity ; 
(4) extensive quantities of salt once in the ocean have been taken from 
it during the deposit of sedimentary rocks, in which there are even beds 
of salt. 

Contrast with Salts in Rivers. — The fact that the salts in the ocean 
water are not in the same proportion as those in the running water of 
the lands might seem opposed to the view of land origin of mineral 
water in the sea. For example, carbonate of lime forms a very com- 
mon and easily recognized constituent of fresh water, while sodium 
chloride is present only in minute quantities. Silica, commonly 
present in fresh water, is present in salt water only in minute quantity ; 
and, on the other hand, the most abundant salts in the ocean (the 
chlorides, sulphates, and bromides) are not common either in the 
rocks of the land or in running water. 

Withdrawal of Lime and Silica. — The excessive amounts of oceanic 
salts which occur only in minute quantities in fresh water is not 
difficult to understand as a result of concentration through constant 
supply from the land, and constant evaporation of ocean water. 
That more common substances, like silica and carbonate of lime, are 
not also concentrated and caused to form dominant constituents of the 
mineral load of the ocean waters, is evidently due principally to two 
facts : (1) that organisms are constantly extracting them, (2) that there 
is precipitation. Increase in salinity diminishes the solvent power of 
water for carbonate of lime, and precipitation is known to be in prog- 
ress in parts of the ocean. Also there are precipitates of glauconite, 
manganese, and other substances on parts of the ocean bottom. A 
great variety of living forms with countless billions of individuals are 
at all times extracting carbonate of lime from sea water and building it 
into bones, shells, tests, and other parts of organisms, both animals 
and plants. Silica is likewise extracted by a great variety of animal 
and plant life, such as sponges, radiolaria, and diatoms. By these 
two processes those mineral substances which animals can use are 
extracted and their quantity kept down ; while substances not needed 
in organic life are left to accumulate. 

Variations in Mineral Content of Oceans. — Throughout the ocean 
there is a fair degree of uniformity in the mineral load, for the ocean 
waters are ever in movement, and there is, therefore, a tendency toward 
mixture. There are, however, some noteworthy variations in composi- 
tion, and consequently in density. Probably there are variations 
near the coast, due to the composition of the waters that run off from 
the land ; and certainly there is decrease in density where large rivers 
pour the lighter fresh water into the sea. Similar decrease in density 
is observed in oceanic areas of heavy rainfall and in the Arctic and 
Antarctic regions, where evaporation is slight, and where melting ice 
returns much fresh water to the ocean. While the average density of 



THE OCEAN 653 

ocean water is 1.026, in the ice fields of the Antarctic a density of 
1.024 has been observed. 

On the other hand, there is increase in density in areas of evaporation, 
for there fresh water is removed and the salts are more concentrated. 
In the Trade Wind belts of the ocean, evaporation is so effective that 
the density of the ocean surface water is notably increased, as in the 
North Atlantic, where a density of 1.0278 has been observed. Even 
more dense is the water in seas enclosed by warm lands and shut off 
from free mixture with waters of the open ocean, as in the Mediter- 
ranean and Red seas. In the latter a density of 1.03 is recorded. 

Since the dense water is heavier, it tends to sink, and, therefore, a 
limit is set to the extent to which surface water may be made dense by 
evaporation. This gives rise to a circulation with a tendency (1) to- 
ward mixture of waters of different densities, (2) toward the stratifica- 
tion of oceanic waters according to density. Were it not for other 
movements of oceanic waters, it is probable that such a stratification, 
with the densest water at the bottom, would be much more pronounced 
than it is. The most notable difference in density with depth are 
those observed where large quantities of fresh water are added to the 
ocean surface, as by rain, by inflow of rivers, and by melting ice. 
There the lighter freshened water floats on the denser salt water, and 
there are often very decided differences in density with depth. 

There is a very common misconception to the effect that the ocean 
water so increases in density that, below a certain level, objects that 
sink in ordinary sea water will float. That this is not true, is proved 
by the fact that even microscopic organic remains sink to the ocean 
bottom, even at depths of several miles. Water is so nearly incom- 
pressible that, even under the enormous pressure of the deep sea, there 
is only slight increase in density as a result of the pressure. In this 
respect the hydrosphere is strikingly different from the atmosphere. 
The density of the ocean bottom water is, however, greater than that 
of the surfaces for a variety of reasons : (1) the settling of dense surface 
water; (2) a measure of compression under the weight of the overlying 
column, notably as a result of the compression of included gases; 
(3) the low temperatures, for the density increases with decrease in 
temperature in sea water down to 2 8° F., the freezing point, while in 
fresh water density diminishes with a fall of temperature after 7 above 
the freezing point is reached. 

Gases in Sea Water. — Besides mineral substances, ocean water 
also contains large quantities of atmospheric gases in solution, nitrogen 
and allied inert gases in greatest amount (3 7 \ %), oxygen next (33 J %), 
and then carbon dioxide (i6\ %). These are absorbed from the air 
on the smooth ocean surface, and in spray and foam of the wind waves. 
Both oxygen and carbon dioxide are also added by the marine organ- 
isms, and probably there is further supply from submarine volcanic 
sources. There is a limit to the amount of each of these gases that 
can be absorbed by the water, and this limit varies with the tempera- 



654 COLLEGE PHYSIOGRAPHY 

ture, cold water being capable of dissolving more than warm. Thus it 
has been found that a little less than twice as much nitrogen and 
oxygen are dissolved in sea water at 3 2° F. than at a temperature of 
86°. 

Although the main supply of these gases comes from the atmosphere, 
and hence is absorbed at the surface, all three are found in all parts 
of the ocean. They are thus distributed to some extent by slow dif- 
fusion, but primarily by movements of the oceanic waters. This is a 
matter of very great importance, since upon the presence of the dis- 
solved oxygen life in the ocean mainly depends, and particularly at 
depths below the surface. In organic processes there is constant 
withdrawal of oxygen from the ocean water ; and, in some of the deeper 
parts of the ocean, animal life is limited by reason of the scarcity of 
this gas. Nitrogen, being little used by marine organisms, does 
not vary greatly. Carbon dioxide is taken from the upper layers 
of the ocean by plants, but since there is no plant life in the deep sea, 
there is no exhaustion from that source. On the contrary, organisms 
at the surface, on the sea floor, and in intermediate depths are con- 
tributing carbon dioxide to the sea; and in all probability there is 
further important contribution from submarine volcanic sources. 
There is no depletion of the supply of carbon dioxide ; but, on the con- 
trary, it is probable that the ocean is one of the sources of carbon 
dioxide in the atmosphere. It is estimated that there is 18 times as 
much carbon dioxide dissolved in the ocean as exists in the entire 
atmosphere. 

Colour of the Ocean Water 

Distribution of Blue and Green Water. — The normal colour of the 
ocean water is blue, and it is often a rich indigo blue ; but in some 
parts of the ocean the blue colour is absent and the water is green 
instead. The bluest of waters are found in the warmer parts of the 
ocean, as in the Gulf Stream, while the colder waters, such as the 
Arctic, are the greenest. Green ocean water is also found along some 
of the coasts. The causes for the differences in colour are not 
thoroughly understood, and they are perhaps of somewhat complex 
character. 

Relation to Colour of Sky. — One naturally thinks of reflection from 
the sky as a cause for the blueness of the ocean, and this is doubtless a 
factor ; but the fact that the blue and even indigo blue may be seen 
with overcast sky, while the deep blue is not observed in the Arctic 
waters, even with bright sunshine, proves that this is not the sole cause. 

Relation to Pureness. — Observations upon distilled water placed 
in a long tube show that its natural colour is blue, while the addition of 
organic or inorganic impurities gives a greenish colour. It seems prob- 
able, therefore, that the bluest ocean waters are the purest, while the 
green waters have a larger proportion of either organic or inorganic 
matter. The white light entering the sea water is diffracted, and the 



THE OCEAN 655 

light waves of shortest length — the violets, indigoes, and blues — 
are scattered and reflected, giving the blue colour. With more impurity 
the coarser green waves are also reflected, and dominate in determin- 
ing the colour. 

Relation to Rivers. — Near coasts a cause for the greenish water 
may well be the suspended sediment that finds its way to the sea from 
the land. In some of the partly enclosed seas like the shallow Baltic, 
the water is already discoloured by sediment ; and off the mouths of 
large, muddy streams, like the Mississippi, the sea is discoloured for 
long distances. It is due to this cause that the Yellow Sea of the 
Chinese coast received its name. 

Selective scattering and reflection of certain colour waves in white 
light is probably the chief cause for the blue and green colour of the 
sea, with reflection of the sky colours as a subordinate cooperating cause 
for ocean water colour. 

Relation to Life. — In all ocean water there is a vast abundance of 
minute and microscopic life ; and it is possible that the difference in the 
nature and abundance of this life is the reason for the fact that the 
water is blue in one part and green in another. If such life is more 
abundant in cold than in warm waters, as seems to be the case, the 
selective scattering of the green rays may be explained. The colour of 
the Red Sea is said to be due to the presence of immense numbers of 
minute reddish algae. 

Relation to Salinity and Dissolved Gases. — There are two other 
possible causes for the difference, — salinity and amount of dissolved 
gases. The cold waters are less saline and contain more included gases 
than the warm waters. In shallow water, the colour is in part deter- 
mined by reflection from the bottom, and in coral reef regions the 
reflected greens and purples from the different areas of sea bottom are 
often very beautiful, in the midst of the normal indigo blue. 

Light in the Ocean 

Depth of Penetration by Sunlight. — The sunlight becomes rapidly 
dimmed in its passage through water, and at great depths no sunlight 
penetrates. By an ingenious apparatus Helland-Hansen succeeded 
in exposing photographic plates during the expedition of the Michael 
Sars in 19 10. He found that at depths of 300 feet during bright 
sunlight all the rays of the spectrum were present in sufficient quantity 
to affect photographic plates exposed for two hours. At 1800 feet, 
blue rays were present, but no sign of red and green rays ; below 1800 
feet and down to 3000 feet light penetrated in the form of ultra-violet 
rays, while rays which the human eye sees were present in only small 
quantity. At a depth of 5400 feet an exposure of two hours failed to 
show the existence of even ultra-violet rays. We may, therefore, as- 
sume total darkness for the ocean bottom at depths of a mile and over, 
in so far as sunlight is concerned. 



656 COLLEGE PHYSIOGRAPHY 

i 

Phosphorescence. — Ocean bottom animals are, however, in many 
cases provided with means for the production of phosphorescent light. 
When a dredge is brought to the surface at night, it is first seen as a 
glowing object, and the cold ocean bottom ooze and the animals in it 
are aglow with phosphorescence. A dim light is, therefore, provided, 
and at least some species of animals carry their own light or develop 
it on need. 

Phosphorescence is not confined to the ocean bottom. Surface 
organisms, both large and small, are capable of developing it, and so 
probably are the animals of intermediate depth. This phosphorescence 
is often seen in the ocean, a boat leaving a trail of phosphorescent 
light, developed by the countless millions of organisms disturbed by its 
passage. The whole water seems aglow because of the immense num- 
ber of minute organisms in it, and here and there a larger phosphores- 
cent animal with brighter light shines out in the midst of the general 
glow. Such phosphorescence is wonderfully developed in the cold 
northern waters ; and it is not always present to the same extent and 
degree. Some nights it is developed on the slightest disturbance of the 
water; at other times it is not to be seen. Neither the cause for the 
phosphorescence nor for its variation are understood ; nor is it known 
what part it plays in the economy of organisms. 

Ocean Temperature 

The temperature of the ocean water varies notably (i) from place 
to place on the surface, and (2) from the surface to the bottom. While 
there is much irregularity, due to special causes, there is, in general, a 
decrease in temperature (1) from the surface downward, and (2) from 
equatorial to polar regions at the surface (Fig. 390). 

Surface Temperatures. — In equatorial regions the average tem- 
perature of the surface waters is about 8o° F., and there is a general, 
though not regular, decrease from this to 2 8°, the freezing point of 
salt water in the polar regions. Salt water differs from fresh water in 
two very important respects in connection with change in temperature : 
(1) while freshwater freezes at 32 F., salt water freezes at 28 , or, if 
its salinity is reduced, at a slightly higher temperature; (2) while 
fresh water ceases to grow denser and sink at 39. 2 F., salt water con- 
tinues to become denser and to sink until the freezing point is reached. 

Water warms much more slowly than land, and it also cools more 
slowly, so that there is less variability in the temperature of the ocean 
surface from day to night and from season to season than on the lands 
in the same zone. Thus, both in tropical and polar zones, the annual 
range of temperature of the surface waters does not normally exceed 
io° at any given locality. The temperature range on the frigid ocean 
may become notably increased when it is frozen, for then the ice sur- 
face may have a temperature far below the freezing point, though it 
cannot rise above it. The temperature of the water beneath the ice, 



THE OCEAN 



657 



however, does not decrease with that of the ice. Between the polar 
and tropical zones there is a greater annual range of temperature, in 
some places where there are cold currents in winter and warm currents 
in summer even amounting to as much as 40 . But even this range, 
which is great for the ocean, is small compared to the range on the 
land in the same latitude, which may amount to as much as ioo° to 
125 . 

As soon as the temperature of the surface water changes, there is a 
corresponding change in its density ; and, since water is a very mobile 




Fig. 399. — Temperature of the surface of the ocean in degrees Fahrenheit. (Challenger 

Reports.) 



liquid, there is consequent movement in order to bring about adjust- 
ment to the new density condition. Some of this movement may be 
vertical, some of it in surface flow. There is thus a tendency to dis- 
tribute the temperature condition of one locality to another, either 
horizontally or vertically. Further distribution is affected by the 
movements resulting from differences in density due to salinity, and 
by the currents of water that move before the winds. In these ways 
the water warmed or cooled in one latitude may transport the tempera- 
ture of that latitude to another part of the ocean. 

Influence of Flow on Surface Temperature. — There is a general 
flow of surface water in a series of well-deiined currents and drifts from 
equatorial to temperate regions, and even into the polar zones. These 
warm currents bend the ocean surface isotherms distinctly northward, 
whereas surface currents from the polar zones bend them southward. 
Thus, instead of a parallel series of isotherms with regularly progressive 
decrease in temperature from equator to pole, we have, as the chart of 
the Atlantic (Fig. 399) shows, a very irregular arrangement of the 



658 



COLLEGE PHYSIOGRAPHY 






*i § 



1 I s 



ocean surface isotherms. This is the greatest cause for disturbance of 
the regularity of the decrease in temperature from the equator to the 
poles ; but the influence of neighbouring lands is also important. 

Influence of Land on Surface Temperature. — The influence of the 
lands is not important directly, but because of the water and air that 
flow from them. Locally the ocean water is warmed by the inflow of 
river water ; and it is locally cooled by the discharge of icebergs from 
tidal glaciers. Much more general, and much more important, is the 
outflow of air from the land, bearing with it temperatures of the land. 
This influence is a cause for lowering the ocean temperature where the 
outflow is from glaciers or from the cold land of winter ; it is a cause 
for raising the temperature where the winds blow from warm lands. 
The first influence must be felt in the partly enclosed Arctic, and in 
such seas as the North Sea and the Baltic. The warming influence is 
distinctly noticeable in the mediterranean seas of the warmer regions, 
such as the Gulf of Mexico, the Mediterranean, and the Red Sea, 
all of which have higher temperatures than the neighbouring ocean. 
The Red Sea is the warmest large area of the ocean water, its tempera- 
ture rising to 90 and, at times, in summer, even higher. The extent 
to which the land exerts this influence depends upon the direction 
of the wind, being least noticeable on coasts toward which the prevail- 
ing winds blow, and being most 
1 I J * noticeable on those seas that 
I I I § are nearly land-enclosed, like 
I J I I the Red Sea. 

Temperatures below the Sur- 
face. — Deep sea exploring ex- 
peditions have made great 
numbers of temperature obser- 
vations, at various depths, in 
all oceans, so that the vertical 
range of temperature is now 
fairly well known (Fig. 400). 
These observations are made 
with a special form of thermom- 
eter attached at intervals to 
the sounding line. Provision 
must be made to record the tem- 
perature at a known depth, and 
this is accomplished by the 
automatic inversion of the thermometer on being drawn toward the 
surface, and the automatic recording of the position of the mercury 
at the moment of inversion. Provision must also be made against 
the enormous pressure in the ocean depths, which is about a ton 
per square inch for every 1000 feet of depth. This is accomplished 
by means of a protecting outer tube on which the pressure is exerted, 
but without pressing on the mercury bulb. 



500 r~7 

__1000 _ Vi 

\37.2 

1500 \ 

2000 


-i^f^r 








JS6 


6o7 tr c 


"^n^: 














40 






^45'" 


0! 

! • 1 

V ol 
\36.2 1 1 

\ 1 | 


^£ 




1 

1 

1 
1 
1 

1 


^ 


1 


2500 


\&£x. 






2800 





Fig. 400. — Profile of the Atlantic between 
New York and Bermuda, showing relations 
of temperature to depth. (Alexander 
Agassiz.) 



THE OCEAN 



659 



Decrease of Temperature with Depth. — In the open ocean it has 
been found that there is at first a rapid decrease in temperature in the 
temperate and tropical zones,. and then a much slower decrease toward 
the ocean bottom, except when surface water is at freezing. Generally 
the temperature is below 40 F. in depths greater than 4000 feet, and 
it is probable that four-fifths of the water of the oceans has a tempera- 
ture of 40 or less, while the average temperature of 
the water of the entire ocean cannot be much, if 
any, higher than 39 . From the 40 level, at a 
depth of 4000 feet or thereabouts, the temperature, 
decreases, slowly, reaching 34 to 36 in the deep 
ocean basins (Fig. 402), and in places, especially in 
the southern oceans, descending to 32 and even to 
31 . Under the equator the ocean bottom tempera- 
ture, at great depths, is close to the freezing point 
(Fig. 401). 

While it cannot be said that the ocean waters are 
characterized by certain temperatures at given 
depths, there is a general stratification, with colder 
water at the bottom, and with the cold layers by far 
the thickest. There are four notable variations from 
this general condition. (1) On the continental 
shelves there are local differences due to currents 
and to seasonal variations. (2) In the paths of 
ocean currents there is often interference with the sm>. 
rate of decrease, and this interference may vary with fig. 401. — Rate of 
the season, or with a shifting of the position of the decrease in tem- 
current. (3) In the polar zones, since the surface equator 6 
waters are never warm, the nature of the change 
from surface to bottom temperature is different from that* of the 
warmer zones. (4) Enclosed seas depart widely from the normal 
rate of decrease. 

Temperatures in Enclosed Seas. — This latter point is one of very 
considerable importance. In the Atlantic Ocean off Porto Rico, at a 




<3\ 00 VO 

ctffrrrwEtfTAL \ ! 

SHELF 



\o °oo *a 

f yC0NTWERTA.L 




Fig. 402. — Diagram to show relation of temperature to depth in the Atlantic. 



depth of 12,000 feet, the depth of the lowest portion of the Gulf of 
Mexico, the temperature is 35 ; but the temperature at the bottom 
of the Gulf of Mexico is only 39^°, which is the temperature of the 
Atlantic water at a depth of 5400 feet. South of Porto Rico, the lowest 
point in the barrier of the Gulf of Mexico is 5400 feet. It appears, 



66o 



COLLEGE PHYSIOGRAPHY 



Atlantic 


68° 


35 <5 




Mediterranean 

75° 




54° 








55° 




52° 


/* -^v 






55° 




38° / 




X 




55° 




37° /■'. 


\ 


55° 




S5Vf:':'vV: yi 




\.55° 



2000 
Ocean Surface 




Fig. 403. — Temperatures in the Mediterranean and Gulf 
of Mexico controlled by the barriers at their mouths. 



therefore, that the coldest water that finds its way into this enclosed 
sea is from this 5400 foot level, and, being denser than the rest of the 
water of the Gulf of Mexico, it settles to the bottom. It appears also 
that coldness of ocean water is not an inherent quality of depth, other- 
wise the bottom waters 
^1 in the Gulf would have 

11 the same temperature 

as the ocean waters at 
the same depth (Fig. 

403)- 

Other partly enclosed 
seas illustrate the same 
condition. The Medi- 
terranean, for example, 
though over 14,000 feet 
deep, has a uniform 
temperature of 55 be- 
low a depth of ^50 feet, 
while in the open At- 
lantic off Gibraltar the 
temperature is 37 or 
38 . The Red Sea has 
a temperature of 70 
from a depth of 1200 feet to its bottom, 3600 feet below the surface. 
On the theory that these bottom temperatures of enclosed seas are 
due to the creeping in of water at the level of the barriers, oceanog- 
raphers have even gone so far as to predict the existence of un- 
discovered barriers to account for unusually high bottom tempera- 
tures, and by later soundings verified the prediction. 

Coldness at Great Depths not Inherent. — The cold waters of the 
ocean depths cannot bean inherent condition, for there are causes pres- 
ent which would slowly raise the water temperature, if permitted to 
operate uninterruptedly. Two of these processes are of special im- 
portance : (1) heat is slowly escaping from the earth, and, as it is 
conducted to the ocean waters, it raises the temperature, and, since 
this decreases the specific gravity, the water rises. In the long geolog- 
ical ages this process of itself would have destroyed any inherent cold 
in the ocean bottom water. (2) The sun shining on the water sur- 
face raises its temperature, and, slow though the process of conduction 
through water is, it may be confidently stated that this cause too 
would have sufficed long since to raise the temperature of the bottom 
waters well above their present condition if no additional store of cold 
water were supplied. 

Creeping of Cold Water toward the Equator. — There is a per- 
fectly simple cause for this cold water and for keeping up its supply ; 
namely, the sinking of cold water in temperate and polar zones and the 
creeping of this dense cold water along the ocean bottom, with a slow 



THE OCEAN 661 

rise at the equatorial belt to restore the equilibrium due to the pole- 
ward surface flow of the warm ocean water. That this is the ac- 
tual cause of the coldness of the ocean waters is clearly indicated by 
several facts : (i) near the equator, temperature records have revealed 
an uprise of the cold waters ; (2) the coldest ocean bottom water is 
found in the oceans most open to the polar zones — notably the South 
Atlantic, the South Pacific, and in the North Atlantic on the western 
side, which is most open to the Arctic ; (3) the temperature conditions 
in partly closed seas which indicate a creep of the cold ocean water ; 
(4) the presence of oxygen in the bottom water, although living in it 
are animals which are using up the supply, and which would exhaust it 
if it were not replenished. 

For these reasons it is generally believed that the cold water of the 
cool temperate and polar zones settles to the bottom and creeps along 
it in a broad, slow drift. This conclusion receives support from the 
Mediterranean, where in winter the water is about 55 from surface 
to bottom, while in summer the surface temperatures are higher. 
Apparently the basin is filled with the coldest water of the successive 
seasons, and only a surface layer is influenced by the summer tempera- 
tures. The Arctic and Antarctic bottom waters show a slight increase 
in temperature, due to the creeping in of cold, dense water of the cool 
temperate latitudes of the sixties. This dense water seems to be 
beneath the cold, but less dense, Arctic water. 

In the Mediterranean there is also some influence from the inflow 
of water from the ocean through the narrow Strait of Gibraltar, but 
this is evidently not the dominant cause for the temperature con- 
ditions, since the water of the Atlantic at the level of the straits is 
slightly above 55 . The Mediterranean, therefore, on a small and 
simple scale, is similar to the great oceans, in being filled with water 
of a temperature determined by the period of greatest cold. In the 
oceans the conditions are more complex, for they are larger and 
more irregular, they extend through all the zones, and they are 
disturbed by a complex system of currents. They have not, therefore, 
and cannot reach the simple condition of the Mediterranean, which is 
in almost complete equilibrium in winter, but is somewhat disturbed 
in summer. 

Ice in the Ocean 

Sea Ice. — In cold temperate latitudes, ice forms on the sea in 
shallow waters along the coast, and especially in partly enclosed bays 
and harbours. In the frigid zone even the open ocean freezes over, 
and thus the sea surrounding the North Pole is covered with ice. Up 
to the point of freezing, which varies from 2 6° to 2 8° according to the 
salinity, the ocean water becomes denser and sinks ; but with freezing 
there comes expansion, so that the sea ice floats. It is, however, 
heavier than fresh water ice, because it includes the salts that were in 



662 COLLEGE PHYSIOGRAPHY 

solution. These salts do not enter into the structure of the individual 
ice crystals, but are separated out from the water during freezing and 
are included in the ice mass as salt or brine. Therefore the sea ice 
is saline, as is the water from which it is frozen. 

Leads and Pressure Ridges. — In the Arctic the sea ice forms to a 
depth of 10 feet or more, and, if undisturbed, forms a broad ice plain 
on which the winter snows fall, and over which it is easy to draw a 
sled. With the movements of the tidal and other currents, this ice 
plain is broken by leads where there is tension, and in these leads open 
water appears; but in the bitter cold of the Arctic winter they are 
soon frozen over by new ice. Where the currents cause compression, 
the ice is thrown into pressure ridges and pack ice, which may rise to a 
height of 50 or 100 feet, consisting of broken and upturned blocks like 
miniature mountains of ice. Sea ice crushes ships which are caught 
in it, unless especially built to evade pressure by rising, as in the case 
of Nansen's ship, the Fram. 

The Arctic sea ice is a highway of winter travel for Eskimos, and it 
has also been followed by numerous Arctic explorers, including Peary 
and Nansen. The pressure ridges and the open leads are the greatest 
obstacles to such travel, and it was in one of the leads that Marvin 
lost his life on returning, during Peary's successful expedition to the 
North Pole. In summer, when the sun is above the heavens all the 
time, the surface of the sea ice melts, the ice breaks apart, and the 
leads no longer freeze. Travel over it is then impossible, and this 
is the reason why, in spite of the great cold, expeditions over the sea 
ice have always started near the end of the winter night, and have 
planned the return before the summer warmth has made the ice im- 
passable. 

Floe Ice. — Slowly drifted about and broken by melting and by 
movement in the currents, the Arctic sea ice finds its way out of the 
Arctic seas, although it may be many years before a given portion 
drifts away. Every summer, therefore, there is a steady stream of sea 
ice down the coasts of Spitzbergen, eastern Greenland, Baffin Land, 
and elsewhere in the south-flowing cold currents. This ice is not in a 
solid mass, but is broken in pieces or floes, and it is commonly called 
floe ice. The eastern coast of Greenland is so constantly bordered by 
a stream of this ice that it is exceedingly difficult for a boat to reach 
that coast. A similar, though less continuous, procession of floe ice 
moves southward in the Labrador Current past Baffin Land and Lab- 
rador. For a thousand miles the boat upon which the author went 
northward to Greenland in 1896 was in this floe ice (Fig. 404). In 
the warmer southern climate the ice rapidly melts, becomes weakened 
so that it breaks up, and finally disappears. 

The Ice Foot. — Both in the Arctic and Antarctic there is, in the 
higher latitudes, a fringe of ice along the land, which has become known 
as the icefoot. It is of somewhat complex origin, being partly sea ice, 
partly snow that has slid or been blown upon it from the land, and 



THE OCEAN 



663 



partly the frozen spray of the waves that break upon the coast before 
the open sea is frozen. In some places rock fragments from the land 
are also incorporated in the ice foot. If the summer conditions do not 
suffice to remove this snow and ice accumulation, it may develop con- 
siderable thickness and width. 

Icebergs. — Both the sea ice and the ice foot include a proportion of 
salt, though in both cases snow forms a part of the accumulation. 
Icebergs, on the other hand, are free from salt, since they are made 
of ice that developed on the land, and flowed down to the sea as a part 
of glaciers. These ice masses are often so abundant near great 
glaciers that they seriously interfere with the passage of boats ; and 




Fig. 404. — Floe ice in the Labrador Current off Baffin Land. 



they often attain great size. In the Greenland waters some of the ice- 
bergs rise from 100 to 200 feet out of the water, and, as already stated, 
since only one-sixth or one-seventh of their mass is out of the water, 
their total height may be 1400 or 1500 feet. Such great ice masses 
float a long time before being melted, and some of those from Green- 
land float as far south as the Banks of Newfoundland, and even 
farther, where they are a menace to navigation (Fig. 405). 

These Greenland icebergs are so large that collision with one 
often results in shipwreck, as when the Titanic was wrecked of! the 
coast of Newfoundland on Apr. 14, 191 2, and 1517 persons lost their 
lives. 

By his invention of the nticr other tnotneter, a self-recording electrical 
resistance thermometer which measures temperature changes of a tenth 



664 



COLLEGE PHYSIOGRAPHY 



of a degree, Barnes has been able to show definitely that the tem- 
perature rises as icebergs are approached. This has been tried out 
in detailed experiments in the Straits of Belle Isle, in a voyage from 
the St. Lawrence to Hudson Bay, and in several trips across the Atlantic 
Ocean. Icebergs 8 to 12 miles from the ship affect the microther- 
mometer notably, and within a quarter mile of the berg the rise of 
temperature is very sharp indeed. 

While icebergs float farther south than 45 in the western Atlantic, 
they are not found south of latitude 70 in the eastern Atlantic, be- 
cause (1) there is no such extensive supply in that part of the Arctic, 




Fig. 405. — Distribution of icebergs (triangles) during the first half of April, 1909, showing 
how steamer routes between America and Europe are deflected southward. (Pilot 
Chart, U. S. Hydrographic Office.) 



and (2) a warm current flows into these northern waters instead of a 
cold outflowing current, such as exists in the west. 

Icebergs are found in front of the Alaskan glaciers which terminate 
as tidal glaciers in the fiords, and also in the Antarctic. In the latter 
locality the icebergs attain a diameter of several miles, forming great 
tabular bergs, like floating islands. The Antarctic icebergs and floe 
ice were well described by Wilkes, 1 838-1 842. These icebergs drift 
northward to latitude 40 or 50 . 

Influence of Floating Ice. — Both icebergs and sea ice from near the 
coast transport much sediment, which falls to the sea bottom as the 



THE OCEAN 665 

ice melts, icebergs being especially important in this respect. Both 
also affect the salinity of the water in which they melt. Again, this is 
especially true of iceberg and ice foot ice from which much fresh 
water is liberated ; but even the floe ice aids in freshening the water, 
since snow that has fallen upon the Arctic ice is incorporated in it. 
Where icebergs are particularly abundant, they cause very notable 
freshening of the sea water. 

All forms of floating ice are important in influencing the tempera- 
ture of the waters in which they are melting. They chill the surface 
water, and this affects not merely the surface layers, but, by the sink- 
ing of the denser water, the influence is extended vertically even to 
the sea bottom. Doubtless a part of the cold of the sea bottom is 
due to this influence of melting ice in Arctic and Antarctic regions. 
The chill of the water is communicated to the air, and in the move- 
ments of the air it is borne to other regions. The chilly climate of 
the Labrador and Baffin Land coasts is partly due to this influence 
of sea ice, transmitted to the land by the onshore winds ; and the 
chill of the melting ice is borne even farther south in the Labrador 
Current and carried by the winds to Newfoundland, Nova Scotia, 
and northern New England. 



References to Literature and Maps 

A. Agassiz. Three Cruises of the Blake, 2 vols., Boston, 1888, 314, 220 pp.; 
many papers in Bull. Mus. Comp. Zool. 

R. Amengual. Lijeros Apuntes Sobre Oceanografia, Valparaiso, 1908, 413 
pp. 

H. T. Barnes. Ice Formation, with Special Reference to Anchor-ice and 
Frezil, New York, 1907, 260 pp. ; Icebergs and their Location in Naviga- 
tion, Smithsonian Report, Publication 2225, Washington, 1913, pp. 717— 
740. 

J. Barrell. Relative Geological Importance of Continental, Littoral and 
Marine Sedimentation, Journ. Geol., Vol. 14, 1906, pp. 316-356, 430- 
457, 524-568. 

G. E. Belknap. Deep Sea Soundings in the North Pacific, U. S. Hydro- 
graphic Office, Washington. 

H. Berghaus. Atlas der Hydrographie, Gotha, 1891. 

A. Buchan. Oceanic Circulation, Challenger Reports, Summary of Scientific 
Results, Part 2, 1895, 38 pp. 

J. Y. Buchanan. On the Specific Gravity of Samples of Ocean Water, Challen- 
ger Reports, Physics and Chemistry, Vol. 1, 1884, 46 pp. 

Challenger Reports. Voyage of H. M. S. Challenger, 50 vols., especially 
vols. 1 to 4 ; Summary of Scientific Results, 2 vols., London, 1897. 

Challenger Society. Science of the Sea, London, 191 2, 452 pp. 

R. Chalmers. Tides of the Bay of Fundy, Ann. Rept. Geol. Survey of Canada, 
Vol. 7, 1896, pp. 14 M-20 M. 

T. C. Chamberlin. On a Possible Reversal of Deep Sea Circulation and its 
Influence on Geologic Climates, Journ. Geol., Vol. 14, 1906, pp. 363-373. 

C. Chun. Aus den Tiefen des Weltmeeres, Jena, 1900, 539 pp. 

L. W. Collet. Les Depots Marins, Paris, 1908. 

V. Cornish. Waves of the Sea and other Water Waves, London, 19 10, 374 
pp. 



666 COLLEGE PHYSIOGRAPHY 

F. W. and W. O. Crosby. The Sea Mills of Cephalonia, Cassier's Magazine, 

Vol. ii, 1896, pp.- 388-397. 

G. H. Darwin. The Tides, Boston, 1898, 378 pp. 

W. M. Davis. Winds and Ocean Currents, Journ. School Geog., Vol. 2, 1898, 
pp. 16-20; Waves and Tides, ibid., pp. 122-132. 

F. de Montessus de Ballore. Seismes Sous-marins et Tsunamis, La Science 

Seismologique, Paris, 1907, pp. 182-225. 
Deutschen Seewarte. Atlas of the Indian Ocean, Hamburg, 1891 ; Pacific 

Ocean, 1896; Atlantic Ocean, 1902. 
H. N. Dickson. Circulation of the Surface Waters of the North Atlantic 

Ocean, Phil. Trans. Roy. Soc, Vol. 196, 1901, pp. 61-203. 
W. Dittmar. Researches into the Composition of Ocean Water, Challenger 

Reports, Physics and Chemistry, Vol. 1, 1884, 247 pp. 
C. K. Edmunds. A Visit to the Hangchow Bore, Pop. Sci. Monthly, Vol. 72, 

1908, pp. 97-H5, 224-243. 
H. Filhol. La Vie au Fond des Mers, Paris, 1892. 
J. W. Flint. A Contribution to the Oceanography of the Pacific, Bull. 55, 

U. S. Nat. Museum, Washington, 1905, 62 pp. 
J. S. Gardiner. The Indian Ocean, Geog. Journ., Vol. 28, 1906, pp. 313-332, 

454-471- 

J. W. Gregory. The Level of the Sea, Scottish Geog. Mag., Vol. 25, 1909, pp. 
311-324. 

E. Haeckel. Plankton-Studien, Jena, 1890. 

R. A. Harris. The Tides : Their Causes and Representation, Pop. Sci. 
Monthly, Vol. 74, 1909, pp. 521-539; Manual of Tides, Part IV B, — 
Cotidal Lines of the World, Appendix 5, U. S. Coast and Geodetic Survey, 
Rept. for 1904, pp. 313-400; Part V, Appendix 6, Rept. for 1907, pp. 
231-545; Cotidal Lines for the World, Nat. Geog. Mag., Vol. 17, 1906, 

PP- 303-309- 

B. Helland-Hansen and F. Nansen. The Norwegian Sea, Report on Nor- 
wegian Fishery and Marine Investigations, Vol. 2, Bergen, 190Q. 

J. Hjort. The Michael Sars North Atlantic Deep Sea Expedition, 1910, 
Geog. Journ., Vol. 37, 191 1, pp. 349~377> 500-523. 

W. H. Hobbs. Origin of Ocean Basins in the Light of the New Seismology, 
Bull. Geol. Soc. Amer., Vol. 18, 1907, pp. 233-250. 

L. Hugues. Oceanografia, Turin, 1904. 

J. Johnstone. The Conditions of Life in the Sea, Cambridge, 1908 ; Life in 
the Sea, Cambridge, 191 1. 

L. Joubin. La Vie dans les Oceans, Paris, 191 2, 334 pp. 

A. Kirchhoff. The Sea in the Life of the Nations, Man and the Earth, London, 
1907, pp. 25-48. 

W. Koppen. Der Ozean, Leipzig, 1902. 

O. Kriimmel. Handbuch der Ozeanographie, 2 vols., Stuttgart, 1907, 
1911. 

A. de Lapparent. Les Oceans, Lecons de Geographie Physique, Paris, 1898, 
pp. 683-700. 

W. Libbey. Relation of the Gulf Stream to the Labrador Current, 6th Inter- 
national Geographical Congress, London, 1895, pp. 461-474. 

W. Marshall. Die Tiefsee und ihr Leben, Leipzig, 1888. 

E. de Martonne. Les Oceans, Mouvements der Oceans, Les Mers, — Traite 
de Geographie Physique, Paris, 1909, pp. 259-317. 

M. F. Maury. Physical Geography of the Sea, New York, 1855, 474 
pages. 

G. W. Melville and H. G. Bryant. Some Results from the Drift Cask Experi- 

ment, Bull. Geog. Soc. Philadelphia, Vol. 4, 1906, pp. 49-56. 
Prince of Monaco. Carte Generale Bathymetrique des Oceans, 1 : 10,000,000, 
8 sheets, Institut Oceanographique, Paris, 1905; ibid., revised to 1912- 

1913- 
W. Moseley. Notes by a Naturalist, London, 1892. 



THE OCEAN 667 

J. Murray. Annual Range of Temperature in the Surface Waters of the 
Ocean, Geog. Journ., Vol. 12, 1898, pp. 1 13-134; Temperature of the 
Floor of the Ocean, ibid., Vol. 14, 1899, pp. 34-51 ; Oceanography, ibid., 
pp. 426-439; The Oceans, Mills' International Geography, New York, 
1899, pp. 60-71; The Deep Sea, Scottish Geog. Mag., Vol. 26, 1910, pp. 
617-624; Exploring the Ocean's Floor, Harper's Magazine, Vol. 122, 
1911, pp. 541-550; The Ocean, New York, 1912, 256 pp. 

J. Murray and J. Hjort. The Depths of the Ocean, London, 191 2, 821 
pp. 

J. Murray and A. F. Renard. Deep Sea Deposits, Challenger Reports, Lon- 
don, 1891, 525 pp. 

F. Nansen. Bathymetrical Features of the North Polar Seas, Christiania, 

1904, 232 pp. 
P. Pelseneer. L'Exploration des Mers Profondes, Paris, 1892. 
A. Penck. Das Meer, Morphologie der Erdoberflache, Stuttgart, 1894, Vol. 2, 

pp. 460-662. 
O. Pettersson. On the Influence of Ice-Melting upon Oceanic Circulation, 

Geog. Journ., Vol. 30, 1907, pp. 273-303. 
J. E. Pillsbury. The Gulf Stream, U. S. Coast and Geodetic Survey, Appendix 

10, Rept. for 1890, Washington, 1891, pp. 461-620; ibid., Nat. Geog. 

Mag., Vol. 23, 1912, pp. 767-778. 
R. Quinton. L'Eau de Mer, Paris, 1904. 
J. Richard. L'Oceanographie, Paris, 1907, 398 pp. 
R. D. Salisbury. The Mineral Matter of the Sea, Scottish Geog. Mag., 

Vol. 21, 1905, pp. 132-136; ibid., Journ. Geol., Vol. 13, 1905, pp. 469- 

484. 

G. Schott. Physische Meereskunde, Leipzig, 1910, 143 pp. ; Geographie des 

Atlantischen Ozeans, Hamburg, 191 2. 
N. S. Shaler. The Depths of the Sea, Sea and Land, New York, 1894, pp. 75- 

152 ; The Resources of the Sea, Man and the Earth, New York, 1905, pp. 

139-149. 
C. D. Sigsbee. Deep Sea Sounding and Dredging, U. S. Coast and Geodetic 

Survey, Washington, 1880, 221 pp. 
J. W. Spencer. The Submarine Great Canon of the Hudson River, Geog. 

Journ., Vol. 25, 1905, pp. 180-190. 
A. Steuer. Leitfaden der Planktonkunde, Leipzig, 191 1. 
Z. L. Tanner. Deep Sea Exploration, U. S. Fish Commission, Washington, 

1892. 
R. S. Tarr. The Arctic Sea Ice as a Geological Agent, Amer. Journ. Sci., Vol. 

153, 1897, pp. 223-229; The Fishing Industry of New England, Bull. 

Amer. Bureau of Geog., Vol. 2, 1901, pp. 1-16. 
Wyville Thomson. Depths of the Sea, London, 1874, 527 pp. ; Voyage of the 

Challenger : The Atlantic, 2 vols., New York, 1878, 391, 340 pp. 
J. Thoulet. Oceanographie, 2 vols., Paris, 1890, 1896 ; L'Ocean, les Lois et ses 

Problemes, Paris, 1904, 397 pp.; Resultats les Campagnes Scientifiques 

du Prince de Monaco, 1902, 76 pp. 
U. S. Census. Fisheries of the United States, Fisheries of Alaska, Washington, 

1911,324 pp. 
F. Viezzoli. L'Adriatico, Parma, 1901, 207 pp. 
J. Walther. Allgemeine Meereskunde, Leipzig, 1893, 296 pp.; Boston, 1899, 

180 pp. 
J. J. Wild. Thalassa, London, 1877. 
C. Wilkes. Antarctic Cruise, Vol. 2, U. S. Exploring Expedition, 1838-1842, 

Philadelphia, 1844, pp. 297-387; Hydrography, ibid., Vol. 23, 1861, 

514 PP- 
R. S. Woodward. On the Form and Position of the Sea Level, Bull. 48, U. S. 

Geol. Survey, 1888, 88 pp. 
For references to other valuable oceanographie data, see the summary by 

Murray in "Depths of the Ocean," pp. 1-21. 



668 COLLEGE PHYSIOGRAPHY 



PERIODICALS 

Memoirs and Bulletins, Museum of Comparative Zoology, Cambridge, Mass. 
Charts, Tide Tables, and Annual Reports, U. S. Coast and Geodetic Survey. 

Washington, D.C. 
Pilot Charts, U. S. Hydrographic Office, Washington, D.C. 
Bulletins and Commissioner's Reports, U. S. Bureau of Fisheries, Washington, 

D.C. 
Charts, British Admiralty Office, and similar bureaus of other European 

countries. 
Pilot Charts, British Meteorological Office. 
Bulletin de Musee Oceanographique de Monaco. 
Publications, Institut fur Meereskunde, Berlin. 
Annalen de Hydrographic, Berlin. 



CHAPTER XX 

LIFE IN THE OCEAN 

Distribution oe Ocean Life 

Conditions Governing Distribution. — The distribution of life in the 
ocean is brought about 'partly by the voluntary movements of the 
organisms, partly by the water currents. Even organisms that are 
fixed are commonly subject to the latter influence in their larval 
stages. Since the ocean water is ever in motion, there is, therefore, 
provision for wide distribution. The extent to which this distribution 
is carried on, depends upon a variety of conditions, some of which 
are stated in the two following paragraphs. 

(i) The Kind of Organism. — Some are fixed and require a special 
kind of habitat, as muddy bottom, or sand, or rock, or surf-beaten 
coast. Others float about with such limited provision for locomo- 
tion that they are practically at the mercy of the currents. Still 
others are free-swimming and are capable of moving at will. 

(2) The Physical Conditions. — Among the physical conditions 
which influence distribution of marine organisms are temperature, 
sunlight, oxygen supply, food supply, and depth. Those that live on 
or near the coast are also influenced by salinity, by the clearness of 
the water, by floating ice, and by the nature of the coast: 

Variety of Organisms. — The organisms include both plants and 
animals, the former being necessarily limited to those upper portions 
of the ocean in which there is sufficient sunlight for plant growth, 
while the latter range from the surface to the bottom of the sea. 
They vary also in size from microscopic forms to the huge whale, the 
largest member of the animal kingdom, and to forms of seaweed, or 
kelp, in the vegetable kingdom which grow to a length of several 
hundred feet, rivalling the height of the highest trees, though having 
less diameter, a few inches only, and much less bulk. The largest 
plants grow on the land. In the ocean only a very few species of higher 
plants, such as abound on the land,. are to be found, and these are living 
in protected waters along the coast. In the open ocean the plant life 
is all of lower orders. Among marine animals, too, the greatest abun- 
dance is found in the lower orders of invertebrates, and next most 
abundant are the fishes, the lowest of the great divisions of the verte- 
brates. All the higher divisions of the vertebrates are represented 
among marine animals, the mammals by the whale, seal, walrus, 
manatee, and others, the reptiles by the turtles and other forms, the 

669 



670 COLLEGE PHYSIOGRAPHY 

birds by large numbers of species which look to the sea for their food 
and spend a greater or iess proportion of their time in the ocean. 
There are no birds that live habitually in the sea, though many spend 
most of their time in it and the remainder on its margin. Some even 
have lost the power of flight, like the auk and the penguin. The birds, 
reptiles, and mammals obtain their oxygen supply from the air, rising 
to the surface for it ; but the fishes, invertebrates, and plants obtain 
it direct from the water. 

Relations of Plants to Animals. — As upon the land, the plants of 
the sea are able to transform mineral substances from the surrounding 
medium into organic tissue. In this process sunlight is necessary, 
and plant life in the ocean is, therefore, confined to the upper layers 
into which sunlight penetrates. The animals of the sea, as upon the 
land, need plant life as a basal food supply, since animal life has not 
the power of transforming mineral substances directly into organic 
tissue. Since the surface organisms, both plant and animal, sink upon 
their death, the range of animal life is extended even to the ocean bot- 
tom, for sunlight is not a necessary part of their life processes, if food 
is supplied to them which has been prepared in the zone of sunlight. 

Contrast with Land and Air. — There is here a very wide difference 
between conditions in the ocean and on the land. The base of the 
atmosphere, where it rests upon the land, is a zone of abundant 
organic life, and this zone is extended into the earth a few inches or 
feet, rapidly grading into a zone where organisms are completely 
absent. The life zone is also extended a short distance into the air, 
again grading into a barren zone. In the ocean, on the other hand, 
while the surface zone of sunlight is the most densely occupied portion, 
organisms are found in abundance both on the dark sea floor and at 
intermediate depths. 

Adaptation to Environment. — In the ocean, as on the land, there 
is an adaptation to environment, with the development of much 
variety of form, colour and habits. A large proportion of these pecul- 
iarities are evidently related to securing food, or to escaping enemies. 
There is the usual great struggle for existence, some forms feeding upon 
others, some rivals for the same food. In the clear, open waters of 
the ocean, the chance of escape from destruction by enemies is far less 
than on the land or even on the coast, where various means of hiding 
are possible. Speed of movement and transparency are common 
means of protection, but the inefficiency of the latter is clearly in- 
dicated by the fact that many fishes produce scores of thousands of 
eggs in order that one may escape the chances of destruction and 
arrive at maturity. 

Cause of Wide Distribution. — There is wider distribution of or- 
ganisms in the ocean than on the land (1) because there are less dif- 
ferences in temperature, (2) because the medium in which they live 
is in motion, (3) because, excepting along the coast, there are fewer 
variations in environment. 



LIFE IN THE OCEAN 



671 



Difference in Bodily Structure. — Also, because of the medium in 
which they live, there is a difference in the bodily structure of the 
marine and land animals, a very large proportion of the former being 
of about the same weight as the water which they displace, or a little 
heavier. On the land the greater number of plants are fixed in place ; 
but in the sea, the plants are mainly floating forms, though this is not 
true along the coast line and in shallow water, where the condition 
is much like that of the land. 

Animals of the land, on the other hand, are freely moving in the 
main ; but while this is also true of a large proportion of the marine 
animals, there is, in the sea, a much greater number of fixed forms 
along the coast and on the sea bottom than among land animals. Among 
these fixed forms are many species which protect themselves against 
enemies or against the waves and currents by a mineral cover, usually 
carbonate of lime, as is illustrated by the corals and by many shells. 
It is out of the remains of these organisms that many of the limestone 
beds of the lithosphere were made in ancient seas. 



Abundance of Life 

The Larger Animals and Plants. — The vast abundance of organic 
life in the ocean, especially in the upper layers and along the coast, 
has long been known. Many coast lines are bordered with luxuriant 
seaweed growth, encrusted with 
barnacles, or fringed by coral 
reefs, mangrove swamps, or salt 
marshes. There are tracts of 
floating seaweed or sargassum ; 
there are great schools of fishes ; 
and there is the huge whale, nar- 
whal, seal, and walrus, as well as 
vast numbers of birds which feed 
upon marine organisms. It has 
also been known that the surface 
water teems with minute organ- 
isms which may be strained out 
by means of tow nets in which, 
after a short interval of towing, 
as many as 50 different species 
and thousands of individuals 
are found. Every pail of water 
dipped from the ocean surface is 
a small world of microscopic and 
sub-microscopic life. 

Invisible Inhabitants of the Sea. — Only very recently, however, 
has it been proved that even this is but a part of the life of the ocean. 
There are forms so small that they sift through the finest meshes of 



k 




Fig. 406. — Small plants of the deep sea. at 
a and b, caught in silk cloth netting. The 
highly magnified'square above is about \ of 
a square millimetre, or about tso of an inch 
on each side. (Murray.) 



672 COLLEGE PHYSIOGRAPHY 

the silk tow net (Fig. 406). This difficulty is met by an ingenious 
machine, the centrifuge, which rapidly rotates a sample of water so 
that even the most minute forms are thrown together by centrif- 
ugal force, the living contents of about 300 cubic centimetres of 
water being concentrated in a single drop of water, where they are 
counted under the microscope. In this way it has been found that 
the surface water down to a depth of from 30 to 160 feet is densely 
populated by minute plant cells. The minute organisms of the surface 
layers of the ocean are known as plankton. 

Upon the abundance of such life, Murray writes as follows : " We 
now know that the whole of the surface waters of the ocean are crowded 
with minute unicellular algae, which are ever busy, under the in- 
fluence of sunlight and chlorophyll, converting the inorganic sub- 
stances in sea water into organic compounds, which in turn supply not 
only the food of the vast majority of marine animals which live in 
surface and intermediate waters, but also of the myriads of creatures 
living near and on the sea floor, miles beneath the level to which the 
sun's rays can penetrate. The surface waters may be regarded 
as vast floating meadows, each great region having its own species and 
a soil (as it were) and other conditions which make for abundance or 
scarcity. The vegetable matter, in the form of phy to plankton, pres- 
ent in the surface waters of the ocean down to a depth of 200 fathoms, 
is probably much more abundant than that in the layer of vegetation 
which covers the land surfaces of the globe. The bodies of these 
minute unicellular algae, which often have calcareous, siliceous, or 
chitinous shells, fall to the bottom after death, together with the dead 
bodies of the animals which browse in these meadows ; accumulating 
on the surface of the deep sea oozes and clays, they supply nourish- 
ment for the creatures that crawl over the bottom of the sea." 

The Four Oceanic Zones 

There are four great zones in the ocean, with conditions sufficiently 
different to give rise to faunas of notably different characteristics. 
There are (1) the littoral zone, or the coast and shallow waters near 
the coast, (2) the pelagic zone, or the upper layers of the ocean water, 
(3) the abysmal zone, or the deep sea, (4) the zone of intermediate 
depths. Each of these will be considered separately for the purpose 
of bringing out some of the noteworthy differences. 

Life Along the Coast (Littoral) 

Resemblances to Life on Land and in Air. — The coast line presents 
conditions intermediate between those of the land and sea; and 
accordingly there are resemblances between the life in the two, while 
some of the species go freely from one to the other. The polar bear, 
for instance, is probably to be classed as a land animal, though it 



LIFE IN THE OCEAN 673 

spends a large share of its time on the ice floating in the sea ; the 
seal and walrus are doubtless to be classed as marine animals, though 
staying out of the water, on the shore or the floating ice, a large part 
of the time. Many birds live in the sea or in the air above the sea 
the greater part of the time, coming ashore mainly for feeding, as also 
do the marine turtles. Some of these forms wander far over the sea, 
as the fur seal does when it makes its journey between breeding 
seasons from the Pribilof Islands even as far as the southern Pacific ; 
but the great majority are confined to the coast region and the shallow 
waters bordering it. This vertical range is limited, because they de- 
pend upon the atmosphere for their organic supply. 

Variation with Nature of Coast. — On the very coast line there is 
normally abundant life, but the abundance and variety vary with the 
nature of the coast, as well as with temperature, food supply, and other 
conditions. The sandy coasts are relatively barren, for the sand 
is too shifting for fixed forms of life, and the variety of burrowing 
animals adapted to life in the shifting sands is not great. Some 
land plants which can withstand the occasional bath of salt spray 
live along the upper margin of the zone of wave action, and a limited 
number of a few species of burrowing animals live in the wave zone. 
Some of these, like certain species of crabs, make journeys to the land 
vegetation for their food. On the whole, and as contrasted with the 
littoral zone in general, the sand beaches may be considered oceanic 
deserts, as may also the zones of shifting sands in shallow water off- 
shore. Even more of a desert is the boulder and pebble beach, in which 
the waves move the surface about with such force and frequency that 
life is practically prohibited. 

Along the shores of protected bays, shifting sands are less extensive, 
and many portions of the coast are of clay or of fine-grained sand, not 
subject to frequent movement. Here land vegetation comes down to 
the sea, and some forms actually invade it with dense growth, as in the 
mangrove swamps, the salt marshes, and the eel grass patches. Such 
luxuriant plant growth supports an abundant and varied marine 
fauna, quite different in character from that on neighbouring, open- 
coast, sand beaches, but including a large number of burrowing 
animals, such as the clam and the scallop. In the shallow waters of 
the bays there is often an abundance of fixed forms, as, for instance, 
the oysters, which live in such numbers that they sometimes build 
layers of shells by the death and accumulation of the shells of succes- 
sive generations. 

On the rocky coasts, whether exposed to the ocean waves or in pro- 
tected bays, the littoral life is mainly fixed, for there is a solid founda- 
tion on which to grow, and the waves would make short work of or- 
ganisms that were not firmly fastened. Various forms of seaweed 
constitute the most noteworthy plant growth on such rocky coasts ; 
and, among animals, the barnacle is conspicuous, — an animal 
which passes through a free-swimming larval stage, then, attaching 
2 x 



674 COLLEGE PHYSIOGRAPHY 

itself to rock or other solid foundation, becomes encrusted in a cal- 
careous armour. Numerous other species live in this zone, some fixed, 
some clinging to fixed forms, some holding on by means of suction and 
moving about slowly, some burrowing into the rock, and some swim- 
ming about when the water is quiet, or in the zone just beyond the 
breakers. 

Other Causes for Variation. — There is much difference in form, 
habit, and abundance of marine organisms in the littoral zone, for 
there are varied conditions, even within short distances. In this 
respect there is resemblance to land life, rather than to other zones of 
ocean life where there is far greater uniformity of conditions. Besides 
local differences due to the variation in environment, there are differ- 
ences due to climate ; for example, the Arctic, temperate, and tropi- 
cal littoral life present wide differences. This may be illustrated by 
contrasting the two extremes, the conditions in the temperate zone 
being intermediate. 

Littoral Life in the Arctic. — In the Arctic the littoral fauna and flora 
are greatly restricted by the temperature, which is unfavourable to an 
abundant and varied life. To this is added the effect of ice, which 
transforms exposed coasts to marine deserts within the zone of ice 
attack. In the shallow waters offshore, where ice does not reach, 
there is abundant and varied life, and in the cold northern waters are 
many large animals which feed upon this life — diving birds, seals, 
walrus, and large fishes, such as cod and halibut. Shallow banks in 
the cold north temperature waters and along the outer margin of the 
Arctic zone are our leading sources of food fish. 

Littoral Life in the Tropics. — In the tropical zone the littoral fauna, 
in places favourably situated for food supply, is wonderfully varied 
and abundant. As upon the land, this abundance and variety can 
be ascribed in part to the warmth and the bright sunshine, which en- 
courages plant growth. Probably no part of the ocean is so densely 
occupied by organisms as the shallow coastal waters of the tropical 
zone, as especially exemplified by the coral reef life. Here the bottom 
is studded with fixed forms in great variety and occupying almost every 
square inch of the surface, while among the fixed organisms are burrow- 
ing, crawling, swimming, and floating species. As contrasted to the 
Arctic life, the marine organisms of the torrid zone are beautiful in 
form and colour. 

Gradation toward the Deep Sea. — From the coast out toward the 
deep sea there is a gradation toward the abysmal fauna, and it is diffi- 
cult to tell where to draw the line. It may, perhaps, be drawn in 
those depths where the penetration of sunlight ceases to be effective 
in encouraging plant growth, say 200 or 300 feet. Beyond it there is 
little plant life, and it is beyond the reach of animals of land origin, 
and the fauna is purely marine in all its characteristics. Some of 
the littoral species extend out into this deeper zone, but in the main 
the fauna is different. 



LIFE IN THE OCEAN 675 

Favourableness of Littoral Zone. — The great variety of life in the 
littoral zone is due to the variations in the environment, — the 
variations in temperature, exposure, nature of coast, salinity, oxygen 
supply, and food supply. Several of these factors also influence the 
abundance of life, but none more effectively than oxygen and food sup- 
ply. In these respects the littoral zone is, in general, favourable to abun- 
dant life, for the water is aerated in the surf zone, and both oxygen 
and food supply are brought to the animals by the waves and currents. 
Along the coasts, currents are more active than elsewhere in the ocean, 
for local differences in temperature, in salinity, and in wind are causing 
constant water movement, and the tidal currents are more effective 
along the coast than elsewhere in the ocean. 

Life at the Surface (Pelagic) 

Wide Distribution of Minute Organisms. — Mention has already 
been made of the great abundance of minute organisms in the surface 
layers of the ocean. These extend from shore to shore of all the oceans, 
and throughout all zones, being apparently somewhat more abundant 
in the colder waters, with abundant oxygen, than in the warmer waters. 
These minute organisms include both plants and animals, and they are 
to be classed as floating forms, for, though provided with some power 
of locomotion, they are essentially at the mercy of waves and currents. 
Accordingly the species have an extraordinarily wide distribution. 

Larger Pelagic Forms. — Besides these minute forms are many 
larger floating and swimming species, and some clinging and fixed 
forms attached to floating bodies such as logs and seaweeds, or to 
swimming animals. In the Sargasso Sea, for example, there is a 
miniature world of plant life and dependent swimming, crawling, and 
fixed forms of animal life. Among the larger animals are numerous 
fishes, some like the herring and mackerel, swimming in great schools, 
others moving singly like the shark and swordfish. The whale also 
roams in the surface and upper layers of the ocean. A multitude of 
floating species of jellyfish and other forms of animal life also inhabit 
this zone, and great numbers of the young of larger animals, especially 
in the coastal waters where many of the fixed forms have a free-swim- 
ming larval stage. 

The Food of Pelagic Animals. — The basis for the existence of 
the pelagic animal life is the abundant plant life, notably the micro- 
scopic algae already mentioned. They measure from 0.0 1 to 0.03 of a 
millimetre and are so small that from 3000 to 12,000 live in each litre 
of water in the upper layers. The abundance of plankton is less in 
the open Atlantic than in the coastal seas, and less at the very surface 
than at a short depth below the surface, being most abundant in depths 
of from 40 to 200 feet. Many of these plants, such as the diatoms, 
have silicious tests, others, Coccolithophoridce, calcareous (Fig. 407). 
Upon the minute plant and animal life of the surface even huge 



676 



COLLEGE PHYSIOGRAPHY 




whales depend for their food, straining out the organisms from the 
water as a tow net does. 

Colour of Organisms. — While there are coloured forms of animal life 
in surface layers of the ocean, and even some that are black, the great 
majority are either transparent, translucent, or blue in colour. This 
serves as a protection, rendering them invisible 
even when viewed from below. 

Reason for Wide Distribution. — Although 
there are many different species in the pelagic 
fauna, one of the most notable facts concerning 
them is their wide distribution. Equally note- 
worthy is the absence of local variations in the 
fauna and flora in a horizontal direction, for 
there are not ordinarily sufficient differences to 
lead to local development of special fauna and 
flora. The chief differences are related to cur- 
rents, cold and warm, and to' variations in the 
conditions of the water, especially in the neigh- 
bourhood of the coasts. Vertically, however, 
there are very striking differences, for there is 
change in temperature, sunlight, and amount of 
oxygen in a very small vertical range. Even 
the lowly algae are found to differ in species with 
depth. 

Predominance of Floating and Free-swimming 
Forms. — The pelagic forms are mainly floating 
or free-swimming, and it is important, therefore, 
that they, especially the former, shall have about 
the same weight as the water which they dis- 
place. Many species actually float, as the sar- 
gassum does, by means of air-filled cells; and 
the Portuguese man-of-war by means of an air- 
filled sac ; others keep afloat by slight movement 
or by swimming ; others still can rise and sink 
at will by means of chambers into and out of 
which water can be forced. 
Lack of Protection from Enemies. — In general the organisms live 
a life of chance, with little or no provision for defence, though some 
are protected by shells, or can swim rapidly, or can discharge cells 
which benumb enemies, as the Portuguese man-of-war does, causing 
a feeling like an electric charge. The smaller are, however, practi- 
cally at the mercy of the larger, and escape from destruction is reduced 
mainly to a mere matter of chance. Only by reason of the wide 
distribution, and abundant provision for succession, as in the great 
number of eggs, the fission of cells, etc., is it possible for the forms to 
exist in such abundance. The chance of survival must be but one 
in many thousands, if one may judge from the extensive provision 



Fig. 407. — A small cal- 
careous plant from the 
sea, one of the Coccolith- 
ophorida. In volume 
these make up the main 
part of the plankton. 
Their diameters may be 
only us© to 5§ of an inch, 
so that they go through 
the finest nets and are 
taken only with the cen- 
trifuge. Such plants are 
found in the stomachs 
of all pelagic animals 
and make up a large 
part of the deep sea 
oozes. (Murray.) 



LIFE IN THE OCEAN 



677 



made for succession by some of the animals which pass through a 
free-swimming larval period. 



Life in Intermediate Depths 

It has been the prevailing belief that while the ocean surface is 
densely populated, and the ocean bottom is also occupied by life in 
considerable variety and abundance, there is an intermediate zone, 
thousands of feet in depth, in which there was a practical, and perhaps 
complete, absence of life. Yet it has seemed strange that among 
the animals brought up in the dredge from the deep sea there are 
some that are coloured, others black, some blind, others with highly 
developed eyes. Even related species presented wide differences 



Neritic Plankton and Nekton 

% ia^pdnt l r u and A ' 




Fig. 408. 



Diagram to show the areas occupied by various types of oceanic plants 
and animals. (Murray.) 



which were difficult to explain on the assumption that they lived side 
by side in the darkness and uniform temperature of the ocean bottom. 
The expedition of the Michael Sars has evidently solved this prob- 
lem in the discovery that the intermediate zone is -inhabited. This 
has been proved by towing nets at different depths and also by using 
a tow net that can be automatically closed at any depth. Thus 
animals were secured in the intermediate layers that had previously 
been thought to belong to the bottom fauna because they were cap- 
tured in the dredge on its way up through the intermediate layers. 
Not enough knowledge is as yet at hand to make possible a very com- 
plete statement of the life conditions in this intermediate zone. There 
is certainly a decrease in abundance of life from the sunlit surface 
layers, through the twilight zone, to the zone of darkness ; and it is 
possible that there is a lower zone in which there is no life. 



678 COLLEGE PHYSIOGRAPHY 

It is quite possible that in some of the deep sea fishes the telescopic 
eyes, having a cylindrical shape, with a convex lens at the end, may be 
useful in the zone of twilight ; and that the red and black colours which 
prevail in the deep sea are of protective value. In hauls between 
600 and 1500 feet, the nets bring up large quantities of deep red 
shrimp and black fishes. These twilight animals, which include 
fishes, worms, cuttlefishes, and crustaceans, may be assumed to be 
invisible from above (Fig. 408). 

Little is known about the distribution and variation of these animals 
of the intermediate zone. One would expect them to be widely distrib- 
uted because of the uniformity of conditions amid which they live ; 
and it would seem probable that variation would be more rapid and 
important vertically than horizontally. Their food supply must come 
from the zone of sunlight ; but whether they obtain it as it falls toward 
the bottom, or whether the animals of one layer prey upon those of 
the layer above and these in turn upon the next layer, thus transmit- 
ting the sunlight influence into the zones of twilight and darkness, 
cannot now be told. It is a great and interesting field for exploration, 
only a mere beginning having been made. 

Life on the Ocean Bottom (Abysmal) 

Relation to Temperature, Oxygen, and Food Supply. — By far the 

greater part of the ocean floor is inhabited, and perhaps it all is. The 
abundance of life varies with the temperature and the supply of oxygen 
and food. Thus where warm ocean currents sweep against the bottom, 
insuring warmth and both oxygen and food, animal life is varied and 
abundant. Such a condition exists southeast of New England, where 
the Gulf Stream flows along the border of the continental shelf. There 
is also abundant life on the Newfoundland Fishing Banks over which 
the cold Labrador Current sweeps. 

In greater depths life decreases in abundance and variety, but it is 
possibly absent from some of the greater ocean depths and in some 
of the colder waters. The great cold, sometimes as low as 30 or 31 , 
necessarily reduces vitality, and the slow supply of oxygen places a 
distinct limitation on life. There seems usually to be oxygen present, 
however, in sufficient quantities for the existence of life, which is 
interpreted as indicating the existence of a slow circulation of cold 
surface waters along the ocean bottom. Where barriers exist to check 
free circulation, there is a diminution of oxygen, and it is probable that 
there are areas in which there is so little oxygen that life cannot 
exist. Such is known to be the case in the Black Sea, where the bottom 
water is so charged with sulphuretted hydrogen that with the exception 
of bacteria no life exists. 

The Monotony of Abysmal Conditions. — The deep sea animals 
exist in the midst of the most monotonous uniformity of conditions, — 
amid uniformly low temperatures, in a nearly motionless medium, 



LIFE IN THE OCEAN 679 

in absolute darkness, excepting for phosphorescent light, on a surface 
that is prevailingly a vast plain of ooze and red clay. Temperature 
differences are the main cause for variation, and these are found 
mainly in a vertical section, along which there is but slight variation 
in the deeper waters of the open ocean. Though it was once thought 
that the great pressures were a dominant factor, and that animals 
could not exist with such pressures, amounting to nearly 5 tons per 
square inch in a depth of 25,000 feet, it is now known that the pressure 
is of no importance ; for once the animal is adjusted to them there is 
pressure equally in all directions both within and without the body, 
and the animals are as unaffected by them as we are by the 15 pounds 
which is pressing on every square inch of our bodies. Only when they 
are raised to regions of lesser pressure, and the expansion from within 
is exerted on the bodies, is this enormous pressure effective. Then 
the skin may be actually cracked open by the internal pressure. 

Absence of Plant Life. — Since no plant life exists in the zone of 
darkness of the ocean, the animals of the sea floor are wholly depen- 
dent upon the supply 
that rains down upon 
them from the densely 
inhabited upper layers. 
This is devoured as it 
falls, and the ocean 
bottom ooze supplies Fig 4og _ A fish from the deep sea with transparent fin3 

lood to burrowing am- (U. S. Fish Commission.) 

mals, while doubtless 

also one form of ocean bottom life preys upon another, as elsewhere 
in the world. Life on the ocean bottom is necessarily limited by this 
food supply, but it is sufficient to support a varied and abundant life 
where other conditions are favourable. 

Adaptations to Abysmal Environment. — Among the ocean bottom 
animals are free-swimming animals, including fishes, some wholly 
blind, some with eyes whose use can be explained only on the assump- 
tion of the existence of phosphorescent light, which, indeed, it is known 
that some of the fishes carry about with them. There is one, for in- 
stance, that has a tentacle-like projection from its head, the end of 
which emits a phosphorescent glow. It has been called a deep sea 
lantern. Many forms burrow in the ocean bottom ooze, passing it 
through their digestive tracts as earthworms do the soil. Others crawl 
over the ocean bottom, and still others are fixed in place ; but on a 
wide expanse of ooze-covered plain there is not an abundance of solid 
pieces upon which these deep sea animals can fix themselves. Thus, 
with the scarcity of suitable foundation, shells are found encumbered 
with clinging species, cables are quickly encrusted, and even bottles, 
thrown over from ships, have been dredged up with a cover of fixed 
forms of ocean bottom life. Some forms of animal life have de- 
veloped special means for fixing themselves in the ocean bottom ooze, 




68o 



COLLEGE PHYSIOGRAPHY 



such, for instance, as enclosing some of the ooze in a bag-like growth 
at the base which serves as an anchor, or the growth of root-like 
branches which spread out through the ooze. These means of fixing 
the animals in place could be successful only in water with little or 
no motion — otherwise the fine-grained ooze would be washed away. 
Survival of Animals of Past Ages. — That the uniformity of condi- 
tions of the ocean floor should be favourable to the survival of animal 

types of bygone ages, elsewhere 
destroyed by species better fitted 
to the environment, seems nat- 
ural. It was, therefore, not a 
matter of great surprise when 
stalked crinoids (Fig. 410) were 
found living in the deep sea, 
though long since exterminated 
in the shallow waters where they 
were once so dominant as to act- 
ually form the bulk of certain 
layers of limestone rock. There 
may be other instances as yet un- 
discovered, for much of the ocean 
depths is yet to be explored. 

The Ocean as a Source of 
Food 

The ocean is important to man 
in a number of ways — as a great 
highway of commerce, as a barrier 
to distribution of life, as a source 
of vapour for the atmosphere, as 
a modifier of climate, and in other ways. It is also an important 
source of food supply. Some communities obtain a very large part 
of their food from the sea, as, for instance, in Norway and in New- 
foundland; and they then have a surplus for distribution among 
other communities. 

In the fisheries it is the coast line and the shallow waters and banks 
of the continental shelf that are the chief sources. Among the 
forms obtained are burrowing animals, like the clam, fixed animals 
like the oyster, crawling kinds like the lobster, and free-swimming 
kinds like the fishes. All of the great groups of the animal kingdom 
contribute to the food supply from the sea, the invertebrates, fishes, 
reptiles, birds, and mammals ; but by far the most important are the 
true fishes. Some of these are surface forms, like the mackerel and 
herring ; but a large number live on or near the shallow bottom, like 
the halibut, codfish, haddock, plaice, etc. Still others are fish that, 
though living in the sea the greater part of their lives, ascend into fresh 




Fig. 410. — The stalked crinoid or sea lily, 
a deep sea animal. (Alexander Agassiz.) 



LIFE IN THE OCEAN 681 

water to lay their eggs, or spawn. Of these the salmon and shad are by 
far the most important. The open ocean, far from land, is not an im- 
portant source of food fish, and the deep sea bottom and intermediate 
layers are not drawn upon at all. 

Besides food, the marine animals furnish a great number of other 
useful products, — whale and seal oil, whalebone, sealskins, walrus 
ivory, coral, sponges, tortoise shell, pearls, etc. The annual value of 
the fisheries of all kinds in the United States is equal to $61,000,000. 
This, however, is only about equal to some of the minor crops on the 
land, such as- sugar beets. The fishery products of Japan and Great 
Britain exceed those of the United States in value, the total for the 
world being nearly $440,000,000 per year. 

For references to literature, see pp. 665-668. 



CHAPTER XXI 

MOVEMENTS OF THE OCEANIC WATER 

Types of Movement 

It is probable that all parts of the ocean water are at all times in 
motion. Some of these movements are slow and imperceptible, 
others are quite obvious. They are due to a complex series of causes 
often completely interrelated. Some of the movements are in the form 
of waves ; others are currents, or drifts of water. Some of the currents 
are not related to the waves in origin, while others, such as the tidal 
currents, develop as a result of the wave, and still others, such as the 
wind drift current, are developed by the same cause that produces 
the wave. 

These various and complex movements may perhaps best be under- 
stood if discussed under three somewhat arbitrary divisions : (i) waves, 
not including the tidal wave ; (2) currents anddrifts, not including the 
tidal currents; (3) tides. 

Waves 

This term is commonly understood to refer to wind waves, but there 
are other forms of wave developed in the ocean, such as the earthquake 
wave, the iceberg wave, the wave due to differences in atmospheric 
pressure, and the tidal wave. With the exception of the latter (p. 700), 
these various forms of wave will now be discussed. 

Wind Waves 

Currents accompanying Wind Waves. — When the wind blows over 
the water, there is friction at the surface of contact, as a result of which 
two quite distinct motions are caused : (1) a forward drift of the water 
in the direction of the wind, (2) the development of a wave translation. 
With continued friction both of these movements are increased until 
(a) a well-defined current is set up, and (b) waves of considerable height 
are formed. Thus, in the trade wind belt, or in other zones of regular 
winds, both currents of surface water and waves move in the direction 
of the prevailing wind, the current advancing much more slowly 
than the wave, and both lagging behind the wind which causes them. 

Relation of Waves to Velocity of Wind. — The wind waves vary in 
height from tiny ripples to great billows, rolling " mountain high." 

682 



MOVEMENTS OF THE OCEANIC WATER 



683 



With a gentle breeze only small waves develop, reaching a height 
of but a foot or two, but if the wind increases to a gale, 
the ocean surface is 
quickly transformed 
to a series of great 
waves, and, if the 
wind continues and 
blows over a large 
expanse of ocean, the 
waves become of 
great size. These are 
formed by the power- 
ful friction of the 
moving air, by the 
adding of wave upon 
wave as the gusts 
strike the water sur- 
face, and by the 
transmission of the 
impulse to the water 
below the surface. 
The wind waves some- 
times attain a speed 
of 50 or 60 miles an 
hour, though com- 
monly less, even down 
to 20 to 25 miles. 
The water itself does 
not move forward at 
this rate, but merely 
the translation of the 
wave form. One may 
illustrate this with a 
rope, when, by giving 
a quick shake at one 
end, a wave is caused 
to pass through the 
rope to the other 
end. As the wave 
form thus rapidly 
advances, another 
follows, with others 
behind, each wave 
consisting of a crest 
or higher part, grad- 
ing down on both sides to a hollow, or trough, the crests and troughs 
being roughly parallel and linear. 





E/I1 


W 1 




m 












% r 




Wm 




mi jd&ij 




I 




t^ 1 


11 


fj 








|r 3 








•■ ?■' 




it 




•*\ 


W, Y> 










mm 


■ *. ; 



684 COLLEGE PHYSIOGRAPHY 

Height and Length of Waves. — The height to which waves rise 
in the open ocean has been greatly exaggerated. A wave 20 feet 
from trough to crest is very high, and it is probable that they rarely 
rise more than 40 or 50 feet. In the southern ocean, where the stormy 
west winds blow over a vast stretch of water, waves are said to be so 
high, and the distance between trough and crest so "great, that sailing 
vessels are temporarily becalmed when sinking into the trough of the 
billows. It is said that exceptional waves 1500 feet long have been 
seen. The wave length, however, is not usually over 600 feet, and 
may be 300 feet or less in length. Indeed, most waves that reach 
the coast are under 25 feet in length. 

While the wave form moves rapidly forward, an object floating on 
the surface is alternately raised and lowered, as crest and trough pass 
it, proving clearly that the water itself undergoes no such motion 
as the wave has. If it did, a vessel would be helplessly carried along. 
The water particles undergo a circular motion. In the trough they 
are moving backward (Fig. 412) ; as the crest approaches they rise, at 












■v' 



IS 



v — . 






Fig. 412. — Movement of water particles in waves, (de Martonne.) 

the crest they move forward, and on the rear of the crest they move 
downward. 

The wave form is a surface feature, but the movement of the water 
extends far below the surface, though with diminishing force. It 
is certain that under large ocean waves there is a motion sufficiently 
powerful to move sand at a depth of from 400 to 600 feet, but even this 
makes the wave motion merely a superficial phenomenon of the deep 
ocean water. 

Ground Swell. — The movement of the ocean, once started, will 
continue even far beyond the exciting cause, especially if there are large 
waves, advancing 25 to 50 miles an hour. It is as a result of this that 
great waves lose their irregularity and are long, deep swells. Such 
waves, called ground swell, or rollers, are often felt even on a glassy sea 
when there is no wind. Generated in some regions where there is 
wind, they travel scores or even hundreds of miles beyond their source 
before they die out. Thus the ocean surface is rarely absolutely 
calm and free from motion, since a ground swell may come from two 
or three distant sources in any direction at the same time. 

Whitecaps and Wind-drift Currents. — The ground swell is a fairly 
smooth, regular wave form ; but when the wind is blowing strongly, a 



MOVEMENTS OF THE OCEANIC WATER 685 

wind wave is far less regular. During a heavy wind the waves con- 
sist of a series of billows whose intervening troughs have much irreg- 
ularity of form. Their surfaces (Fig. 411) may have a series of 
small waves superimposed upon them, and their crests may be broken 
by the force of the wind, forming whitecaps, or great foaming crests, so 
steep on the side away from the wind that the crest often breaks and 
forces forward a wave of foaming water. The friction "of the air sets 
up a current, or an actual movement of the surface water, quite 
independent of the wave itself. Therefore, a boat adrift in the sea, 
though rising and falling with the passage of each wave, will also be 
carried forward, both by the wind and by the wind drift current of 
surface water. 

Whitecaps sometimes break against a ship and wash the decks from 
end to end. Normally a vessel rises and falls with the passage of the 
crests and troughs, but such combers may temporarily quite submerge 
the boat. To small boats they are exceedingly dangerous. During 
heavy gales the wind actually blows the water from the wave crests 
in sheets of spray. 

Slight Influence on Life. — In the open ocean the wave doubtless 
has influence on oceanic life, and the breaking of the wave is certainly 
a means of aerating the water from which the marine organisms obtain 
their oxygen. Aside from this influence, the energy of the open ocean 
wave seems lost, excepting when encountered by vessels. The cease- 
less activity of the ocean as the waves pass in succession over it, is 
one of the great dynamic forces of the earth's surface ; and seemingly 
one of the least effective forces in relation to life and to earth change. 
Only where the waves enter shallow water and break against the shores 
of the islands and continents do the ocean waves exhibit their full 
force and enter a work of notable character. There they cause a 
series of phenomena which profoundly influence plant and animal life, 
which have distinct importance to man, and which cause great 
changes in the land margin, as we have already seen (Chap. XI). 

Waves Due to Winds and to Pressure Difference 

Similarity to Seiches'. — In the chapter on Lakes it was pointed out 
that there are undulations of the water level, known as seiches, due 
to differences in atmospheric pressure on different parts of the lakes. 
Similar movements must be of common occurrence in the ocean, 
though ordinarily masked by other movements. Every time an area 
of low pressure passes over the water surface the level of the ocean 
must be very slightly raised, and with the passage of high pressure 
areas it must be depressed. It is quite probable that undulations of 
this origin are constantly passing through some part of the ocean. 

Inundations accompanying Hurricanes. — When the difference in 
atmospheric pressure is great, there may be such a local distortion of 
sea level as to be plainly noticeable along the coast. Usually such 



686 COLLEGE PHYSIOGRAPHY 

a wave develops in storm centres where there is a pronounced diminu- 
tion in pressure, and it progresses with the movement of the storm. 
Sometimes the sea level rises several feet during the passage of such 
storm, and low coast lands are inundated by it, causing much damage. 
Such was the case during the passage of the hurricane in which Gal- 
veston was destroyed in 1900, the sea level rising into the city. Other 
cases of inundation have been reported from the low coast lands of 
southeastern Asia. 

Such waves, which are roughly circular, with a diameter of several 
score of miles, are not due solely to pressure differences. For in low 
pressure areas winds are blowing spirally toward the centre, and the 
drift of water toward the centre still further raises the sea level. Thus 
the wave is due to a double cause, the relative value of the two causes 
doubtless varying greatly in individual instances. When the pressure 
differences and wind velocity die down, there must be a readjustment 
of sea level, from which seiches-like undulations develop. There must 
be all gradations between extreme instances like that referred to at 
Galveston, and minute variations due to slight pressure differences. 

Other Wind-formed Waves. — In a similar way the wind, driving 
water before it, must cause differences in sea level, which, with the 
cessation of the wind, must find adjustment. The gravitative ten- 
dency is to maintain the oblate spheroidal form ; but local, temporary 
causes give rise to departure from this spheroidal form so long as they 
operate ; and, with their cessation, gravity proceeds to establish the 
normal condition. We have less knowledge of these oceanic move- 
ments than of most other kinds, for they are irregular in occurrence, 
usually slight in amount of vertical range, and, excepting in the more 
notable instances, are masked by the other, more regular movements, 
yet they are known to be of common occurrence, and, as already 
pointed out, sometimes of very great importance because of their 
destructive inundations. 

Other Waves 

Minor Disturbances. — Waves are generated in the ocean by any pro- 
nounced disturbance of the water. For instance, a fish leaping from 
the water or swirling rapidly through it, or a stone thrown into the 
water, or a boat passing through it, are all causes for waves. The waves 
of a rapidly moving steamboat, for instance, may extend several miles 
from their source. 

Avalanche Waves. — When great masses fall into the water, waves 
of considerable size may be generated. For instance, an avalanche 
falling into the water may sweep over the neighbouring coast with 
great force and destructiveness. Such a wave in an Alaskan fiord 
in 1905 rose no to 115 feet on the adjacent coast, and from 15 to 20 
feet at a distance of 1 5 miles. Had the coast been inhabited, great 
destruction would have been caused. 



MOVEMENTS OF THE OCEANIC WATER 687 

Iceberg Waves. — Such avalanches are uncommon, but falls of 
ice from the cliffs of tidal glaciers (p. 210) are of frequent occurrence 
in certain localities, as in the Alaskan fiords and along the Greenland 
coast. There they are locally the most important kinds of waves, 
sweeping through the fiords every few moments or hours, according 
to the activity of the glaciers. Being profound disturbances of the 
water, these iceberg waves sweep through the fiords as low, deep undula- 
tions, which, upon reaching the coast, form powerful breakers, even 
when the water surface is perfectly calm. 

Earthquake Water Waves. — When earthquake shocks occur on 
the sea floor, a wave undulation is started which affects the water 
from surface to bottom (pp. 433-435). This is the tsunami. Though 
low in vertical range, such a wave is profound, and it may sweep across 
the broadest of oceans. For example, earthquake waves generated 
on the Asiatic coast are not uncommonly recorded on the tide gauges 
of the American coast. After travelling such distances, the earth- 
quake water wave has so diminished in size that it is not perceptible 
to ordinary observation ; but on coasts near the centre of origin, as 
it piles up in the shallow coastal waters, it sometimes becomes a wave 
of great height and destructiveness, doing vast damage to life and 
property on low-lying coasts. Fortunately such waves are rare and 
local in their influence. 

Ocean Currents 

Causes for Currents in the Ocean. — Besides currents resulting from 
modification of the tidal movements, there is a complex circulation 
of ocean waters due to the combination of a number of causes. The 
following are among the more important of these : (1) The addition of 
water to the sea, either from the inflow of rivers, or the melting of 
snow or ice, or the fall of rain ; (2) the abstraction of water from the 
sea by evaporation ; (3) raising of the level of the sea surface by winds 
or by atmospheric pressure ; (4) the drift of water before the winds ; 
(5) change in density of the sea water, either through variation in 
salinity or in temperature. 

All of these causes are in operation to a greater or less degree, and 
the water is in a constant state of movement as a result of the dis- 
turbances in equilibrium thus produced. Sometimes the movements 
are slight and merely local, sometimes of general extent and easily per- 
ceptible, sometimes due to a single cause, sometimes to a combination of 
causes. There is also a difference in significance of the several causes. 
For example, evaporation and rainfall, one of which tends to lower 
the surface, the other to raise it, operate slowly, and adjustment 
commonly takes place without giving rise to readily perceptible currents. 
Rivers, adding water to the sea, and tending to locally raise the sur- 
face, often give rise to noticeable local currents, which are, however, 
lost sight of in the broad ocean a short distance from the river mouth. 



688 COLLEGE PHYSIOGRAPHY 

Melting glaciers cause perceptible currents in fiords ; but these also 
are only local phenomena. It is, however, probable that a very 
notable proportion of the water that flows in surface currents out of 
the Arctic basin is due to the raising of sea level there by the large 
amount of fresh water that is poured into this basin by the rivers 
and from the melting snow and ice. 

Importance of Wind Drift and of Density. — Two causes are 
apparently of greater importance in determining oceanic circulation 
than either of these. One of these is the drifting of water before the 
winds, the other the differences in density of the ocean water. If 
water sinks in one part of the sea, because of greater density, there 
must be an inflow of water to take its place. Or, if there is a drift 
of water away from a given area, there must be inflow to take its place ; 
and, on the other hand, there must be an outflow of water from the 
area to which the water drifts, otherwise the sea level would be per- 
manently raised. 

By these disturbances in the equilibrium of the ocean waters, and 
the necessary adjustments to which they give rise, an oceanic cir- 
culation is caused which is world-wide in extent, and which affects the 
ocean from surface to bottom. The nature of this circulation can best 
be explained if it is considered under two distinct headings; but it 
must be understood that there is a complexity of causes, so that in 
nature there is no such arbitrary division as is adopted for clearness 
of presentation. These two headings are: (i) the great planetary cir- 
culation, (2) the surface wind drift currents. 

The Planetary Circulation 

Wind Drift, Temperature, and Salinity. — Three widespread 
causes are at all times at work on the ocean surface, disturbing the 
equilibrium of the ocean waters, and thereby inducing circulation. 
These are: (1) the drift of water before the winds, (2) variation in 
temperature, (3) variation in salinity. The other causes mentioned 
in the preceding section are contributory to the movements generated 
by these causes. It is probable also that there is a cause of circula- 
tion operating on the ocean bottom, as a result of the slow conduction 
of earth heat from the lithosphere to the hydrosphere. 

These several causes give rise to a great planetary circulation which 
affects all the oceans, and water at all depths in the oceans. This 
circulation is slow, but general and continuous, though varying in 
speed and continuity. We are as yet not possessed of a sufficient body 
of fact to permit a determination of the relative importance of the 
contributing causes ; nor is the course of this circulation mapped, 
nor its speed determined. There are, however, a number of -facts 
which point to temperature differences as the most important factor 
in this planetary circulation, while the winds are the prime cause for 
the surface currents. 



MOVEMENTS OF THE OCEANIC WATER 689 

Circulation between Poles and Equator. — In the open ocean, the 
planetary circulation appears to consist of a sinking of cold waters in 
the cold temperate and polar zones, a flow equatorward along the 
bottom and in intermediate depths, a rising and a surface flow pole- 
ward. It may be repeated that the influence of differences in density 
due to salinity, the wind drift of surface water, and other causes enter 
into this circulation. Of the surface movements we have considerable 
knowledge, and that phase of the circulation is treated in the next 
section. The evidence of the other parts of the planetary circulation, 
though less direct, is convincing. In the first place, it is a well-known 
physical fact that cooling of salt water causes increase in density and 
consequent settling. This necessarily means movement away from 
the settling water, to give it place, and toward the point of settling, 
to replace that which sinks. That such movements occur there can be 
no doubt. That they extend widely through the oceans is proved by 
several facts, as follows : (1) As has already been shown, no other ex- 
planation of the cold water on the ocean bottom is possible. (2) Only 
by some such circulation can oxygen be moved to the ocean bottom in 
sufficient amount to support the life there. (3) The coldest waters 
are found in the parts of the ocean most open to the frigid waters, 
as represented in the contrast between the South and North 
Atlantic, and in the North Atlantic between the bottom temper- 
atures of the eastern and western sides. (4) In the equatorial 
Atlantic a wedge of cold water has been found rising above the 
normal level, as if the water here were rising toward the surface. 
(5) The temperatures of the bottom of partly enclosed seas are 
those of the level of the lowest point in their rim, pointing clearly to 
a circulation by which water of the open ocean enters from the level 
of the rim. 

Resemblance to Atmospheric Circulation. — The great oceanic 
planetary circulation, in general slow, but here and there accelerated 
for one reason or another, bears a certain resemblance to the circula- 
tion of the atmosphere, in which there is a movement equatorward, 
a rising there, and a poleward outflow. But there is a noteworthy 
difference, for while the atmosphere is warmed at the bottom, where it 
rests on the lithosphere and hydrosphere, the ocean is warmed only 
at and near the top. The atmospheric circulation depends primarily 
upon warming in the lower layers in the equatorial belt, with resulting 
decrease in specific gravity of the air and rising ; while the oceanic cir- 
culation is primarily due to cooling at the surface in the higher latitudes. 
In both cases there is an overturning of the mobile medium, though 
the primary cause is different in the two cases. If the sun's heat 
warmed the ocean bottom, as it does the land surface, the planetary 
oceanic circulation would undoubtedly be much more vigorous. Such 
rising as results from the escape of terrestrial heat must be very slow, 
and it is widespread over the ocean floor, not concentrated along any 
given belt. 
2 Y 



690 COLLEGE PHYSIOGRAPHY 

Circulation of Enclosed Seas. — The Mediterranean. — In a 

way the circulation in enclosed seas illustrates the phenomena of 
oceanic circulation. The Mediterranean may be taken as the typical 
illustration. It is a basin 14,400 feet deep and with a temperature of 
55 from top to bottom in winter, though with higher surface tempera- 
tures in summer. It is, therefore, believed that the temperature of 
the deep waters is due to sinking of cold water in winter, and not to 
inflow of ocean water, for the water in the outside ocean at the level 
of the Strait of Gibraltar is above 55 . Owing to evaporation, the 
water of the Mediterranean is more saline and consequently more 
dense than the Atlantic water outside the Strait, and its surface is 
actually lower than that of the ocean. This condition is due to the 
fact that the rainfall is only about a quarter as much as the evapora- 
tion from the water surface. Since the surface of the Mediterranean 
is lower than that of the open ocean, there is an inflow of surface water 
through the Strait of Gibraltar (Fig. 413) ; since the Mediterranean 
water is denser than that of the open ocean, both because it is Salter 

and colder, there is an out- 
sea surface flowing undercurrent . The 

relative amount of outflow 

Atlantic ^ //W"?k s " J • n '• 4.1 

Ocean ^.'V ^Werranean Sea '§ %. *ud inflow VaHCS greatly 




according to the state of tide 

and other conditions; but 

. there is a constant attempt 

Fig. 413. — Diagram showing the contrast of , . f +U A 

inflowing and outflowing currents from Medi- at adjustment 01 tne OlS- 

terranean and Black Seas, that at Gibraltar turbed equilibrium of the 

flowing in at the surface while that at the rnrmprt : ncy watpr hnrlip* 

Bosphorus and Sea of Marmora flows in at Connecting water DOdieS. 

the bottom. (Murray.) Circulation in Red Sea. — 

The Red Sea shows similar 
conditions. Its density (1.03) is greatly increased by evaporation, 
which amounts to from 10 to 25 feet a year in a region of moderate 
rainfall. If there were no compensating movements of water, the 
Red Sea would evaporate and precipitate its salt. There is a 
surface current of warm water in the Indian Ocean, and an out- 
flowing bottom current of denser water constantly passing through 
the Strait of Bab-el-Mandeb. It is certain that this bottom 
current must carry out as much salt as is brought in by the surface 
current, otherwise the Red Sea would grow steadily Salter. That 
there is no inflow of water from the deeper Indian Ocean is proved 
by the temperature conditions. The Red Sea, which occupies a basin 
7200 feet deep, with a rim rising to within 1200 feet of sea level in 
the Strait, has a winter surface temperature of about 70 and a 
summer temperature of 85 or more. Below the influence of 
summer warming, the water is uniformly at 70°, but in the Indian 
Ocean outside the Strait of Bab-el-Mandeb, the temperature is 37 at 
a depth of 7200 feet. There can be little doubt that the temperature 
of the Red Sea waters is determined by circulation in its own area. 



MOVEMENTS OF THE OCEANIC WATER 691 

Circulation in Black Sea. — In the Black Sea there is the reverse 
condition. Here the rainfall and inflow of river water exceeds evap- 
oration, and the surface level is on the average about 2 feet above that 
of the Mediterranean. Therefore a surface current of less dense water 
flows out through the Bosphorus. A reverse current of denser water 
enters in the Black Sea along the bottom of the strait (Fig. 413). The 
reason for this current is that a column of water of such salinity as 
that of the Black Sea must be higher than a column of greater salinity, 
like the Mediterranean, if the two are in balance. That is to say, 
the less saline water of the Black Sea needs to stand higher than the 
Mediterranean water ; but water is so mobile that it will flow away 
from a higher to a lower level, even down a grade as low as that be- 
tween the surface of the Black and Mediterranean seas. This outflow 
therefore disturbs the equilibrium between the columns of different 
density, and there is a bottom inflow of saline water to restore the 
equilibrium. Thus there is constant outflow at the surface, and inflow 
at the bottom. 

The Surface Currents 

Causes of Currents. — Movements of the surface water of the ocean 
are induced by a variety of causes, such as inflow of rivers, differences 
in density, differences in level due to evaporation and rainfall, and 
to the friction of moving air. Among these causes the last is, without 
doubt, by far the most important, and it appears to be the chief 
underlying cause for the great system of surface currents and drifts 
of the several oceans (Fig. 416). 

This cause may be imitated by blowing upon a water surface, when 
a miniature drift is almost immediately started. On lakes and partly 
enclosed seas it is a matter of general knowledge that the surface 
water drifts in the direction of the wind, and that the drift continues 
even after the impelling cause dies out, and may extend as a current 
well beyond the area in which the wind is blowing. Such a drift is 
due to the friction of moving air upon the mobile water, but it is not 
confined to the actual surface, for the moving surface water also drags 
along the layers below. Thus the wind, though operating only on 
the actual water surface, may start a movement which involves water 
to a depth of scores and even hundreds of feet. 

The Indian Ocean. — In the oceans the relation of surface currents 
and drifts to the wind is often well illustrated, but nowhere better than 
in the Indian Ocean. There, in summer, the southeast trade winds 
extend across the equator and blow upon southern Asia as the sum- 
mer monsoons (Chap. XXV) ; in winter the wind direction is reversed, 
and the winds blow off the land as winter monsoons. South of the 
equator the winds are fairly constant in direction, and there is a great 
eddy of ocean water similar to that in other oceans, as described later. 
But north of the equator the ocean drifts and currents are quite com- 



692 



COLLEGE PHYSIOGRAPHY 



pletely reversed in the two opposite seasons. In both seasons there 
is an irregular eddy in the equatorial region and extending northward 
into the Bay of Bengal and the Arabian Sea, but the direction of the 
water movement is opposite in the two seasons. In summer the water 
in the equatorial belt flows westward before the trade winds, then 
circles into the Arabian Sea and the Bay of Bengal, moving northward 
under the influence of the summer monsoons (Fig. 414). But in the 
winter the water sweeps southward out of these bays, then eastward 
in the equatorial belt, under the influence of the offshore winter 
monsoons. The east-moving equatorial current, which flows op- 




FiG. 414. — Currents of the Indian Ocean (arrows in the water) in relation to southwest 
monsoon of summer (arrows on land in left-hand figure) and the northeast monsoon of 
winter (right-hand figure). 



posite or counter to the normal westward-moving equatorial currents 
in the oceanic eddies, is called a counter current. 

The Southern Ocean. — Between Antarctica and the southern tips 
of Africa, Australia, and South America is a broad ocean belt, some- 
times known as the Southern Ocean, including the Antarctic Ocean 
and southern portions of the Atlantic, Indian, and Pacific oceans. 
In the larger part of this Southern Ocean the wind direction is prevail- 
ingly from the west, and, accordingly, there is a rather regular and 
steady eastward drift of water, encircling the earth. Slowly moving 
drifts set off from this current toward the north, especially along the 
western sides of South America and Africa, but the great body of 
surface water drifts eastward with a fair degree of regularity. In a 
large part of the southern west wind drift there is floating ice, but it is 
rarely met with north of 45 south latitude. 

The Atlantic Ocean. — In the Atlantic Ocean, while there are areas 
of variable winds and resulting wind drift currents of varying extent 
and direction, especially near the coasts, there are four belts of wind of 
sufficient uniformity of direction and persistence to generate regular 



MOVEMENTS OF THE OCEANIC WATER 



693 



drifts of ocean water. One of these is the southern belt of west winds, 
already referred to as the cause of the east moving current of the south- 
ern ocean. The second belt of regular winds is the northern west 
wind belt ; the third and fourth' are the trade wind belts, one on either 
side of the equator. 

Atlantic Eddies. — In the trade wind belt the wind blows with 
marked steadiness from the southeast south of the equator and from 
the northeast north of the equator. This causes a drift of water to- 
ward the equator from either side, and, since the impelling winds are 




Fig. 415. — The North Atlantic Eddy, Gulf Stream, and Labrador Current. The 
boundaries are not really as sharp as in this diagram, and the existence of a certain 
portion of the Gulf Stream in the western part of the Gulf of Mexico has been ques- 
tioned. 



from an easterly quarter, with a westward movement toward the 
South American coast. Were the land not in the way, this west- 
moving equatorial current would probably form a west-moving 
drift sweeping completely around the earth. As it is, the equatorial 
drift divides on the South American coast, part sweeping northward, 
part southward. Further deflection is caused by the influence of 
terrestrial rotation (Ferrel's Law, Chap. XXV). The equatorial drift 
swings out into the Atlantic in each hemisphere, forming a great eddy 
in the North Atlantic by right-hand deflection, and another in the 
South Atlantic by left-hand deflection. On the eastern side of the 
eddy the water again comes under the influence of the trade winds, 



6 94 COLLEGE PHYSIOGRAPHY 

and thus the surface waters slowly eddy about (Fig. 415). In the 
centre of the eddy extensive masses of floating seaweed, or 
sargassum, accumulate, and this is sometimes spoken of as the 
Sargasso Sea. 

While the equatorial water is in general westward movement under 
the influence of the southern and northern trades, there is an east- 
moving counter current near the centre, consisting of warm water of 
low salinity. This counter current may be due in part to the excessive 
rainfall in the equatorial belt, but it is thought to be due mainly to 
the return flow of some of the water that is piled up against the South 
Atlantic coast by the equatorial currents. 

From this, it is evident that the great oceanic eddies, which are 
repeated in the South Indian Ocean and in the North and South 
Pacific, are due primarily to the motive power of the wind, but that 
their circular course is due to the interference of (1) the continent 
barriers, (2) the deflective effect of earth's rotation. Doubtless 
other factors of importance enter to aid in the circulation, such as the 
expansion of the water due to heat, the raising of the surface by rain- 
fall in one part and lowering it by evaporation in another part, and the 
change in level due to the piling up of water along the coasts. It has 
just been pointed out that one result of this is a return or counter 
current at the surface. Doubtless also there is partial compensation 
by vertical circulation ; but, with the surface water varying in density, 
this compensation can be only partial, since the water surface will be 
actually higher where less dense than in contiguous areas of greater 
density. Then, as in the case of the Black Sea, there may be compen- 
sating bottom movements as well as surface currents. That there is 
actual vertical circulation is indicated by the fact that there are cold, 
uprising currents along the coasts away from which the trade winds 
are blowing, that is, along the west-facing coasts. In this way a part 
of the cold water of the Peruvian Current off South America and the 
Benguela Current along the western coast of South Africa is accounted 
for. Very likely the surface drifts are also influenced by the slow up- 
welling of water in the great planetary circulation. 

The Gulf Stream. — In the North Atlantic the oceanic circulation 
is complicated by important well-defined currents, some from the 
Atlantic, and one from the Gulf of Mexico. The latter, known as the 
Gulf Stream, is one of the most important of oceanic currents. It has 
its origin in the Caribbean Sea and Gulf of Mexico, into which a part 
of the equatorial drift passes by way of the Antilles and the South 
American coast. Here the warm tropical waters are warmed still 
more. Thus the level of the water in the Gulf of Mexico is raised by 
indrift of water and by expansion due to warming. Probably also 
there is additional increase in height due to rainfall and the inflow of 
streams from the land. It has been estimated that the surface of the 
Gulf of Mexico is about three feet higher than sea level at New York. 
Since there is a continuous submerged barrier forming the rim of the 



MOVEMENTS OF THE OCEANIC WATER 695 

Gulf of Mexico, there can be no adequate compensating vertical cir- 
culation, and the outflowing water must escape as a surface current, 
the available outlet being in the strait between southern Florida and 
Cuba. Thus the entire Strait" of Florida is occupied by a stream of 
very warm, salt water, with a surface temperature of 81 °, a width of 
about 50 miles, a depth of 1800 to 2000 feet, and a velocity of 5 miles 
an hour. Through the Strait it flows so fast that the bottom is 
swept clean of mud. 

Beyond the Strait of Florida the Gulf Stream broadens rapidly, and 
its velocity decreases, being reduced to about i| miles per hour east of 
New York. Here the Gulf Stream waters, together with a portion 
of the western part of the North Atlantic eddy, is within the belt of 
the west winds of the northern hemisphere, and before them the warm 
southern water is drifted northeastward to the European coast and 
into the Arctic Ocean. An eastward course is also provided for by 
the deflective influence of the earth's rotation, which turns this current 
toward the right. 

It was formerly the custom to speak of the warm drift of water which 
bathes northwestern Europe as the Gulf Stream. With better knowl- 
edge of oceanic movements it has been found that it is really a West 
Wind Drift in which Gulf Stream water is but a part, and the im- 
mediate' propelling cause is the prevailing west winds. It might 
seem fitting to continue the use of the term Gulf Stream for this water 
as we continue the use of the word sunrise, without being open to a 
charge of ignorance. There has, however, been strong objection to 
this, some of it of a more or less pedantic nature, and the warm current 
on the northwestern European coast is now without a better name than 
the cumbersome North Atlantic West Wind Drift. 

The Terms Drift, Current, and Stream. — On the ocean current map, 
and in the preceding description of oceanic circulation, we have made 
use of three terms descriptive of parts of the oceanic circulation : (1) 
drift, (2) current, (3) stream. There is no real distinction between 
these terms, though it i§ quite generally recognized that a " drift " 
is a slow motion of the upper layers without any very definite boun- 
daries ; a " current " is more rapid and more definite ; and a " stream" 
is still more definite. The Gulf Stream, for example, is in some parts 
quite definitely bounded, so as to suggest a stream of warm water 
in the ocean. There has, however, been much exaggeration as to 
the definiteness of the Gulf Stream boundary. In most parts there 
is gradation of such slow degree that hours of steaming are necessary 
to pass from the normal ocean water to the unquestioned Gulf Stream 
water. The most definite boundary is on the western side, where a 
cold, south-moving current parallels it on the landward side. 

A drift may move no faster than 10 to 15 miles a day, while a current 
or stream may attain a velocity of 4 or even 5 miles an hour. The 
drift is usually a broad movement, due in large part mainly directly 
to the wind ; but a current or stream is commonly a part of the oceanic 



696 COLLEGE PHYSIOGRAPHY 

circulation, so modified as to concentrate the movement along a more 
or less definite belt, with consequent increase in velocity. 

The Arctic Ocean. — Little is known of the movements of the waters 
in a large portion of the Arctic. But it has long been known that tree 
trunks from northern Asia have stranded on northern America, in- 
dicating a trans-polar drift of the Arctic waters. The wreck of the 
Jeanette likewise drifted across the Arctic Ocean. It was this that 
led Nansen on his north polar journey to push his boat into the Arctic 
ice with the hope that he might drift over or near the North Pole. 
While his boat drifted westward, as expected, it did not cross the pole, 
though perhaps it would have done so had he started farther east. 
One of the Melville-Bryant casks, placed in the Arctic Ocean near 
Pt. Barrow, Alaska, floated across the Arctic ocean to Iceland be- 
tween 1899 and 1905, conceivably passing near the North Pole. 
Another of these casks drifted from Alaska to northern Norway. 
North of Greenland Peary found the floe ice drifting eastward, as if 
under the influence of the prevailing westerly winds. During the 
winter of 1913-14 the Karluk drifted westward about 800 miles from 
Colville River, Alaska, nearly to Wrangell Island, Siberia, moving at 
the rate of 7 miles a day. These furnish the basis of Amundsen's 
plan of drifting to the North Pole in 191 5 in the Fratn. 

It is well known, however, that the warm current of the eastern 
North Atlantic sweeps into the Arctic north of Norway, keeping the 
sea clear of ice as far north as latitude 70 N., and greatly modifying 
the climate of northern Europe and the contiguous Arctic. There is 
also a north-moving current along the western coast of Greenland. 

Labrador Current and other Outflowing Streams. — On the other 
hand, cold, ice-laden currents sweep down the coast of eastern Green- 
land, and along Baffin Land and Labrador. The latter, fed from be- 
tween the Arctic Islands north of North America, is known as the 
Labrador Current, and is scarcely less important to eastern America 
than the Gulf Stream. Turned to the right by the deflective in- 
fluence of the earth's rotation, it hugs the eastern coast of North 
America as far south as Cape Cod, then, continuing southward off- 
shore, it sinks and is lost as a surface current. It is ice-laden as far 
south as northern Newfoundland and bears icebergs even farther south. 
The chill of its waters influences the coastal climate as far south 
as Massachusetts, and affects the water temperature even farther 
south, especially below the surface. 

There is no really analogous current from the Arctic to the Pacific 
through the narrow, shallow Bering Strait. It is true that a cold cur- 
rent passes through Bering Strait, but although it is reinforced by 
cold water flowing southward from Bering Sea, it attains no such size 
as the Labrador Current, and does not carry its influence so far south. 
Nor is it ice-laden to so southern a point as the Labrador Current. 
Like the Labrador Current, this cold, south-moving current in the 
North Pacific keeps to the right under the deflective influence of rota- 



MOVEMENTS OF THE OCEANIC WATER 697 

tion, and therefore hugs the North Asiatic coast and affects its climate, 
as the Labrador Current does that of North America, though to a less 
degree. 

These outflowing Arctic currents doubtless represent in part a return 
of the water that flows in along northern Europe, in part an outflow 
of the fresh waters that enter the Arctic basin from the great rivers 
and from the melting glaciers and snows of the north. Vertical cir- 
culation to dispose of these waters is seriously interfered with by sub- 
marine ridges, which to a very large extent cut off the Arctic basin 
from the North Atlantic basin. 

The Pacific Ocean. — On a larger scale the circulation of the Pacific 
repeats that of the Atlantic in its general features. There is a northern 
and a southern eddy, and there is a counter current between the north 
and south equatorial drift, extending as far east as the Gulf of Panama. 

In the maze of islands in the western Pacific there is complex move- 
ment, due to deflective effects of the islands and submarine ridges, 
and in the North Pacific there is a current which closely simulates 
the Gulf Stream. This, the Kuro Shiwo y or Japanese Current, repre- 
sents the equatorial drift, which, after entering the East Indian region, 
is warmed in the seas between the East Indies, the Philippines, and 
the Asiatic coast, and escapes as a well-defined current between the 
Philippines and Formosa. It passes out into the North Pacific, broad- 
ening as it flows, and by the west wind drift is propelled toward the 
American coast, being deflected that way also by the influence of 
terrestrial rotation (Fig. 416). 

Since the North Pacific is nearly cut off from the Arctic, first by 
the Aleutian Island mountain range, then by the shallow Bering Sea, 
Bering Strait, and bordering lands, there is, as we have seen, no com- 
parable south-moving Arctic current. Nor is there any escape for 
the warm waters drifted northwestward by the west winds. These 
waters, therefore, circle southward in even greater quantity than in 
the North Atlantic eddy. They raise the temperature of the north- 
ern Pacific and the Gulf of Alaska, but, flowing thence southward 
they bring cooler water off shore from the coast of United States. 

The Influence of Ocean Currents 

Relation to Marine Life. — The circulation of the ocean waters 
is of importance to ocean life in influencing its distribution, in trans- 
portation of food supply, and in distributing the needed oxygen. 
Without such circulation ocean life would doubtless be very different. 
The significance of this may be illustrated by the distribution of reef- 
building corals, which abound on coasts against which definite cur- 
rents of sufficiently warm water sweep, while they are limited or 
even absent from less favourably situated coasts. 

Relation to Navigation. — Ocean currents have distinct influence 
on navigation by aiding or retarding the movement of ships, especially 



698 



COLLEGE PHYSIOGRAPHY 




MOVEMENTS OF THE OCEANIC WATER 



699 



sailing vessels. The strong currents along the South American coast 
were a very decided factor in the movement of the small ships in which 
Columbus sailed. Even the west wind drift of the North Atlantic 
produced sufficient effect upon the sailing vessels of colonial days 
to attract the attention of Benjamin Franklin, then postmaster- 
general of the Colonies. The investigation which he undertook to 
explain the fact that west-going boats went faster than east-going 
led to the first correct explanation of the Gulf Stream. It goes with- 
out saying that a current flowing at the rate of 4 or 5 miles an hour, 
as the Gulf Stream does, is a factor to be reckoned with in navigation ; 
and even the slower drifts are 



of importance. The accom- 
panying diagram (Fig. 417) 
showing the drift of a derelict, 
illustrates both the direction 
and rate of movement of an 
object at the mercy of wind, 
waves, and currents in the 
North Atlantic. Many obser- 
vations have been made on 
floating derelicts, and on bottles 
placed in the ocean for the 
purpose, in order to determine 
the rate and direction of the 
ocean currents and drifts. 

Relation to Climate. — The 
outflowing water from the Arc- 
tic and Antarctic regions is of 
great importance in modifying 
the climate of those regions, for 
the cold water that sinks to 
the bottom creeps to other 
zones, and the surface currents 
carry both cold water and ice to 
warmer regions. Without such 
distribution of cold water and 

ice the conditions in the polar zones would doubtless be far different. 
Every season vast quantities of sea ice and glacier ice are drifted out 
of the Arctic. These cold surface currents naturally affect the tem- 
perature of the sea in the regions to which they flow, and the winds 
which blow over them are chilled in their passage. Since the deflec- 
tive effect of rotation swings these currents against the east-facing 
coasts of the northern hemisphere, their influence is mainly felt in 
the western parts of the ocean and the contiguous east-facing lands. 
The cold climate of Labrador, Newfoundland, and New England is 
in part due to the effect of the cold Arctic current transmitted to the 
land by the ocean winds. 




BORMAY & CO., 



Fig. 417. — The path of a wrecked vessel 
which drifted across the Atlantic in ten 
months, being occasionally turned back by 
storms. 



7 oo COLLEGE PHYSIOGRAPHY 

The warm currents produce influences essentially the opposite of 
those caused by cold currents. They distribute the tropical heat 
and diminish the temperature of the tropical waters. Partly enclosed 
seas, like the Red Sea, the Mediterranean, and the Gulf of Mexico, 
have higher temperatures than the open ocean in the same latitude, 
partly because of the lack of opportunity for distribution of the heated 
water. We may be certain that without ocean currents the temper- 
atures of the tropical waters would be far higher than they are. This 
warmth is distributed in the great oceanic eddies well up into the 
temperate zones, and in the North Atlantic even into the polar zone. 
The effect of rotation swings these currents to the eastern side of the 
ocean, so that the west-facing coasts are bathed by warmer currents, 
as is so well illustrated on the northwestern coasts of North America 
and Europe. The winds blowing over these warm waters produce a 
profound influence upon the contiguous lands. Thus Europe is in- 
habited by an agricultural population up to the Arctic Circle, while 
in eastern North America a bleak, barren land is found in the same 
latitude as flourishing farm land and dense industrial population in 
Europe. 

Relation to Rainfall and Fog. — The ocean drifts and currents have 
also important influence on precipitation. Winds blowing over 
warm waters become charged with vapour, and this is, of necessity, 
partly precipitated when the air is chilled either in passing over cooler 
water or in rising over the land. It is due to the latter cause that there 
is so heavy a rainfall on the western coasts of Europe and America ; 
while the former cause explains numerous fog belts. One of the 
foggiest places on the earth is on and near the Banks of Newfoundland, 
where the warm southern current and the cold Labrador Current flow 
side by side in opposite directions. A vessel rarely crosses this region 
without encountering fogs. At San Francisco and offshore from it 
fogs are common, because a cool south-flowing current exists immedi- 
ately offshore, while beyond is warmer water over which the winds 
blow, obtaining abundant vapour which is condensed into fog parti- 
cles as the air is chilled in the passage over the cold current. 

Tides 

Distortion of the Hydrosphere. — Both the sun and the moon exert 
an attraction on the earth, as a result of which the liquid hydrosphere 
is distorted, and this distortion causes the phenomenon of tides. It is 
too complicated a phenomenon for complete discussion in a book of 
this scope, requiring for its adequate treatment mathematical discus- 
sion. Those desiring to read more about this should consult a text- 
book of astronomy, the Encyclopaedia Britannica, or the reports of the 
U. S. Coast and Geodetic Survey. 

Lunar Distortion exceeds Solar. — Since the attraction of gravi- 
tation varies with the mass and inversely as the square of the distance, 



MOVEMENTS OF THE OCEANIC WATER 701 

the moon, 240,000 miles distant, although so small, has far greater 
tide-producing power than the larger sun, 93,000,000 miles distant. 
If there were no moon, there would still be a tide, though much 
smaller and much less complex than the present tide, whose main char- 
acteristics are dominated by the moon. We, therefore, commonly 
speak of it as the lunar tide, though it must not be forgotten that the 
tidal movement is a complex combination of lunar and solar tide. 

The moon distorts the hydrosphere, raising a broad swell on the 
surface nearest the moon, and another, somewhat lower, on the 
opposite side of the earth, while between these two swells are broad de- 
pressions or troughs. If the earth were completely liquid, this distor- 
tion would be in the nature of an ellipse. A similar but lower dis- 
tortion is caused by the sun. As the earth rotates, this distortion 
follows the inciting cause. It therefore sweeps around the earth 
once in about twenty-four hours, so that two solar and two lunar 
waves, with intervening troughs, pass around the earth approxi- 
mately during each rotation. 

The passage of these tidal waves is subjected to a complex series of 
modifying influences. Some of these are due to irregularities of ocean 
bottom and coast, others to astronomical causes, such as varying 
distance between earth and moon or sun, and varying relative posi- 
tions of earth, sun, and moon. Therefore the tide varies greatly, 
both in interval and in height ; and both in a given locality and in 
different localities. 

Period between Tides. — The tide does not coincide exactly with 
the moon's position, but lags behind it. Nor does it sweep around the 
earth in a general wave; but apparently develops in the different 
oceans, as it does in small degree in large lakes and enclosed seas, for 
the lands stand in the way of the free sweep of the tidal wave. In its 
passage around the earth the tide does not recur at the regular interval 
of half a rotation, 12 hours, but in half a rotation plus the forward 
movement of the moon in its orbit around the earth, or 12 hours and 
26 minutes. Thus there is a retardation of 26 minutes between each 
tide or 52 minutes in the day, so that if high tide comes at 12 o'clock 
noon on one day, it will appear at 26 minutes past 12 that night, and at 
8 minutes before 1 o'clock the next noon. 

Height of Tides. — The exact height of the tide in the open ocean 
is not known, and doubtless varies from place to place, as it certainly 
must from time to time. It is not a recognizable movement of ocean 
water in the open ocean, though on oceanic islands it registers itself as 
a slow rise and fall of the ocean surface twice each day, and to a height 
of 2 or 3 feet. The highest reach of the water is called high tide, the 
lowest reach low tide (Fig. 418), and when the tide is rising it is com- 
monly said to be coming in or flowing, since the rising water advances 
upon the land. The falling tide is said to be going out or ebbing. 
The motion is really that of two great waves, with broad, low crests 
and intervening troughs, sweeping through the ocean and causing a 



702 



COLLEGE PHYSIOGRAPHY 



slow, rhythmical rise and fall of the surface, while the entire ocean 
water from surface to bottom is involved in the motion. 




Fig. 418.— High tide and low tide near Bourne on the coast of Massachusetts. 
(J. L. Gardner.) 



Varying Relationships to the Moon. — The range between the sea 
level at low and high tides, known as the tidal range, is subject to con- 



MOVEMENTS OF THE OCEANIC WATER 



703 



® 




© 



® 



LUNAI^TIOE 



siderable variation according to the distance of the moon. When in 
its orbit around the earth, the moon is farthest from the earth, or 
in apogee, the tidal pull is less than at the opposite parts of the orbit, 
or perigee. There is, therefore, a rhythmic variation in tidal range, 
with a period of about two weeks between the higher or perigee stage 
and the lower or apogee stage. Since the distance between earth and 
sun also varies during a complete revolution, being nearest in the peri- 
helion and farthest in aphelion, there is a similar, though slighter, 
semi-yearly variation in the solar tide. The tidal range is also in- 
fluenced by the position of the moon in the heavens, for with the change 
in season the moon is vertical at different latitudes, and since the ocean 
water is disturbed irregularly over the earth, the variation in in- 
fluence of lunar pull is 
very considerable. 

Even more note- 
worthy than these causes 
for variation in the 
height of the tidal wave 
is the relative position 
of sun and moon. When 
the sun, moon, and earth 
are in nearly the same 
line, the solar and lunar 
tides are combined, and 
the tidal range is high. 
These high ranges of 
tide are known as spring 
tides, and they occur at 
new and full moon. The 
opposite condition, when 

moon and sun are out of line, occurring in the periods of the moon's 
quarters, give rise to a lower range of tides because the solar and lunar 
tides are not combined. These are known as neap tides. Therefore 
once each lunar month there are two spring and two neap tides 
(Fig. 419). 

To go much farther with a statement of tidal variation would demand 
mathematical treatment. Enough has been said, however, to indicate 
the main fact of great variability in inciting cause. Summarized, we 
have (1) a daily, 24 hour and 52 minute rise and fall of the ocean surface 
with two periods of high and two of low water, the range of one of 
the tides being greater than that of the other; (2) twice each lunar 
month the range is higher than normal — spring tides — and twice 
lower — neap tides ; (3) the range varies also with distance between 
earth and moon, between earth and sun, and according to the latitude 
where the moon is vertical ; (4) the tidal range varies with different 
combinations of these causes for variations. It is, therefore, an ex- 
ceedingly complex phenomenon that is included under the term tide. 




© 



Fig. 419. — Diagrams to show positions of moon and 
sun at spring tide (above) and neap tide (below). 



7 o4 COLLEGE PHYSIOGRAPHY 

Varying Relationships to the Lands. — Even greater complexity is 
introduced by the complex environment in which the tidal movement 
takes place. There is, first of all, the fact that the ocean waters are 
irregularly distributed, and that they are separated by lands and by 
submerged ridges. It was formerly postulated that the tide was 
generated in the great southern ocean and that it swept up into the 
Atlantic, Pacific, and Indian oceans, advancing successively to more 
and more remote parts of the branching oceans. The movement is 
now found to be much more complex, and it is thought by some 
students of the subject that the tide is generated in the main in the 
individual oceans. The exact mode of origin of the tidal waves, and 
the relation of the tidal wave of one ocean to that of another, is not yet 
clearly demonstrated. 

A second important influence of environment is the effect of shallow- 
ing water. Every continent is surrounded by a continental shelf, 
and, as the profound tidal movement reaches this shallowing area, 
it is increased in height. Thus there are few places on the exposed 
coasts of continents with a tidal range as low as 2 or 3 feet. It is 
quite possible that the assumed height of the ocean tide from measure- 
ments on oceanic islands is also somewhat too great because of piling 
up of water as the tide advances upon the islands. Where the influence 
of shallowing water is most felt, the tidal range may become as much 
as 5, 10, or 15 feet, and in limited areas even more. At the same time 
that the tidal wave is thus locally raised, horizontal movement is 
also frequently introduced, causing tidal currents. These are common 
in the shallow waters surrounding most continents, and they are also 
proved to exist in shallow areas in the open ocean. Such tidal currents 
are not properly the tidal wave, but a modification of it by interference 
with the wave motion due to shallow water. 

There is, thirdly, a very complex modification of the tide by 
the irregularities of coast lines. This subject is so complex that 
a complete analysis of it is impossible here ; but a general view of 
the influence of this cause may be gained by selecting a few typical 
instances. 

Tides in Partly Enclosed Seas. — Along many coasts there are bays 
with the entrance more or less enclosed. The Mediterranean, for 
instance, is open to the sea only by a narrow strait. When the Atlantic 
tide rises outside the Straits of Gibraltar, there is an inflow of water 
into the Mediterranean ; but manifestly this cannot be sufficient to 
cause tidal rise and fall in so large a sea. Therefore there is no tide 
in the Mediterranean, excepting a very small one generated in this sea 
itself. A more open bay, like the Gulf of Mexico, is less isolated, but 
even here the open ocean tide causes only slight rise and fall. A very 
small bay, with a narow opening, can be filled as the tide rises, and 
lowered as it falls, but a large bay cannot be. In each case, however, 
powerful tidal currents flow through the narrow opening as the outer 
ocean level varies. 



MOVEMENTS OF THE OCEANIC WATER 



705 



Tides in Broadly Open Bays. — On the other extreme, there are 
many bays with broad mouth and narrowing toward the head. Into 
these the tidal wave advances, and its height is increased not merely 
by interference due to shallowing bottom, but also by the convergence 
of the margins. It is in such places that we get the greatest tidal range, 
as in the Bay of Fundy, where there is a range of from 30 to 53 feet, 
in Ungava Bay, where there is a similar tidal range, and in Turnagain 
Arm of Cook Inlet, Alaska, where the vertical difference between 
high and low tides is 54 feet. 

Tidal Races. — Between the two extremes of broad-mouthed 
and narrow-mouthed bays there is every intermediate form, and each 
has its own influence upon the tidal wave. Accordingly there is great 
variety in the tidal range, 

even within narrow limits. l ,. H0 , URS AF T! R TRANSIT 

There is also variation in the 
time at which high or low tide 
reaches points, for as the 
wave advances upon an irreg- 
ular coast it reaches the head- 
lands first, then progresses into 
the indentations at a rate vary- 
ing with their form and depth. 
On very irregular coasts it 
therefore sometimes happens 
that the tide is high in one 
bay at a different time than 
in a contiguous bay. If a 
strait connects such bays, 
rapid currents or races sweep 

through them with both rate FlG - 420. — Different time of arrival and differ- 

1 j" , • • •,! ,1 ent height of tide on the two sides of Hell 

and direction varying with the Gate ne ar New York City. (U. S. Coast 

State of the tide. Or the same and Geodetic Survey.) 

condition may be the result of 

the fact that in one bay the tide rises higher than in the other. Such 
races often occur in the gap between New York Bay and Long Island 
Sound at Hell Gate ; also in the straits between Buzzard's Bay and 
Vineyard Sound, especially at Wood's Hole. Rapid currents de- 
velop also in the Bay of Fundy, in the English Channel, and the North 
Sea, along the coasts of Alaska, British Columbia, and Norway, and 
in many other places (Figs. 420, 421). 

These currents often become very rapid and complex, even inter- 
fering with navigation. Where such currents develop, they may flow 
side by side in different directions, or at different rates, or one current 
can flow above another. The water is set into rapid and irregular 
motion, increasing during some stages of tide and decreasing during 
others. At dead low or high tide they may nearly or quite cease, 
and the direction of motion during the incoming tide is reversed during 



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706 



COLLEGE PHYSIOGRAPHY 




JM4 j& 

0™/ ^$>P _.^ 







the outgoing tide. Where such currents develop, the terms coming 
in and going out are much more applicable than on more open coasts 
where there is little if any current but a gradual rise and fall of the 
oceanic surface. Even where the tidal currents exist, however, the 

surface gradually rises 
and falls with the tide. 

In favourable locali- 
ties the tidal currents 
develop a rough choppy 
wave surface known as 
the tide rip, which is 
greatly intensified when 
the wind blows against 
the current. Sometimes 
the rip is due to friction 
with a shallow bottom, 
but at times it occurs 
w T here the water is too 
deep for that explana- 
tion. In such cases it 
seems to be due to fric- 
tion of the surface cur- 
rent with the lower 
layers which are either 
stationary or flowing in 
a different direction. 

Tidal Influence in 
Fresh Water. — Where 
the tide enters river 
mouths, the salt water 
extends up a certain dis- 
tance, depending upon 
the slope of the river 
bed, the height of the 
tide, and other factors. 
But the tidal effect 
reaches still higher, for 
the salt water dams the 
stream, holding back its current, and causes a tidal rise even in the 
fresh water. In large streams this effect may extend ioo miles or more 
up-stream. In the St. Lawrence the tide is felt nearly to Montreal and 
in the Hudson above Albany. In such long, narrow stretches the tide 
is usually unequal, the high tide coming rather quickly, while the period 
of low tide is prolonged. With considerable bodies of fresh water 
thus ponded back, the outflow after the high tide stage is naturally 
extended. And the time of high and low tide is not simultaneous 
with that at the river mouth, for an interval is required for the tidal 




FlG. 421. — The high tides in Buzzard's Bay, Massa- 
chusetts, and in Long Island Sound, in contrast with 
the lower tides near Martha's Vineyard Island and in 
New York harbour. This inequality of level results 
in tidal races between the islands south of Buzzard's 
Bay and at Hell Gate. 



MOVEMENTS OF THE OCEANIC WATER 707 

effect to pass up-stream. But even with such causes for variation 
there is a rhythmic swing of tidal rise and fall here as elsewhere. 

The Bore. — A peculiar modification of the inflow of the tide in estu- 
aries on coasts with a high range is the bore, as illustrated in the Petit- 
codiac River, New Brunswick (Fig. 422) . Here the tide rises high in the 
Bay of Fundy, and during the low water stage the " river," or estuary, 
is a broad mud flat, bordered with marshes and with only a small 
stream flowing down it. The tide rises into the estuary, but it cannot 
advance up it as fast as it rises in the open estuary mouth. There is, 
therefore, such a difference in elevation between the sea level in 
the estuary mouth and the floor of the estuary higher up that the 
water breaks into a wave with foaming crest, which rushes rapidly up 




Fig. 422. — The bore, at Moncton, New Brunswick. 

the estuary. Day after day this bore wave rushes up the river at the 
proper stage, and with such regularity that its period of arrival is pre- 
dicted within a minute or two. It varies in height according to the tide 
in the bay, but, even when lowest, is an impressive sight. After the 
arrival of the bore the water flows rapidly in, and high tide is soon 
reached. The rise is so rapid that one can see it rise on the shores. 

Similar bores occur in other places, differing in height and in detail 
from this one, but in essential features being similar. Among the 
places where the bore occurs are the Severn in England, the Seine 
in France, the Amazon, and some of the Chinese rivers. In some of 
these cases the bore appears only at certain states of the tidal wave 
and not with the regularity of the Petitcodiac bore. In the Seine 
the bore is known as the mascarat; in the Amazon as the powcoa. 



708 COLLEGE PHYSIOGRAPHY 

Where developed as a high wave, as in China, the bore is sometimes 
very destructive, and special means are taken to protect the shipping 
and the river banks from its effects. 

Tidal Prediction. — From the preceding it is clear that the height 
of the tide varies from time to time and from place to place, and 
that there are also variations in the time between tides, and in the 
behaviour of the tidal rise and fall from mere surface swing to rapid 
currents and even to the tidal bore. There are places where there is 
no tide, or only one tide, or one high range and one low range, or one 
long and one short tide. Yet, in spite of all these variations, there is 
such underlying regularity that it is possible to make accurate predic- 
tions for any place on the earth. It is, however, first necessary to 
know the local influence, and this requires a series of observations. 
But having made these observations, since the regularly recurring 
cycle of rise and fall will be repeated rhythmically, a series of accurate 
predictions can be made for years in advance by taking into account 
the various astronomical causes for variation. The local influences 
are constant for a given locality ; the astronomical causes for variation 
are regular and calculable. Therefore, knowing the local peculiarity, 
and understanding the astronomical causes, gives the necessary basis 
for tide prediction both as to time and range. It is on this basis that 
tide tables are prepared for all parts of coasts visited by ships of 
civilized nations. 

Importance of Tides to Man. — Tides and tidal currents are among 
the most important phenomena of coast lines, but in the open ocean 
are of little importance. The time and height of tide must be reckoned 
with in all navigation near the coast, and the force and direction of 
tidal currents must be known. Often vessels are drifted out of their 
course by the tidal currents, and these are among the most frequent 
causes for shipwreck. The tides are important in coastal cities as 
a means of removing waste products and as a cause for maintaining 
a supply of pure salt water. Without the efficient aid of the tides the 
problem of life in seaport towns would be much more serious than it is. 

Tidal currents are important means of distribution of sediment, and, 
as we have seen, are factors of significance in the development of coast 
lines. By them sediment is often drifted into and across bay and har- 
bour mouths, and one of the problems of civilized nations is how best 
to combat the tidal influence, which is helping to seal up harbours with 
sediment. The building of jetties, dredging, and other means are 
being employed by commercial nations, with an annual expenditure of 
millions of dollars, to combat this phase of tidal work. 

The tide, like the ocean waves, represents a vast store of energy, 
most of it apparently going to waste. In a very few places dams 
have been made to impound the water of the high tide stage, but, with 
this exception, man has made no direct use of this vast store of energy 
expending itself along the continental coasts. 

For references to literature, see pp. 665-668. 



PART III. THE ATMOSPHERE 

CHAPTER XXII 
CHARACTERISTICS OF THE ATMOSPHERE 

General Description 

Relation of Atmosphere to Earth. — The earth is enveloped by a 
gaseous mantle known as the atmosphere, though the substance of 
which it is composed is usually called the air. The atmosphere is 
quite as much a part of the earth as are the solid rock and the water of 
the oceans and rivers. It contains the same elements as those which 
make up the land and sea, only it exists in the gaseous instead of the 
solid or the liquid form, just as water may exist as solid ice, liquid 
water, or gaseous water vapour. 

The atmosphere, as we know it, is quite different from that other 
transparent substance which separates the earth from the sun, stars, 
and planets and which is called the ether. The atmosphere travels 
with the earth on its journey around the sun, being held in place 
through the earth's attraction of gravitation. 

Thickness of the Atmosphere. — The thickness of the earth's at- 
mosphere is not known, but we do know that the part of the atmos- 
phere which is dense enough to support life is limited to about 5 or 6 
miles from the earth's surface. Aviators in aeroplanes have ascended 
to a height of over 4 miles. The atmosphere, however, in more or 
less modified form, extends higher than the loftiest mountain top 
(Mt. Everest, 29,002 feet or about 5I miles). The ascent of balloons 
has shown that it extends even higher. Balloons with aeronauts have 
been over 6 miles from the land surface. No ascents of this sort have 
been made over the oceans. Unmanned (sounding) balloons have 
been up to an altitude of about 18 or 20 miles. The phenomena of 
twilight indicate that the atmosphere extends to a height of at least 
45 miles. The glowing of meteors at an elevation of nearly 200 miles 
above the earth's surface shows that the atmosphere is present there, 
for, in the ether of space, meteors do not burn. It is thought from 
the lights of the aurora that the atmosphere is present over 200 
miles above the earth, though at this height it must be extremely 
attenuated. 

Atmospheric Pressure 

Density near Earth's Surface. — The weight of the atmosphere at 
sea level is about 15 pounds to the square inch, the equivalent of about 
34 feet of water or 30 inches of mercury. This is because the air, 

709 



710 



COLLEGE PHYSIOGRAPHY 



although light and invisible, has perceptible weight ; and each particle, 
drawn down by gravity, presses on those below it, as stones in a pile 
press on those beneath. The air, extending to a height of 200 or more 
miles above the earth's surface, has a weight which can be measured. 
The average weight at sea level is convenient for use as a unit. We, 
therefore, say that the weight of air on each square inch of sea level 
is about 15 pounds. 

Every square inch of the surface of the human body bears a great 
weight of air. The pressure within the body, however, is equal to 
that outside, so that we do not notice this pressure. If the outside 
pressure were suddenly removed, the expansion of the air within our 
bodies would probably burst many of the tissues and cause the eyes to 
protrude and the skin to crack, as is the case with fish which are 
hauled up from the deep sea, where they were under great pressure of 
water, to the surface, where this pressure is relieved (p. 650). 

The column of air resting on the top of a mountain a mile high is of 
course 5280 feet less in height than a similar column (Fig. 423) at sea 

level, and, therefore, of 
such diminished weight 
that the atmospheric press- 
ure on the mountain top 
is less than that at sea 
level. Pressure pushes the 
molecules of a gas 3 such 
as the atmosphere, closer 
together, so that the air 
is denser near the sea 
than on mountain tops. 

Fig. 423. — Diagram, to show why the atmosphere is . ,. , f ,, x 

more dense at sea level than on a mountain top. Accordingly, IUlly tWO- 

thirds of the atmosphere, 
by weight, is within 6 miles of sea level, and the air is not nearly as 
dense at the top of a high mountain, like Mount McKinley or Mount 
Everest, as it is at sea level near the base. On mountain tops the 
thinness or rarefied character of the air is such that it is difficult to 
breathe enough oxygen for the needs of the body. Some men and 
animals accustom themselves to this rarefied air so that they are able 
to live on high plateaus and mountains ; but the rarefied air furnishes 
the principal reason why the higher mountains of the world have not 
yet been ascended. Persons living at the lower levels, however, find 
that when they are on mountains they must breathe more rapidly in 
order to get enough oxygen, and frequently they become exhausted 
in the effort. It is also difficult to sleep at great altitudes. 

Relation to Temperature. — Because of the elasticity of air, its 
weight or density also changes with difference in temperature. The 
air filling a room 10 by 20 feet weighs 301 pounds when the tempera- 
ture is 6o°. When the temperature is increased to 8o°, the air is so 
expanded that some of it will be expelled from the room if opportunity 




CHARACTERISTICS OF THE ATMOSPHERE 711 

is given, and the amount left in a space of this size weighs only 291 
pounds. 

Relation to Gravity. — Because of the different pull of gravity on 
light air and on heavy air, atmospheric movements are started ; and 
this movement of air from place to place is known as wind (Chap. 
XXV). 

Barometric Pressure. — The temperature at the earth's surface is 
always changing and, consequently, the weight of the air changes also. 
This weight or pressure of the air is measured with an instrument 
known as the barometer ; and the weight of a column of air at any 
given place is known as barometric pressure. The barometer takes 
advantage of the principle that atmospheric pressure will push the 
liquid up into a tube having a vacuum in the top, displacing it until a 
column is formed that equals the weight of the air column pressing 
on it. This pressure, for example, pushes water up from a well into the 
tube of a pump. The stroke of the pump exhausts the air from the 
tube, tending to make a vacuum into which the water may be pushed 
by atmospheric pressure. Because of the fact that a column of water 
34 feet high balances the air pressure, an ordinary pump cannot raise 
the water from a well more than about 34 feet deep. 

Mercurial Barometers. — Barometers can be constructed with 
water columns a little less than 35 feet long. Usually mercury is 
employed in the tube, because it takes a column of mercury only 30 
inches high to balance the atmospheric pressure. 

A rough barometer may be made by using a glass tube about 35 
inches long, sealed at one end. If the tube is rilled with mercury 
and inverted with the open end in a small dish of mercury, the mercury 
in the tube will first fall a few inches and then remain stationary, 
being kept there by the air pressure. By fastening the tube to an 
upright stick it is possible to watch the mercury rise and fall from day 
to day with the variations of atmospheric pressure. If a scale is 
marked on the glass of the tube, the amounts of variation may be roughly 
measured. With the coming and passing of storms there is a varia- 
tion in atmospheric pressure. This is recorded by the height of the 
mercury column, which is measured in ordinary mercurial barometers* 
in inches and tenths of inches, or, with a scale called the Vernier, 
in hundredths or thousandths of inches. When the air is heavy, the 
column of mercury in the barometer is high ; 30. 20 inches for example, 
is a relatively high barometer. With light air the column of mercury 
in the barometer is low, but the range between high and low pressure 
for a given altitude is slight, and 29.30 inches is a relatively low barome- 
ter. However, 30.20 inches is not always to be regarded as High, nor 
29.30 inches as always Low, for high and low barometers are not asso- 
ciated with definite fixed values. 

Aneroid Barometers. — Because of the disadvantage of carrying 
a mercurial barometer, another instrument, called the aneroid barome- 
ter, is more often used. An aneroid, as usually made, is small enough 



7 i2 COLLEGE PHYSIOGRAPHY 

to be carried in the pocket and has a metal diaphragm inside a metal 
case. The differences in air pressure cause this diaphragm to move, 
and the movement is communicated to a hand which moves over a 
dial. 

Use of Barometers in Measuring Elevations. — Because there is 
less air and, therefore, less pressure above plateaus and mountains 
than above plains, the barometer is low on highlands and high on 
lowlands. This makes it possible to use the barometer in measuring 
elevations. By graduating the dial in feet, it is possible to measure 
changes in elevation with an aneroid barometer. A disadvantage in 
the use of any barometer for measuring altitudes is that it is affected 
not only by variations in pressure measured by a person who travels 
from lowlands to highlands, but also, as is explained later, by (a) 
changes in air pressure during the passage of storms, and (b) those due 
to the heating of the atmosphere. In using a barometer for the ac- 
curate measurement of elevation, therefore, it is necessary to compare 
its record with that of another barometer which is kept at a fixed 
point, and to make corrections accordingly. 

Barographs. — Another form of barometer, the barograph, is self- 
recording, having a pen point continuously pressed against a cylindrical 
roll of paper which is revolved by clock work. The barograph gives a 
continuous record of changes in atmospheric pressure (Fig. 447). 

Composition of the Atmosphere 

The Atmospheric Mixture. — The atmosphere is a mechanical mix- 
ture, not a chemical compound. The most important components 
for our study are, (a) oxygen, (b) nitrogen, (c) carbonic acid gas, 
(d) water vapour,,and (e) dust. Of these, oxygen and nitrogen make up 
the greater part, and the air is chiefly a mixture of these two gases, 
about 21 per cent oxygen and 79 per cent nitrogen (Fig. 424). In 1894 
argon, and, subsequently, several other inert new elements were dis- 
covered in the atmosphere. They are so much like nitrogen that the 
discovery of their presence does not change our views regarding the 
behaviour of the atmosphere in any essential way. 

Oxygen. — Oxygen is a necessary element in the atmosphere for 
man and all animals. Man could not live in an atmosphere of pure 
oxygen, and it is, therefore, important that the oxygen be diluted 
with the nitrogen, for otherwise the rapid changes we know as combus- 
tion, and with which we are familiar in connection with the burning 
of a fire, would cause rapid changes in the tissues of the body and make 
it impossible for men and animals to live. 

Nitrogen. — Nitrogen, in addition to its importance in diluting the 
oxygen of the air, is used by some plants. 

Carbon Dioxide. — Carbonic acid gas, or carbon dioxide, forms 
only 0.03 per cent of the air under ordinary conditions, but is exceed- 
ingly important. It is composed of one part of carbon and two of 



CHARACTERISTICS OF THE ATMOSPHERE 



713 



oxygen, and plants have the power of separating these two gases, build- 
ing the carbon into their tissues and releasing the oxygen to be breathed 
by men and animals. 

In the bodies of animals, oxygen unites with carbon by a process of 
slow combustion, so that, with every breath, we exhale a small quan- 



ALTITUDE 

km.o 10 20 




10 20 30 40 50 60 70 

VOLUME PER CENT. 



Fig. 424. — Proportions of the atmospheric gases at the earth's surface, and calculation of 
change with altitude. (Humphreys.) 



tity of carbon dioxide, which is a poisonous gas. In such a rapid form 
of combustion as fire, the oxygen combines rapidly with the carbon of 
the wood, or coal, or oil, and produces heat. This we use to form 
steam in locomotives or engines which run machinery. It is likewise 
true in the slow combustion within the bodies of men and animals, 



7 i4 COLLEGE PHYSIOGRAPHY 

that heat is formed, producing some of the energy which animals need 
for life. 

Water Vapour. — It is a familiar fact that water vapour is taken as an 
invisible gas from the surfaces of water bodies, so that a pool of water 
evaporates under the heat of the sun, and a dishful of water is con- 
verted into water vapour on the surface of a stove. Likewise the wet 
surfaces of sidewalks become dry when the air is moving over them, 
even if the sun is not shining, and wet clothes which are hung on a 
line become dry because of the evaporation of the water. This process 
of evaporation constantly introduces water vapour into the atmosphere, 
but the amount of water vapour varies from place to place, so that some 
places have very dry air, while others have damp or humid air. Like- 
wise, the amount of vapour differs in the same place from time to time, 
some days being dry, others humid. Evaporation in dry air is rapid 
and is usually accompanied by a clear sky, but when there is much 
vapour there may be clouds and rain. Such forms of water as dew, 
frost, fog, clouds, rain, snow, and hail (Chap. XXIV) are due to the 
condensation of the water vapour in the atmosphere. 

Dust. — The solid particles that float in the air are known as dust. 
Dust is introduced into the air (a) from chimneys and from forest fires, 
in the bits of carbon which we call smoke, (b) in small particles of pollen 
from plants, (c) in the dust that blows up over dry places, (d) in the fine 
particles which are thrown into the air from volcanoes, (e) in the salt 
from the oceans, and (f) the meteoric dust that comes from the burn- 
ing and disintegration of shooting stars. 

Around cities, dust particles are exceedingly abundant because of 
the large amount of smoke which rises from chimneys. Thus a dull, 
hazy atmosphere is exceedingly common near large centres of popula- 
tion. During periods of drought, the roads and fields in the country 
contribute a good deal of dry material which may float away in the 
air. Accordingly there are times when the air in the country becomes 
as hazy with dust as in the vicinity of the cities. The dust is washed 
from the air in rain storms, and on this account it is usually clearer 
after a rain. Upon high mountains and over the ocean the air is 
fairly free from dust particles. Although dust is everywhere present, 
and while it is invisible under ordinary circumstances, it may be seen 
clearly when a beam of light shines into a darkened room. There 
are, of course, quite as many dust motes floating in the air everywhere 
as are seen in the beam of sunlight, but they are not visible under or- 
dinary conditions. 

The dust in the atmosphere furnishes solid particles around which 
the water vapour condenses to form fog and rain, and gives us the 
colours of the sky, and the phenomenon of twilight. Microorganisms 
are also included under the general term, atmospheric " dust," and 
these are often related to the occurrence of disease. 

For references to literature on General Characteristics of the Atmosphere, 
see pp. 744-745- 



CHAPTER XXIII 

LIGHT AND WARMTH IN THE ATMOSPHERE 

Light in the Atmosphere 

The Nature of Light. — It is a familiar fact that the light of the 
earth is supplied by the sun and is transmitted through the 92,750,000 
miles from the earth to the sun at great speed, traversing this distance 
in about 8 minutes. Light is also emitted by other bodies having 
high temperature, for example, by burning coal and red-hot iron. 

Colours of the Rainbow. — Sunlight travels in a series of waves, 
which differ in length and colour, but whose union forms white light. 
When a beam of sunlight is passed through a glass prism, these light 
waves are turned, each at a slightly different angle. The sunbeam 
enters the prism as white light, but comes out of it with the colour 
waves separated, so that violet, indigo, blue, green, yellow, orange, 
and red may be recognized. These colours are known as the colours 
of the spectrum, or, because of the fact that the light waves are similarly 
separated in the drops of water of a rainbow, the colours of the 
rainbow. 

Refraction and Selective Scattering. — The bending of the rays 
of light is known as refraction. In their passage through the atmos- 
phere the waves of light are interfered with by the dust and water in 
the air, and colours are, therefore, produced. The dust in the air 
produces colours through selective scattering, the dust in the air inter- 
fering with the passage of light waves, as small pebbles in shallow water 
interfere with water waves. The dust thus causes some of the waves 
which make white light to be turned aside or scattered, and the waves 
having the shortest length, those in the violet end of the spectrum, 
are most easily turned aside. That is, they are selected for scattering. 

Because of this selective scattering of the short blue waves the sky 
has a blue colour. There is a great deal of dust in the air, however, 
and the more dust, the greater the loss of the blue, and hence the greater 
the predominance of the reds and yellows, giving the sky the red and 
yellow colours, as at sunrise and sunset when the rays of light pass 
through a great thickness of the lower, dust-filled layers of the air 
(Fig. 425). The varied colours of clouds at sunrise and sunset are 
mainly a result of the reflection of colours caused by refraction and 
selective scattering. 

Reflection. — The phenomenon of reflection of rays of light from a 
body is familiar in the reflected light from smooth surfaces, like water 

715 



716 



COLLEGE PHYSIOGRAPHY 



LATE AFTERNOON 



SUN'S RAY8 I N 
EARLY MORNING 




or the glass of a mirror ; but irregular surfaces like the ground also 
reflect light, and it is reflected sunlight which makes the moon appear 
to give light. The earth would have the same appearance if seen from 
the moon ; and some of the other planets have their light for the 

same reason. The stars, however, 
give light as the sun does, because 
they are hot. 

Mirage. — Among the changes in 
light as it is refracted and reflected 
in its passage through the atmos- 
phere, is the phenomenon of mirage. 
This is caused by refraction and re- 
flection when layers of air have differ- 
ent temperatures and consequently 
different densities. In deserts and 
on the sea, mirage and a related 
phenomenon called looming are espe- 
.. .. . , . ,, cially perfect, but it commonly shows 

Big. 425. — At sunset and sunrise the . . J " . ' . . * 

sun's rays pass through the dust-filled objects inverted, — for example, a 

vessel with the masts downward. 
The phenomenon of mirage is espe- 
cially deceptive in deserts, where, 
owing to the reflection of the blue of the sky, it sometimes gives the 
appearance of water and often leads travellers astray. 

Halos. — The drops of water in the rainbow cause refraction and 
reflection of the light which is passing through the raindrops. Similar 
changes in the light rays sometimes cause halos around the sun or 
moon, due to the refraction of the light which passes through the icy 
crystals of thin clouds high in the air. 

Colours due to Reflection and Absorption. — Reflection also causes 
colours of leaves, flowers, and other objects. For example, when 
light reaches white paper, all the waves are reflected and the paper 
appears white, but when light reaches black cloth, most of the rays are 
absorbed and very little light is reflected. Still other objects absorb 
some of the waves and reflect others, thus giving colour, as in a red 
flower which reflects an excess of red waves, or green leaves, which 
reflect an excess of green waves. 



air for the distances DA and BA in 
contrast with the small distance CA 
at noon. 



Supply of Warmth to the Atmosphere 

Radiant Energy. — The fire in a stove causes the iron of the stove 
to be warm, so that we feel its warmth at a distance of several feet. 
This is because waves of heat from the stove have passed that distance 
through the air. When the top of the stove is very hot, the iron becomes 
red because the waves produce, not only heat, but the sensation of 
light as well. This form of energy which we call heat and light is 
known as radiant energy, and the process of emitting it is called 



LIGHT AND WARMTH IN THE ATMOSPHERE 717 




r, 



</!>>\\ 



jIVvv 



Fig. 426. — The earth and the sun's rays, showing how 
small a portion are intercepted by this small planet. 



radiation. The sun is a great centre of radiant energy, but some of 
the stars may be even larger and hotter, although they do not influence 
the earth on account of being much farther away from it. The radiant 
energy from the sun which reaches the earth is called insolation. 

Radiation causes the loss of heat and bodies become cooler, as in 
the case of a stove which will radiate all its heat and become cold 
in a few hours after the 
fire is out. Although VvVtlli 

the sun has been radiat- 
ing its heat outward in 
all directions for millions 
of years, a very long 
time will be required for 
it to radiate all of its 
heat and become cold. 
Only a small proportion 
of the heat radiated 
outward by the sun is 
intercepted by the earth 
(Fig. 426), but this radiant energy has fundamentally important 
effects upon the earth's surface. 

Transparent and Diathermanous Substances. — Air, glass, and 
certain other substances allow light to pass so freely that they are called 
transparent. They also allow heat to pass freely and are, therefore, 
called diathermanous. Because the atmosphere is diathermanous, 
the sun's rays reach the earth's surface at midday with comparatively 
little loss. Dust particles interfere with the passage of the rays 
of heat as well as light, in the latter case causing the brilliant colours 

at sunrise and sunset, 
and, in the former, the 
cooler atmosphere when 
the sun is low and 
passes through a great 
thickness of dust-laden 
air near the horizon. 

Fig. 427- -The cross-sections of sun's rays AB and EF J^te m the afternoon, 
are equal, but at noon they are concentrated on the for example, we may 
width of ground CD while at sunset they are spread ac tuallv look at the set- 
over the greater width GH. . •> , 

ting sun, because many 
of the rays are intercepted by the particles of dust. Quite as im- 
portant as the presence of dust, however, is (a) the greater thickness 
of air traversed by the sun's rays (Fig. 425) and (b) the angle of inso- 
lation (Fig. 427), the rays spreading over a broader surface than at 
midday and, therefore, heating it less. Water vapour is also very 
important in absorbing sunlight. 

Heat from Direct Passage of Radiant Energy. — Very little heat is 
absorbed by the atmosphere during the direct passage of radiant 



SUN'S BAYS 

REACHING EARTH SUN'S RAYS REACHING EARTH IN 

AT NOON FROM AFTEROON WHEN SUN IS LOW IN 
NEARLY ABOVE HEAVENS. 




SURFACE OF THE EARTI 



7 i8 COLLEGE PHYSIOGRAPHY 

energy through it. This is demonstrated by the fact that, instead of 
being much warmer on the tops of high mountains, which are nearer 
the sun than the plains at their base, the atmosphere is usually cooler 
there. As a matter of fact, light and heat rays pass through the at- 
mosphere without heating it, except as these rays are interfered with 
by particles of dust and drops of water, which cause the absorption 
of heat rays. Consequently, the earth's surface would be cold if it 
depended for its heat upon the warming of the adjacent atmosphere 
as the radiant energy passes through it. The ether of space is ex- 
ceedingly cold because there is no dust or water vapour there to in- 
terfere with the passage of radiant energy through space. The heat 
which reaches the earth's surface, however, supplies warmth to the 
atmosphere by indirect means. 

Heat by Reflection and Absorption. — Water not only reflects 
light, but it also reflects a large percentage of the heat rays which 
reach its surface. This is the reason we often become sunburned 
when we are out in a boat. The streets of a city or the stone walls 
of a quarry are warmer than the open country, because the sun's rays 
are reflected from the pavements and walls and rocks. Some bodies, 
however, reflect little heat, and the sun's rays are mainly used in warm- 
ing them directly. Such bodies are said to absorb heat. This is 
especially true of black objects, and because white cloth reflects heat 
and black cloth absorbs it, it is cooler to wear white clothing than 
black or blue in the summer. Accordingly, the uniforms of United 
States navy officers, who are in service in the tropics, are made of 
white material ; and men go without their coats in summer in the 
temperate zones because, entirely aside from the weight of the coat, 
the reflecting white clothing is, in most cases, cooler than the ab- 
sorbing black clothing. This can be readily proved in winter by 
placing two pieces of cloth, one black, the other white, on a snow 
bank in the sun. The black cloth soon sinks into the snow, because 
the sun warms it, but the white cloth remains on the surface, because 
it reflects the heat rays and is not warmed. 

Heat by Radiation. — A coil of iron pipes containing steam or hot 
water is warmer than the air surrounding it in a room, and, therefore, 
radiates its heat out into the room and is known as a radiator. The 
earth radiates into space the heat that comes to it from the sun. In- 
deed, the earth would otherwise become warmer and warmer, instead 
of maintaining a fairly constant temperature. During the day more 
heat is absorbed than can be radiated, but at night radiation cools 
the ground. In summer when the days are longer than the nights, 
the ground grows warmer, and in winter when the opposite condition 
is true, radiation so far exceeds the supply of heat that the ground 
becomes cold. 

The rocks and soil of the earth radiate heat and hence" cool more 
quickly than water. They are said to be better radiators, and they 
are also more effective absorbers of heat than water is. Accordingly, 



LIGHT AND WARMTH IN THE ATMOSPHERE 719 

in winter the land becomes cooler than the sea, and on frosty nights 
those objects which radiate their heat most rapidly generally have 
the most frost. 

Heat by Conduction. — When a flatiron is placed on a stove, the 
handle of the flatiron very soon becomes so hot that it is unpleasant to 
pick it up. This heat has been conducted to the handle from the 
bottom of the flatiron, which is the only part in contact with the hot 
stove. In a similar way some of the sun's heat is conducted below 
the surface of the ground or water, and some of it into the air that 
rests upon them. Water, air, and ground, however, are not as good 
conductors as iron, and ground is so poor a conductor that below a 
depth of from 30 to 40 feet there is practically no difference in tem- 
perature from summer to winter. 

Heat by Convection. — The lower layers of water in a kettle are 
heated by conduction, since they are directly in contact with the hot 
metal. Cool water is heavier than warmer water, and the cool upper 
layers of water in the kettle, therefore, tend to sink and displace the 
warm lower layers, which are crowded up by the settling of the cooler 
layers from above. This is convection. If the water continues to 
warm, it will finally boil, but not until all of the water in the kettle 
has been heated by conduction and moved away by convection so 
that the cooler water may take its place. Similar convection takes 
place in a lake in the autumn, with the opposite result. The surface 
layers of water are gradually cooled in the autumn by radiation. 
These cool layers settle, and the warmer lower layers of water rise 
to the surface and are there cooled by radiation and then settle to 
the bottom to give place to the warmer water. Until all of the 
water in the lake has been cooled by radiation so that it has 
approximately the same temperature, it is impossible for the lake 
to freeze. It is for this reason that shallow ponds and bays of 
slight depth freeze before deep lakes are covered by ice. This also 
is convection. 

Similar convection occurs in the air. Near a lamp, for example, 
the air is warmed and becomes lighter and is pushed out of place by the 
settling' of the heavier surrounding air. This movement of heavier 
air crowds up warm air in the vicinity of a lamp or stove, and causes a 
draft in a fire. The crowding upward of the warm air causes an up- 
ward movement in the chimney. 

Heat from the sun is the cause of extensive convection upon all parts 
of the earth. The air is warmed in one place by radiation and con- 
duction of heat from the ground, or water, and is pushed out of place 
by the settling of the heavier cool air drawn down by gravity. In this 
way the air is set in motion and we have wind. When air rises its 
temperature may decrease notably without appreciable loss of heat 
through conduction or radiation, but wholly through expansion. 
This is adiabatic cooling. With compression of descending air a 
corresponding heating occurs. 



7 2o COLLEGE PHYSIOGRAPHY 

Measurement of Temperature 

Thermometers. — The measurement of temperature of the air is 
made with the thermometer. The commonest type of thermometer 
is a hollow, sealed glass stem or tube, of small calibre, with a bulb con- 
taining mercury at the bottom. The air in the tube has been removed 
before the tube is sealed, and the mercury is, therefore, free to rise and 
fall in the vacuum of the tube. In the thermometer we take advan- 
tage of the principle that mercury or alcohol expands and requires more 
space when warmed, and contracts and takes up less space when cooled. 
It would be possible to use many different liquids in the thermometer, 
but mercury or alcohol is commonly used, chiefly because it does not 
freeze at ordinary temperatures. Mercury is ordinarily used in the 
thermometers which are not to be exposed to cold greater than the 
temperature of about — 40 F., the freezing point of mercury. Al- 
cohol or other fluids, such as certain light oils, are used for thermom- 
eters which are to be exposed to lower temperatures. 

With the change of temperature, the mercury in the bulb expands or 
contracts and thus causes a tiny thread of mercury to rise and fall in the 
tube. The measurement of the temperature by the rise and fall of the 
liquid in the tube makes it necessary to have the tube graduated in 
degrees. 

Fahrenheit and Centigrade Scales. — There are several methods of 
division of thermometer tubes, the one most commonly used in America 
and England being the Fahrenheit scale (F. or Fahr.). In the Fahren- 
heit scale the boiling point of water is placed at 212 and its freezing 
point at 3 2 . A more simple scale of graduation is known as the 
Centigrade (C. or Cent.), which is most commonly used on the con- 
tinent of Europe. In this, the freezing point is placed at o°, and the 
boiling point at ioo°. To convert Centigrade to Fahrenheit at tem- 
peratures above freezing, multiply by i.8° and add 32 . For example, 
io° Cent. = 50 Fahr. (io° X i.8° = 18 + 32 = 50 ). The Fahr- 
enheit scale was perfected about 17 14 by Fahrenheit, and the Cen- 
tigrade scale 28 years later by Celsius and Linnaeus. Various other 
scales have been proposed. The Reaumur scale is based upon a 
freezing temperature at o° and the boiling temperature at 8o°, while 
the inverted scale of Celsius probably had o° for the boiling, point 
and ioo° for the freezing point. The latter is obsolete, and the Re- 
aumur is used only in Russia and parts of Germany. 

Metal Thermometers and Thermographs. — There are also metal 
thermometers, based upon the same principle of contraction and expan- 
sion with changes of temperature. Thermometers of this kind, made 
of metal strips connected with a hand that moves over a graduated 
dial, are often to be seen in front of city stores. 

. Metal thermometers are also used in connection with self-recording 
temperature records. They have an arm bearing a pen which is 
moved as the temperature changes. The pen is placed so that it 



LIGHT AND WARMTH IN THE ATMOSPHERE 721 

presses against a piece of paper on a cylinder which is revolved by 
clock work. With the daily and seasonal variations of temperature 
the pen rises and falls, while the paper on the cylinder revolves regularly, 
so that the pen draws a line recording the temperature continuously. 
These self-recording thermometers are called thermographs. 

Maximum and Minimum Thermometers. — Another type of ther- 
mometer is used for observing the extremes of heat and cold at times 
when the observer is away from his instruments or at places to which 
he is unable to go, as in the case of thermometers sent up in balloons 
or lowered beneath the sea with sounding apparatus. The maximum 
thermometer has a constriction in the tube just above the bulb. The 
thermometer rests horizontally rather than vertically. When the 
temperature rises, the expanded mercury will be pushed up through 
this constriction, but when the temperature falls there is no such force 
to push the mercury back through this constriction and the thermom- 
eter, therefore, records the maximum temperature which has been 
reached. Later the mercury is sent back into the bulb by whirling the 
thermometer rapidly about a pin provided for that purpose. 

In the minimum thermometer alcohol is used rather than mercury, 
and the tube contains a small piece of coloured glass known as the index. 
The surface tension at the top of the column of alcohol keeps the index 
in position and pulls it down when the temperature falls. When the 
temperature rises, the alcohol flows freely around the index, and, 
therefore, when the temperature increases and the alcohol flows back, 
the index remains in the lowest position which it has reached. Accord- 
ingly the position of the top of the index indicates the lowest tempera- 
ture which has occurred since the minimum thermometer was set. 
After making an observation the minimum thermometer is set again 
by lifting the bulb until the index slides back to the terminus of the 
alcohol column. 

Instrument Shelters. — In keeping accurate meteorological records 
it is necessary to take care to place the instruments where they are 
not influenced by local conditions. Thermometers, for example, 
give very different readings, depending upon whether they are in the 
shade or in the sun. The usual method is to use an instrument shelter 
with the sides made of slats, so that the air will circulate freely and the 
sun and rain will not reach the thermometer. It should be placed 
either on open ground or on a roof. 

Warming of the Land 

Effect of Absorption and Radiation. — During the day, the sun's 
heat causes the land to be warmed by absorption. The heat absorbed 
at the surface is conducted a few feet into the ground, although 
generally not much farther than the roots of plants reach. It is 
thought that the reason the ground nowhere becomes excessively 
warm by absorption is because so much of the heat is lost by rerlec- 
3 a 



722 COLLEGE PHYSIOGRAPHY 

tion, by radiation, and by conduction to the air. Everywhere, how- 
ever, the ground is warmed during the heat of a sunny day and may be 
far over ioo° F. It is then cooled off at night by radiation, especially 
if the sky be clear. 

In the tropical region, during the long hot days, radiation is unable 
to remove all the heat that is absorbed, and the ground does not become 
very cool at night, except in places like the trade wind deserts, where 
water sometimes freezes at night. In the temperate zones the ab- 
sorbed heat probably accumulates during the summer when radiation 
does not remove it all, but in winter the radiation during long nights 
removes so much of the heat from the earth that the ground freezes. 
In the polar regions the radiation during the long winter is so exten- 
sive that it causes the ground to freeze to depths of hundreds of feet, 
and the short cool summer results in the absorption of so little heat 
that the frost goes out of only the upper layers of the ground. In 
central Alaska near Fairbanks, for example, the frost extends to a depth 
of more than 175 feet and is present in summer and winter, while in 
the northern United States the frost in winter rarely extends more 
than 4 or 5 feet into the ground, and is entirely removed during the 
summer. 

There are still other reasons for minor local differences in the warm- 
ing of the lands. Dark-coloured surfaces absorb more heat and are, 
therefore, warmed more quickly than light-coloured soil and rocks. 
Bare earth is warmed more quickly than that covered by plants. 
Sunny, south-facing slopes will absorb more heat than shady, north 
slopes. Valley sides reflect heat and interfere with radiation from the 
valleys and with winds. They are, therefore, warmer than the adja- 
cent hilltops. 

Warming of the Water 

Reasons for Warming more Slowly than Land. — The reason that 
water warms less rapidly than the land is, first, because of reflection. 
The surface of the water, especially when calm, reflects more heat 
under bright sunshine than the land, so that there is less heat left 
to warm the water. Moreover, because of its circulation the movable 
water is set in motion when one part of it is warmed, so that the heat 
is distributed in a way that is impossible in the motionless land. 
Transparency of water also results in the transmission of heat below 
the surface. So some heat goes to warm the deeper layers, while in 
the land all of the heat which is absorbed is used to warm the upper 
layers which, therefore, become much warmer than the upper layers 
of water. Sunlight penetrates dimly to depths of several hundred 
feet in the water, and, although the water is not warmed appreciably 
at this depth, the opaque land, into which the sunlight cannot pene- 
trate, is never warmed to a depth of more than a few feet by the 
absorption of heat. Further, much of the heat is expended in evaporat- 
ing the water. This is called latent heat or heat of vaporization. Lastly, 



LIGHT AND WARMTH IN THE ATMOSPHERE 723 

it requires twice as much heat to raise the temperature of water i° as 
it does to raise the temperature of land an equal amount. 

Reasons for Cooling more Slowly than Land. — On these accounts 
even the small bodies of water, such as ponds and lakes, warm more 
slowly during the day and during the summer than the adjacent land. 
They likewise radiate their heat more slowly at night and in winter 
than the adjacent land, because water is such a poor radiator that it 
cools more slowly than soil and rock. There is, therefore, a smaller 
range of temperature from day to night and from summer to winter in 
large bodies of water, and the climate over them and at their borders 
is characterized by less extremes of heat and cold than the climate over 
the land. 

Warming of the Air 

Effect of Radiation, Conduction, and Convection. — As already in- 
dicated (p. 717), some of the sun's rays are intercepted in their passage 
through the atmosphere from the sun, and some of the heat rays radi- 
ated from the earth are, likewise, intercepted by the dust in the air. 
The air is, therefore, not perfectly diathermanous. In addition to 
this direct heating of the air by the passage of radiant energy is the 
warming by conduction from the ground to the lower layers of the 
atmosphere. Radiation is even more effective in warming the air than 
conduction, which acts slowly, over short distances. These warmed 
lower layers of air are lighter and are, therefore, displaced by the set- 
tling of cooler, heavier, upper layers, so that the higher portions of the 
atmosphere are heated chiefly by convection. 

Thus the atmosphere is seen to be warmed by radiation, by conduc- 
tion, and by convection, just as the stove warms the air in a room in 
these three ways. At night and in winter the air is cooled by radia- 
tion and also cooled by contact with the ground. Radiation is inter- 
fered with by vapour and dust in the air, so that more heat is retained 
in the lower atmosphere on hazy and muggy days than in clear, dry 
weather. It is partly because of the fact that radiation fails to cool 
the ground that a hot, damp day may be followed by an oppressively 
warm night. Most of our unpleasantly warm summer weather comes 
in connection with just this sort of interference with radiation. 

Distribution of Temperature over the Earth 

Isotherms. — The distribution of temperatures upon the earth's 
surface is usually represented by lines known as isotherms. An 
isotherm may be defined as a line connecting places which have the 
same temperature. An isothermal chart is a map showing the tempera- 
ture of a given area such as a state, the United States, or the world, 
for a given period or for a moment of time. Isothermal charts may be 
drawn to represent mean temperatures for the year or for a part of the 
year, or for a moment of time. For example, an isothermal chart of 



724 



COLLEGE PHYSIOGRAPHY 



the world for January (Fig. 428) has isotherms passing through all 
places whose average temperature for the month of January is the same, 
and it will differ decidedly from an isothermal chart for the year, where 




Fig. 428. — Isothermal charts of the world for January (upper map) and July (lower map). 

the lines pass through places having the same average annual tem- 
perature. 



LIGHT AND WARMTH IN THE ATMOSPHERE 725 

The Zones. — The distribution of temperature from place to place 
on the surface of the earth is not simply a matter of heating of the land 
and water as a result of absorption, conduction, convection, and radia- 
tion, but also has to do with the distribution of heat in relation to 
(a) winds, (b) ocean currents, (c) position of the sun (Fig. 429), and (d) 
altitude. If the earth were heated by the sun's rays with relation to its 
spherical form, but with none of the four complications listed above, 
we should have three simple results : (1) all places between the 
equator and the tropics would be warmest, because they would receive 
the vertical rays of the sun ; (2) all places between the tropics and the 
Arctic and Antarctic circles would be intermediate in warmth, be- 
cause they would receive the rays of the sun at some time during each 




90 %\ 



Dec. 
Fig. 429. — Davis's diagram to show the variation in insolation with latitude and with 

the season. 



day in the year, while (3) the regions between the poles and the 
Arctic and Antarctic circles would be coldest, because the poles would 
have six months of sunlight and six months of darkness, while the areas 
within 23J of the poles would have a variable number of days during 
the year in which the sun's rays did not reach the earth's surface at all. 
It is usual in grammar school geography to have a map of the zones 
with one equatorial or torrid, two temperate, and two polar or frigid 
zones (Fig. 481). The boundaries between these zones are the 
Arctic and Antarctic Circles, and the Tropics of Cancer and Capricorn. 
The actual temperature within these zones, as we may now study it in 
college geography, differs very much because of the distribution of heat 
in accordance with the four features listed above. 



726 



COLLEGE PHYSIOGRAPHY 






:SOUTH P.OLE. 



Fig. 430. — The sun's rays always reach the 
poles at an angle, therefore passing through a 
greater thickness of air, slanting over a broader 
area, and heating the earth's surface less than 
in the temperate and tropical regions. 



Effect of Winds. — The effect of distribution of heat by winds is to 
carry the temperature of warm lands to the cooler ocean in some places 
where the wind blows from the land to the sea, and to carry the tem- 
perature of the temperate zone 
into the polar region, or vice 
versa. A city on the sea coast, 
for example, might have a lower 
temperature than a city some 
distance inland, if the wind were 
from the ocean in summer at a 
time when the sea was cooler 
than the land. The lower tem- 
perature of the coastal city 
would then be due to the influ- 
ence of the wind in carrying the 
temperature of the cool ocean 
inland to the warm land. The 
opposite might be true at an- 
other season. At times, when a 
breeze is blowing from the warm land to the cooler sea, the temperature 
upon a vessel at anchor in a harbour is warmer than would be the case 
if there were a calm, because the higher temperature of the land is 
being carried out upon the water. 

T „ ° . . ,, Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan. 

Likewise in the temperate zone 
of the northern hemisphere, a 
north wind is likely to be cool, 
or even very cold in winter, be- 
cause it carries southward the 
temperature of the land at a lati- 
tude which is cooler because it 
receives the sun's rays at a lower 
angle and because the nights are 
longer. 

Effect of Ocean Currents. — 
The ocean currents also distrib- 
ute the temperature from place 
to place ; for example, Iceland, 
which is on the Arctic Circle, has 
a temperature similar to that of 
New England and Newfoundland 
because the warmer water of the 
temperate zone is carried north- 
ward to the latitude of the Arctic 
Circle. In a similar way a cold 

ocean current from Greenland and Labrador bathes the 
the Maritime Provinces of eastern Canada and gives it 













Pole 
























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33* 


\ 














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7. 


Eq 


uato 


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l2 










4„« 


.-- 


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A. 

6 


x 


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v 


.. 




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j 


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b. 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan. 



Fig. 431. — The annual variation in insolation 
at the upper limit of the atmosphere with 
latitude (solid lines), 1 at the equator, 2 at 
latitude 45°, 3 at the north pole. Curves 
4, 5, and 6 (dotted lines) show the value of 
insolation at the earth's surface, in the same 
three regions, after passage of the heat 
through the atmosphere. (Angot.) 



coast of 

a lower 

temperature than it would receive from the direct heat of the sun 



LIGHT AND WARMTH IN THE ATMOSPHERE 727 

in this latitude. The cold current from the Antarctic Ocean has a 
similar effect upon the coast of Chile in South America. 

Effect of Position of the Sunl — The heating power of the sun is 
greater when it is high in the heavens at noon than in early morning 
or late afternoon. It is less in winter than in summer and less in the 
temperate than in the tropical zones. The reason why the sun's 
heating power is less when it is low in the heavens is because (1) the 
heat rays pass through a greater thickness of dust-laden air when the 




Fig. 432. — Isotherms in the north polar region for February, 1878-1887. 

sun is low (Figs. 427 and 430), and (2) fewer rays reach and heat a 
given surface. Accordingly, we have three results of variable positions 
of the sun: (1) the amount of heat given by the sun's rays varies 
each day as the angle at which the sun's rays pass through the air is 
changed ; (2) the seasons of summer and winter occur in both hemi- 
spheres as the sun is first high and then low in the heavens ; (3) the 
climate is hottest in the tropical zone, where the sun is vertical at 
some point every day in the year, and is cooler between the tropics 



728 COLLEGE PHYSIOGRAPHY 

and the Arctic and Antarctic circles, where the sun's rays are never 
vertical and are inclined at lower and lower angles as the North and 
South poles are approached (Figs. 429, 431, 432). 

Effect of Altitude. — There is an average decrease in temperature 
at the rate of about i° F. for every 300 feet of ascent, as we know from 
observations upon high plateaus and mountains and the records of 
thermometers which have been carried up in balloons. This is 
because there is less warm ground to radiate heat into the upper layers 
of the atmosphere, and the warm air, which carries some heat to the 
upper layers by convection, expands and cools as it rises. It there- 
fore supplies less and less heat as the altitude increases. Accordingly, 
at the equator a mountain 15,000 feet, or three miles high, has a frigid 
climate because of the coolness of the upper air and in spite of the fact 
that the sun's rays are never very far from the vertical. The smaller 
conduction to the air from the limited summit area of a mountain top 
also results in less heating of the air near mountains. It is also a 
matter of common observation that highlands are sometimes cooler 
than neighbouring plains. The fact of the cooling of expanding air may 
be observed by the use of a pump in inflating a bicycle or automobile 
tire, for the air pumped into the tire is compressed, or made more 
dense, and is, therefore, warmed. If the cap is taken off the tire, and 
the finger held in the air which rushes out, it may be noted that this 
air is cooler. Its coolness is due to the fact that it expands as it 
escapes. 

In spite of the fact that cool air surrounds the tops of mountains 
and high plateaus, they may become quite warm at noon and in the 
early afternoon as a result of exposure to the direct rays of the sun. 
Persons ascending mountains often observe that they are very warm, 
if sitting in a protected sunny place, but if they sit down only a few 
feet away in a shady spot or where the wind is blowing^ they feel very 
cold. As soon as the sun ceases to shine, however, radiation goes on 
so rapidly in the clear, thin, upper layers of the air, that even the 
warm places quickly cool off. The temperature on a highland may be 
as much as 90 at midday and as low as io° at night. This is the basis 
of the fact that some of the people dwelling upon the plateau of 
Mexico habitually wear a blanket, the sarape, because the altitude of 
the plateau is such that if the sun goes behind a cloud the air is so 
cool that it is necessary to wrap the sarape around the head or 
throat to avoid catching cold. 

The Stratosphere or Isothermal Layer. — The decrease in tempera- 
ture with increase in altitude was formerly thought to continue upward 
more or less indefinitely. About 1901, however, it was discovered 
that at an altitude of over 6 miles in the north temperate zone the 
temperature of the atmosphere becomes stationary and even increases 
slightly. This warmer portion of the upper air (Fig. 433) is called 
the stratosphere, but the name isothermal layer has also been applied to 
it. Its upper limit is unknown. Its lower limit increases in altitude 



LIGHT AND WARMTH IN THE ATMOSPHERE 729 

toward the equator, at least in the northern hemisphere. It may be 
as low as 23,000 feet in the Arctic region, 35,000 to 40,000 feet in 
England and central Europe, 43,000 feet near St. Louis in United 
States, and something in excess of 50,000 feet over the Atlantic Ocean 



KILOMETERS 
18 

17 


i CENTIGRADE 

s % ISOTHERMAL 

-ssk R 


IS 


;;;fc aove.ctive 

«| ZONE 

-55 h 


13 


IS 


-53 \ c Ce/7/ng of connective zone 


to 

9 


•~ . . V\ _..-,■ (7 mff&s ohroMie s»jale-v»/.) 

.^SoNj^yT UPPE R CO N G EjATLE D ^VA PO R; 

" -4tS^ layer / 


a 


" J7 C\o N V E C T 1 V E 


6 


-^Z ONE 


5 


drv ,5 \middle layer 


3 


LOWER 5 \ -- " V^^Y '^"'-^ 


£ 
1 

sea 


level '^^^s^' / ' ^^^^o 



Fig. 433. — The temperature gradient near the earth's surface and in the isothermal layer, 
from explorations with sounding balloons and kites. (Hobbs.) 



in the tropics. Its average summer temperature ( — 6o° F.) is warmer 
than the winter temperature ( — 7 1 ° F.) . Its cause is not yet well under- 
stood. 

Within this upper portion of the atmosphere convection does not 
go on freely as in the lower air. Practically all the clouds are within 
the convective zone. 

Other Causes for Variations in Temperature. — There are minor 
influences upon the distribution of temperature: for example, (a) ac- 
cording to the situation, as in the case of exposure to the wind,(&) in 
accordance with the nature of the rock, which results in greater heating 
of dark-coloured rock, and (c) as a result of the influence of water 
bodies. 



73o 



COLLEGE PHYSIOGRAPHY 



16* 20*? 



Daily and Seasonal Temperature Changes 

The Normal Daily Range. — As may be seen from Fig. 434, the 
warmest part of the day is not at noon when the sun is highest in the 
heavens, but at about 3 o'clock in the afternoon. This is because 
the heating of the ground in the morning was delayed because of the 
necessity of warming what had been cooled off by radiation the night 
before. After the ground is warmed there is a continued rise in tem- 
perature until the sun is so low in the heavens that radiation goes on 
at a rapid enough rate to exceed ^ 
the heating of the ground. Ac- 
cordingly, the ground and air 
commence to cool two or three 
hours after the sun's rays are 
vertical, and continue to do so 
until sunrise. This results in the 
coldest period being just before 
sunrise rather than at midnight. 

A number of conditions fre- 
quently interfere with the normal 
daily range, as, for example, a 
cloudy sky, which prevents the 
temperature from rising because 







A.M. 










P.M.. 








M 2468 10 12 2468 10 M 


























26' 
25 : 
24" 
23' 
22 
!?1 
N 20 
19 
I 8 
17 






































/ 
























/ 






















/ 
























/ 














V^ 


























































































































ITHACA 

Fig. 434. — The normal daily range 
temperature in eastern United States. 



of 



NOON 16? 20? 

Eiffel Tower 

Fig. 435. — Daily range of temperature in 
winter and in summer, at Paris (solid line) 
and Eiffel Tower (dotted line), showing the 
influence of altitude. (Angot.) 



the clouds interfere with the passage of the sun's rays, or the blowing 
of a cold or warm wind, which may cause the temperature to fall 
during the noon hours, or to rise during the night (Fig. 500). 

The amount of temperature change from day to day differs from 
time to time and from place to place. Thus when cool nights follow 
warm days, the range exceeds that when cool nights are followed by cool 
days. In winter, the daily range is generally smaller than in summer, 
in parts of the temperate zones it is less than at the equator, and at sea 
it is less than on the land. The normal curve of daily range of tem- 
perature is shown in Fig. 435. Figure 436, showing the change in tem- 
perature for six successive summer days, illustrates departures from 
the normal curve. Likewise, Fig. 437 illustrates the variation in daily 



LIGHT AND WARMTH IN THE ATMOSPHERE 731 

range for selected stations from the Arctic region to the temperate 
zone and near the equator. 

The Seasonal Range. — The normal curve of seasonal range is 
similar to the normal curve of daily range. A record of average 



iod D 

95° 
90° 
85° 
80° 
75° 
70° 
65° 
60° 
55° 


NOON 


NOON 


NOON 


NOON 


NOON 


NOON 


100° 

95° 

90 b 

85° 

80° 

75° 

70° 

65° 

60° 

55° 












7] 

























L -/ 










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-V 


--/--V 


/- 




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U- 




ZZT" 


x 


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— — 





/- 










































































Fig. 436. — Record of the changes in temperature at Ithaca, N.Y., for six successive days. 



temperature from day to day based on many years of observation 
shows that in the northern hemisphere there is a steady increase in 
temperature from January to July or August and a gradual decrease 

from July or August to January 
(Fig. 438). The middle of the sum- 
mer is June 21, but, in the long run, 
the warmest month of the year is 
July, because, as in the case of the 
daily range, the ground radiates so 
much of its heat during the winter 
that it must be warmed before the 
temperature begins to rise, and the 



M 3 A.M. 6 9 NOON 3 6 9 P.M. 


too 

90 

70 

50 
40 
30 
20 



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-30 


































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s v 




s s 2 


,^ 


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"-" 


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Fig. 437. — The daily range of tempera- 
ture of the normal sort for winter 
(dashed lines) and summer (continuous 
lines). (1) Arctic; (2) St. Vincent, 
Minn.; (3) Djarling, India; (4) Jaco- 
babad, India; (5) Key West, Fla. ; 
(6) Galle, India ; 5 and 6 are near the 
warm ocean. 




Fig. 438. — The seasonal range of temperature. 
(1) St. Vincent, Minn.; (2) New York State; 
(3) Yuma, Ariz. ; (4) Key West, Fla. ; (5) 
Galle, India. 4 and 5 are near the equable 
ocean. 



73 2 COLLEGE PHYSIOGRAPHY 

maximum is reached after the direct rays of the sun cease on June 2 1 
and before radiation exceeds heating in amount. The middle of the 
winter is in December, but the coldest part of the winter is in Jan- 
uary, somewhat as the coldest part of a day is later than midnight. 

Figure 438 shows the curves of seasonal range in various parts of the 
world such as Minnesota, Arizona, New York, Florida, and India. 
It will be noted that the range of temperature in Minnesota is very 
much greater than in India. Although the December and January 
temperature at the equator is high in contrast with the low tempera- 
ture in the north temperate zone, the seasonal range is less than in the 
higher latitudes. Moreover, India is a peninsula projecting far into 
the ocean, and the range of temperature over the equable ocean is far 
less than over the land. In the southern hemisphere the coldest part 
of the year is the summer of the northern hemisphere. There are 
similar differences in seasonal range of temperature as a result of 
altitude, deserts, and various other factors. 

For references to literature on Light and Warmth in the Atmosphere, see 
pp. 744-745- 



CHAPTER XXIV 
RAIN AND OTHER FORMS OF WATER 

Water Vapour and Humidity 

The Source of Vapour. — The water vapour in the air is chiefly sup- 
plied by evaporation from the surface of the ocean, rivers, and lakes, 
and from damp surfaces. The water vapour is everywhere diffused 
through the air, and even desert regions like the Sahara have some 
water vapour, or humidity. 

Absolute Humidity. — The actual amount of water vapour in the 
atmosphere is spoken of as the absolute humidity; and this refers to 
the amount of water vapour, expressed in grains per cubic foot of air. 
The term absolute humidity is also applied to the vapour pressure, 
expressed in inches or parts of inches of the mercury column. When 
air contains as much water vapour as it can possibly hold, the air is 
said to be saturated. A room 10 by 15 by 15 feet at a temperature of 
70 F. contains about 2.6 pounds (avoirdupois) of water in the form 
of invisible water vapour, when the air is saturated. Two and six- 
tenths pounds, therefore, represents the absolute humidity of a room 
of that size. 

Relative Humidity. — The term relative humidity is used to repre- 
sent the proportion of water vapour in the air in relation to the maxi- 
mum amount which the air can contain at a given temperature. This 
is measured, not in grains, but in percentages. The relative humidity 
of completely saturated air is 100 per cent. Absolutely dry air would 
have o per cent relative humidity, and air containing only half as much 
moisture as is possible would have a relative humidity of 50 per cent. 
It is always necessary to state in the definition of relative humidity 
that this represents the proportion of water vapour in the air at a 
given temperature. The room referred to above, which contains 2.6 
pounds of water, when the air is saturated at a temperature of 70 F., 
could contain much less water at a temperature of 6o° F. 

Relation to Evaporation. — In deserts, where the relative humidity 
is likely to be small, the air is so dry that evaporation goes on rapidly. 
In the tropical forest, on the other hand, the relative humidity is great, 
there can be only a little evaporation and, therefore, surfaces are likely 
to remain damp. The same conditions apply to the temperate zones, 
as in the regions of moderate rainfall in United States and Europe. 
In summer this lack of evaporation affects our comfort, because some 
days are humid or muggy, and at such times the heat is oppressive. 

733 



734 COLLEGE PHYSIOGRAPHY 

We perspire easily and are very uncomfortable because little evapora- 
tion can take place from the surface of the body when the air is humid. 
Evaporation from the skin cools us because some of the heat needed to 
change the perspiration into water vapour comes from the surface of the 
body. On clear, dry days we feel more comfortable because evaporation 
from the skin removes the perspiration, the percentage of relative hu- 
midity being so low that the air can readily evaporate a great quantity of 
water vapour. It is because of this that the temperatures of 90 to ioo° 
or more in Arizona are not accompanied by as uncomfortable condi- 
tions as we experience under equal temperatures in the Mississippi 
valley or near the Atlantic coast, where the relative humidity of the 
air is greater. 

Measurement of Humidity and Evaporation 

The Hygrometer. — Several types of instruments are used for 
determining the humidity of the air. Among these is the hair hygrom- 
eter. This consists of a bundle of hairs from which the oil has been 
extracted. The hairs absorb the water vapour in the air and change in 
length with the changes in amount of absorbed vapour. This property 
is frequently observed by people whose hair becomes straight in damp 
weather. The hair hygrometer has a hand on a graduated scale, 
moving in one direction if the humidity is high and in the other if it is 
low. 

The Psychrometer, or Wet and Dry Bulb Thermometers. — Another 
instrument for the measurement of water vapour is the sling psychrom- 
eter. This consists of two thermometers attached to a wooden or 
metal back. One of the thermometers has a piece of wet cloth 
around the bulb. The sling psychrometer takes advantage of the 
principle (1) that evaporation is more rapid in dry than in humid 
air, (2) that evaporation lowers the temperature. The method of 
using the sling psychrometer is to whirl the thermometers around for 
a minute or two so that the thermometers may come into contact with 
a large body of air. If the air is saturated, there will be no evaporation 
from the wet muslin, and the two thermometers will read the same, 
indicating a relative humidity of 100 per cent. If, however, the air is 
dry, the wet bulb thermometer will register a slightly lower tempera- 
ture. The relative humidity of the air can be calculated from tables 
which show all common differences in temperature between wet and 
dry bulb thermometers and the corresponding variations of water 
vapour. Such tables may be obtained from the United States Weather 
Bureau. 

The Evaporating Pan. — The commonest method of determining 
the rate of evaporation is with an evaporating pan. This consists of a 
dish of water in which is placed a ruler, graduated in inches and tenths 
of inches. Since evaporation varies from day to day and from place 
to place, this device makes it possible to tell how much water in the 



RAIN AND OTHER FORMS OF WATER 



735 



form of vapour is taken from the evaporating pan in a given time. It 
is of course necessary to prevent the rain from falling into the pan, or to 
allow for the rainfall, and to keep the pan freely open to the air. 

Precipitation of Moisture in the Air 

Relation to Increased Temperature. — It was stated above that 
the absolute humidity of the air depends upon the temperature. If 
saturated air, with its ioo per cent relative humidity, is warmed, it 
ceases to be saturated, because its capacity for moisture is increased. 
Accordingly, its relative humidity falls, and increased evaporation may 
take place. The desert region of the Sahara shows this, for the winds 
there are blowing toward a warmer region and consequently their 



MONDAY 


TUESDAY 


WEDNESDAY 


THURSDAY 


FRIDAY 


SATURDAY 


SUNDAY 


6 XII 6 


6 XII 6 


6 XII 6 


6 XII 6 


6 XII G 


6 XII 6 


6 XII G 
































/ 


1 


/ 


h 






f\ 


r 


Wr 


/ 


1 


( 


\ 


( 




/ 


I 


/ 




J* 


i 




V 


\ 


J 


V 


\ 


\ 


V 






V 


J 


I 


/ 








v«r 




V 












N/ 




J 















































AUG. 14 15 16 17 18 19 AUG..2.0, 1,893, 

Fig. 439. — The changes in relative humidity with temperature, from a week's record at 
Ithaca, N.Y. The relative humidity is low, 30 to 60 per cent, at or soon after noon 
(XII) ; it is nearest 100 per cent at night when the air is cooled. 

relative humidity is being lowered. This makes the air so dry that 
the ground is dried and a desert is produced. 

Relation to Decreased Temperature. — The opposite condition is 
found where damp air is cooled so that its relative humidity increases 
to a point when the air becomes saturated. The amount of water 
vapour in a room with a temperature of 6o° may represent a relative 
humidity of only 80 per cent. If, however, the air is cooled from 6o° 
to 40 , the relative humidity of the room may increase to 100 per cent 
without the introduction of any more water vapour, because the capac- 
ity of the air for moisture is decreased by cooling. After the relative 
humidity reaches 100 per cent, and the air is saturated, any further 
cooling forces some of the water vapour to condense into liquid water, if 
the temperature is above freezing, or into snow or ice if the tempera- 
ture is below freezing. This is known as precipitation. 

Illustrations of Precipitation. — A person who wears glasses observes 
this phenomenon on cool days, for in walking out of doors the tempera- 
ture of the glass of his spectacles is low, but the cool glass has no effect 
upon the cool air out of doors. As soon as he enters a warm building, 
however, the cool glass decreases the temperature of the warm air 
with which it comes in contact, and at the same time decreases its 



736 COLLEGE PHYSIOGRAPHY 

capacity for moisture. If the relative humidity of the warm building 
is 40 per cent, the air immediately in contact with the glasses may have 
its relative humidity increased to 100 per cent by cooling, and a little 
water vapour may be condensed, precipitated upon the glasses, causing 
them, as we say, to " steam." The same thing is shown by breathing 
against a cool window-pane. The breath is cooled to the point of 
saturation, and some of the vapour is condensed upon the glass of the 
window. Similarly, a glass of water ' ' sweats ' ' in warm, humid weather 
because the cool glass reduces the temperature of the air near it and 
raises its relative humidity to 100 per cent. As this is the point of 
saturation, some of the vapour must condense, forming drops of water 
on the outside of the glass. The point of saturation is often called 
dew point because dew is formed when this point is reached. When- 
ever air is chilled to the dew point, condensation takes place (Fig. 
439)- 

Forms of Water 

Formation of Dew. — When the ground is cooled by radiation, as it 
usually is at night, the lower layers of the air are chilled by contact 
with the cool ground. If the relative humidity of the air is fairly high, 
that is, if the air is damp, some of its water vapour will be condensed 
upon the ground as dew. It is rarely the case that the air is suffi- 
ciently humid so that dew forms before sunset. There are three condi- 
tions which check the formation of dew : first, exceedingly dry air ; sec- 
ond, the movement of the air, so that the cool air is moved away before it 
reaches the dew point; and third, the checking of radiation by clouds. 

The reason that dew forms so commonly on grass is that vegetation 
radiates its heat rapidly and, hence, cools early in the evening. A 
further factor, however, is the rise or transpiration of water from plants; 
there is sometimes also a slight supply of vapour from the ground. This 
water from the surface of leaves and grass and from the ground is 
removed during the day by evaporation, and it is only during the even- 
ing or night, when the cooling of the ground is sufficient to saturate the 
air, that evaporation is checked and small drops of water gather as 
dew on the surfaces of leaves and grass. 

Formation of Frost. — It is a familiar observation that fantastic 
crystal forms often appear on a window-pane on cool nights in winter. 
This is commonly known as frost. Frost is formed upon leaves and 
plants and in general upon the earth's surface at low temperatures. 
Frost is a solid form of water which is made by the condensation of 
water vapour at temperatures below freezing. It is not always frozen 
dew, but may result from the direct freezing of water vapour at tempera- 
tures below 32 F. Usually it is a solid form of water vapour due to 
direct precipitation with the temperature below 32 F. Frost is also 
sometimes formed in favourable localities when the general tempera- 
ture of a region is above freezing, and this is because either (a) the air 
is damper above low, swampy ground, or (b) cooled air settles down 



RAIN AND OTHER FORMS OF WATER 



737 



into the valleys, and frost forms there when there is none on the adja- 
cent hills. Air also cools by radiation and conduction as it slowly 
descends the cool slopes. 

The growing season for plants is commonly determined by the num- 
ber of days between the last severe frost in the spring and the first 
severe frost in the fall. These are spoken of as killing frosts, because 
they stop the growth of plants or even kill them. Late spring frosts 
often do great damage to buds, and early frosts in autumn may destroy 
fruit that is not yet ripe. They are apt to come during nights when the 
air is so clear that radiation is exceedingly rapid. Sometimes in the 
spring plants are killed after having budded or leaved out, and, while 



PERCENTAGES OF FOG 
60 to 65 X.. 
50 to 60 °4_ 
40 to 50 
30 to 40 V 
20 to 30 % 
10 to 20 X 




Fig 



440. — Map to show percentages of foggy weather on the Grand Banks of Newfound- 
land in June, 1913, in relation to the Labrador Current and Gulf Stream. Small arrows 
indicate ocean currents. Feathered arrows terminating in circles indicate winds : (o) 
the length of the shafts being proportional to the number of hours in 100 with wind in 
a given direction ; (b) the number of feathers showing the force of the wind ; and (c) 
the percentage of calms, light and variable winds being shown by the figure inside the 
circle. (From Meteorological Chart of the North Atlantic Ocean, U. S. Weather 
Bureau.) 



they may recover after a light frost, a heavy frost usually results in 
there being nothing left to grow the next season except the seeds, 
bulbs, or roots. 

A well-known phenomenon in connection with frosts in the autumn 
is the change in the colour of the leaves. The beautiful red and yel- 
low colours of autumn foliage are due to frost. Later this kills the 
leaves and causes them to fall off most of the trees. During the cold 
winter season in the temperate zone, the trees and shrubs remain 
dormant and do not burst out into new life until, with the return of 
warmth in the spring, frosts are no longer common. 

Formation of Fog. — If we breathe out into cool air, the breath 
becomes visible. What really takes place, however, is the condensa- 
tion of the water vapour of the breath into tiny particles of liquid 

3 B 



738 COLLEGE PHYSIOGRAPHY 

water of such minute size that they float and form a fog. Damp air is 
chilled in other ways besides being breathed out from the lungs, how- 
ever. For example, fog is formed at night when the air over damp 
plains is chilled to the point of saturation. There is frequently fog 
because of the mixing of two currents of air, one of which is cool and 
the other warm and damp. This is the most common cause of sea fog. 

Two places in the world which are famous for dense fogs are the 
Grand Banks of Newfoundland, and the vicinity of London. In the 
former locality (Fig. 440), on the path of trans-Atlantic steamers be- 
tween the north of Europe and New York, the warm Gulf Stream and 
the cool Labrador Current are near together. The warm, damp air 
moving across the cool Labrador Current from the Gulf Stream is 
chilled so that its relative humidity is increased to 100 per cent and 
water vapour is condensed into particles of fog. The same thing occurs, 
though less commonly, when cool air from the Labrador Current moves 
into the region of warm air over the Gulf Stream. Consequently, 
this part of the ocean is nearly always foggy. Vessels going through 
this fog sometimes collide. Large ocean steamers are likely to run down 
the small fishing schooners which frequent the Grand Banks, fishing 
for the abundant cod and halibut and other fish there. Indeed, fog is 
one of the most dangerous features on the sea, in spite of the fact that 
cautious captains reduce their speed and blow fog horns to warn other 
vessels of their approach. Even then there is sometimes disaster, as 
in the wreck of the Empress of Ireland, during a fog on the lower St. 
Lawrence on May 29, 1914, when over 1000 people were drowned. 
Ships are also in danger of running aground in a fog. In entering 
harbours ships frequently have to stop and anchor because of the 
dense fog. 

Another occurrence of dense fog is due to a different cause. In the 
vicinity of a large city, as in London, dust particles aid in the forma- 
tion of fog by supplying solids upon which the water vapour may be 
condensed. It is thought that the large amount of dust, chiefly smoke 
particles, in the vicinity of London helps to produce the dense fogs 
there, though radiation would cause some fog near London, even if 
there were no smoke there. Fogginess has increased, however, with 
the growth of London and the increase in production of smoke. The 
fog is frequently so dense as to stop all traffic upon the streets, and not 
infrequently results in the closing of the stores in the city during the 
day. 

Formation of Clouds. — Clouds are of much the same nature as fog, 
and, indeed, the lower clouds are fog. The higher clouds, however, 
caused by the condensation of vapour with a temperature far below 
3 2 F. are composed of particles of snow and ice. During the summer 
many clouds are caused by the rise of damp, warm air to such altitudes 
that the air is cooled and the relative humidity increased to the dew 
point. The formation of clouds is chiefly due to the cooling which 
accompanied the expansion of the rising air. Another cause of cloud 



RAIN AND OTHER FORMS OF WATER 



739 



formation is the blowing of damp air over cold surfaces, such as the 
top of a mountain (Fig. 441 ) . Another cause is the coming in contact of 
a warm and a cold current of air, one above the other ; and clouds of 
this sort are common on days when the warm air is also very damp. 

Types of Clouds. — The forms of clouds are varied, and, due to 
accidental relationships of air currents, they are sometimes fantastic, 
sometimes beautiful. The clouds which overspread the sky with an 
appearance of layers of strata are called stratus clouds. They are 
usually not very high above the earth, and often come so low as to lie 
upon the tops of the hills (Fig. 442). 

On warm summer days the clouds formed by the rising and cooling 
of damp air assume a different shape and are called cumulus clouds. 
At an elevation of several thousand feet above the surface of the earth 




Fig. 441. 



Clouds in process of formation on the western slope of the St. Elias Range in 
Alaska and over the ice plateau of the Malaspina Glacier. 



the vapour in the rising air begins to condense. Thus the cumulus 
clouds are apt to have a fiat base. Above this may rise a series of 
domes and billows, sometimes a mile in height and often very beautiful, 
especially when lighted and coloured at sunset. On hot summer after- 
noons cumulus clouds often develop into what we call thunder-heads. 
They are then called cumulo-nimbus. 

A type of cloud which forms still higher in the heavens is called the 
cirrus cloud. It differs from the other two types in being made up of 
transparent particles of ice, so thin that the sun shines through them. 
Rings around the sun or moon are often seen in cirrus clouds, which 
vary greatly, often having delicate feathery or plumed forms. 

The gradations between these three types of clouds are given com- 
pound names, such as cirro-stratus, cirro-cumulus, or strato-cumulus. 
The rain cloud is called nimbus. 

Precipitation of Rain. — The most important topic in connection 
with the discussion of the forms of water in the atmosphere is what 



74Q 



COLLEGE PHYSIOGRAPHY 






Fig. 442. — Types of clouds. Cirrus, above; cumulus, middle; stratus, below. 
(From Encyclopaedia Britannica.) 



RAIN AND OTHER FORMS OF WATER 741 

makes it rain. This is, in general, a simple matter, following the 
explanation of the condensation of water vapour into dew, frost, fog, 
and clouds. The formation of fog, as already explained, is due to the 
condensation of water vapour as a result of the cooling of humid air 
until its relative humidity reaches the dew point. When fog particles 
are of small size, they float in the air in the fog or cloud ; but they some- 
times grow to such size that they fall as rain drops. The growth of 
rain drops to such size that they can no longer float in the air is due to 
(a) continued condensation of vapour, (b) the uniting of cloud particles. 
Rain, therefore, is only a continuation of the process of cloud or fog 
formation, but when water vapour condenses rapidly, as in thunder 
clouds in summer, the rain drops may assume great size. 

It sometimes happens that there is cool, damp air at one level and 
warm, dry air at a lower level so that rain drops which are formed 
above may be evaporated on their way from the clouds and never 
reach the ground. Streamers of rain evaporated in this way are often 
seen in summer, descending part way to the earth. 

Measurement of Rainfall. — The amount of rainfall is recorded by 
the rain gauge. Any cylindrical measurer, such as a pail with vertical 
sides, can be used to measure approximately the number of inches of 
water that fall on a given surface. The rainfall is usually so slight, 
however, that provision must be made to measure it accurately by 
collecting the water in a smaller space than the surface on which it 
falls, thus magnifying the depth of water. 

The usual way of making a rain gauge is to have two cylinders one 
inside the other, the inner cylinder having a diameter of 2.53 inches, 
the outer one 8 inches. A funnel fits over the outside cylinder, and the 
opening at the bottom leads into the inside cylinder. The rain that 
falls in the funnel collects in the bottom of the inner cylinder to a 
depth of ten times that of the actual rainfall. By measuring this 
-depth, the actual rainfall may be obtained. There are also self- 
recording rain gauges, the one most commonly used having a balanced 
pair of small receptacles so arranged that when the rain has filled one, 
this tips down and empties out the water, bringing the other receptacle 
in position to be filled. When this is filled, it likewise tips down and 
places the first receptacle in position, and an electrical connection 
records each time that the gauge is filled, so that the total rainfall is 
automatically registered. 

The amount of precipitation for a region is commonly given as if 
it were all rain. Snow is melted, and then the depth of water is meas- 
ured and considered as " rainfall." Instruments are sometimes used 
for measuring snowfall, but usually the snowfall is measured out of 
doors in some place where it has not drifted. It is usual to allow one 
inch of rainfall for every ten inches of snow. 

Formation of Sleet. — Rain drops sometimes freeze on their way 
towards the earth's surface as they fall through a cold layer of air. 
This frozen rain is called sleet. Some sleet, however, is formed by the 



742 



COLLEGE PHYSIOGRAPHY 




Fig. 443. — Snowfall map of the United States in late autumn, winter, and spring, showing 
variations with latitude, altitude, and the season. (U. S. Weather Bureau.) 



RAIN AND OTHER FORMS OF WATER 



743 





Fig. 444. — Crystals of snow. 



partial melting of snow that is precipitated at higher levels, melted 
midway and then frozen before reaching the ground. 

Formation of Snow. — Snowflakes bear somewhat the same relation- 
ship to rain that frost 
does to dew, being 
formed by the condensa- 
tion of water vapour in a 
cloud at a temperature 
below the freezing point. 
Snowflakes are not frozen 
rain drops, as is the sleet, 
but are crystals, formed 
from water vapour with- 
out going through an in- 
termediate liquid form. When snowflakes grow without interference, 
they form beautiful, regular crystals whose varied forms may be 
observed by allowing them to settle on the sleeve of the overcoat in 
winter, before they have had any tendency to melt on touching the 
ground. Snow crystals grow as regularly as salt and alum crystals in 
a slowly evaporating solution (Fig. 444). The feathery frost patterns 
on window-panes are also caused by a crystal growth when water va- 
pour condenses at temperatures below 32 , but this is what we usually 
call frost and not snow. 

Snowflakes are usually irregular because of one of several causes : 
(a) the crystals are often broken ; (b) several crystals may unite by 

falling on one another, 
forming a matted mass, 
and most snowflakes 
are due to the forma- 
tion of several snow 
crystals; (c) snow is 
sometimes partly 
melted in falling 
through a warm layer 
of air. It often hap- 
pens that snow melts 
entirely before falling 
to the earth and 
reaches the ground as 
rain, as is often noticed in hilly or mountainous countries, where the 
hilltops are covered with snow, while only rain falls in the valleys a 
few hundred feet below. 

Formation of Hail. — During severe thunder storms and violent 
tornadoes the air is whirled rapidly about in strong currents, and 
hailstones are likely to fall to the surface of the earth. They are some- 
times as much as two inches in diameter (Fig. 445). If a hailstone 
is cut in two, it is seen to be a mass of snow and ice, usually more or 




Fig. 445. 



Hailstones. _ Figures on the scale are in 
inches. 



7 44 COLLEGE PHYSIOGRAPHY 

less spherical and built up of several alternate layers of shells. This is 
because the hailstones are whirled up and down in the violent air 
currents, passing from cold to warm and back to cold currents of air. 
They often grow to considerable size, because they are kept suspended 
for a long time, and they may even rise in the air after falling part 
way to the earth. As soon as the air current ceases its rapid motion, 
however, the hailstones fall rapidly through the air because of their 
weight, and when they fall they often break window glass and do great 
damage to crops. Conditions favouring the formation of large hail- 
stones are so uncommon that their destructive effects are limited to 
small areas and rare occasions. 

References to Literature 

Cleveland Abbe. The Aims and Methods of Meteorological Work, Vol. I, 
Maryland Weather Service, pp. 219-330; Treatise on Meteorological 
Apparatus and Methods, Annual Report of the Chief Signal Officer for 
1887, Appendix 46, Washington, 1888, 392 pp. 

Alfred Angot. Traite Elementaire de Meteorologie, Paris, 1899, 1907, 417 pp. 

D. Archibald. The Story of the Atmosphere, London, 1901, 210 pp. 

S. A. Arrhenius. Lehrbuch der Kosmischen Physik, Leipzig, 1903, pp. 473- 

925- 
J. G. Bartholomew and A. J. Herbertson. Physical Atlas, Vol. 3, Meteo- 
rology, London, 1899. 

F. H. Bigelow. Circulation of the Atmosphere of the Earth and Sun, Pop. Sci. 

Monthly, Vol. 76, 1910, pp. 437-461. 
Alexander Buchan. A Handy Book of Meteorology, London, 1867, 204 pp.; 

Report on Atmospheric Circulation, Report on the Voyage of H. M. S. 

Challenger, London, 1889, 263 pp. 
A. W. Clayden. Cloud Studies, London, 1905, 184 pp. 
H. H. Clayton and S. P. Ferguson. Measurements of Cloud Heights and 

Velocities, Annals Astronomical Observatory Harvard College, Vol. 30, 

Part 3, 1892. 
H. C. Cox and J. P. Goode. Lantern Slide Illustrations for the Teaching of 

Meteorology, Bull. 3, Geog. Soc. of Chicago, 1906. 
W. M. Davis. Elementary Meteorology, Boston, 1894, 355 pp. 
Henry Gannett. Rainfall Map of the United States, PI. I, Water Supply 

Paper 234, U. S. Geol. Survey, 1909. 

G. K. Gilbert. A New Method of Measuring Heights by Means of the Barom- 

eter, 2d Ann. Rept., U. S. Geol. Survey, 1882, pp. 403-566. 
J. Hann. Lehrbuch der Meteorologie, Leipzig, 1901 and 1906. 
A. J. Henry. Salton Sea and the Rainfall of the Southwest, Nat. Geog. Mag., 

Vol. 18, 1907, pp. 244-248. 
A. J. Herbertson. Distribution of Rainfall over the Land, London, 1901, 70pp. 
A. J. Herbertson and E. G. R. Taylor. Oxford Wall Maps, 1909-1911 : 

Rainfall Maps of the World, and of each continent separately; The World 

— Thermal Regions; The World — Pressure and Winds. 
W. J. Humphreys. Origin of the Permanent Ocean Highs, Bull. Mt. Weather 

Observatory, Vol. 4, 1911, pp. 1-12; Vertical Temperature Gradients, 

ibid., Vol. 2, 1909-1910, pp. 1-18,183-192; Holes in the Air, Smithsonian 

Report for 1912, Publication 2198, Washington, 19T3, pp. 257-298. 
Mark Jefferson. Rainfall Map of the World, in an Atlas of Commercial 

Values, Boston, 1912, p. 62. 
S P. Langley. Researches on Solar Heat and its Absorption by the Earth's 

Atmosphere, Bull. 15, U. S. Weather Bureau, Washington, 1884, 139 pp. 



RAIN AND OTHER FORMS OF WATER 745 

C. F. Marvin. Measurement of Precipitation, Circular E, Instrument Divi- 
sion, U. S. Weather Bureau, 1903; Barometers and the Measurement of 
Atmospheric Pressure, ibid., Circular F; also Circulars A, G, and K. 

W. I. Milham. Meteorology, New York, 1912, 541 pp. 

J. W. Moore. Meteorology, London, 1894, 1910, 466 pp. 

W. L. Moore. Descriptive Meteorology, New York, 1910. 

A. Lawrence Rotch. Sounding the Ocean of Air, London, 1900, 184 pp.; 
Charts of the Atmosphere for Aeronauts and Aviators (with A. H. Palmer), 
New York, 191 1, 96 pp. 

Thomas Russell. Meteorology, New York, 1895, 277 pp. 

Smithsonian Meteorological Tables. Smithsonian Misc. Collections, No. 
1032, 1907. 

J. Tyndall. The Forms of Water, New York, 1872. 

U. S. Hydrographic Office. Illustrative Cloud Forms, Washington, 1897. 

U. S. Signal Service. Professional Papers and Notes. 

U. S. Weather Bureau. Daily Weather Maps ; Rainfall and Snow of United 
States ; Rainfall of United States ; Snow and Ice Charts ; Lettered 
Bulletins; Numbered Bulletins; and other publications. 

Frank Waldo. Modern Meteorology, New York, 1893; Elementary Meteo- 
rology, New York, 1896. 

R. de C. Ward. Practical Exercises in Elementary Meteorology, Boston, 1896, 
199 pp.; Sensible Temperatures, Bull. Amer. Geog. Soc, Vol. 36, 1904, 
pp. 129-138; Relative Humidity in our Houses in Winter, Journ. Geog., 
Vol. 1, 1902, pp. 310-317. 

PERIODICALS 

Monthly Weather Review, Washington, D.C. 

Bulletin Mount Weather Observatory, Washington, D.C. 

American Meteorological Journal, Boston. 

Quarterly Journal Royal Meteorological Society, London. 

Symons' Meteorological Magazine, London. 

Journal of the Scottish Meteorological Society, Edinburgh. 

Die Meteor ologische Zeitschrift, Brunswick. 

Das Wetter, Berlin. 

del et Terre, Bruxelles. 

Bulletin de la Societe Beige a" Astronomic, Bruxelles. 

Annuaire de la Societe Meteor ologique de France, Paris. 



CHAPTER XXV 
WINDS 

The Movement of Air 

Relation between Winds and Air Pressure. — Wind is simply air in 
motion, usually in a horizontal direction. The cause of the wind is 
the pull of gravity upon air of different weights and the resulting dis- 
placement of lighter by heavier air. Wind may, therefore, be said to 
be due essentially to differences in the weight or pressure of different 
parts of the atmosphere. The atmosphere may be thought of as 
composed of a great many columns of air, held in place above the earth's 
surface by gravity. The column of air at one place, warmed by 
the sun's heat, is expanded, and becomes lighter than the columns of 
air near by which are not warmed and expanded so much. Accordingly 
the settling of the cooler, heavier air causes the rising of the expanded 
lighter air, just as it does around a lamp or in the neighbourhood of a 
fire. Heavy air, which is said to have high pressure, disturbs the 
equilibrium of light air, which is said to have low pressure. The 
heavy air moves or flows from places of high toward places of low 
pressure, forcing the light air to rise. In this way a circulation is set 
up which we know as the wind. 

Barometric Gradients. — Since the variations in weight or pressure 
of the air are known as barometric pressure, the difference in air press- 
ure which causes the wind is called the barometric gradient. This is so 
named because of the fact that the heavy air flows from a region of 
high pressure, or high barometer, to a region of low pressure, or low 
barometer, as if it were going down a gradient exactly as flowing water 
does. There is not a real slope or grade as in the case of a river, how- 
ever, but merely lighter air in one place than in the other. Just as 
water flows rapidly down a steep grade in a river valley, so the air 
flows swiftly, or the wind has a high velocity, if the difference in press- 
ure is great, because the barometric gradient is steep (Fig. 448). 

Measurement of Air Movement. — The measurement of the direc- 
tion and rate of movement of the wind is important in connection with 
the study of the atmosphere, wind direction being commonly deter- 
mined by the ordinary wind vane and the rate of movement of the air 
by an instrument known as the anemometer. The commonest form 
of this instrument has four light metal cups mounted on cross 
bars. The wind strikes the hollow side of the cups and causes the 
cross bars to revolve. Each revolution is communicated by a vertical 

746 



WINDS 



747 



shaft to a cog-wheel, which is connected with a moving hand upon a 
dial. In this way the velocity of the wind is recorded in miles. The 
dial is graduated so that the movement of the wind indicates the num- 
ber of miles and tenths of miles the wind has moved. The anemometer 
is sometimes equipped with a self-recording apparatus connected by an 
electric wire. 

The velocities of the wind may be roughly given as follows : a light 
breeze commonly has a velocity of from i to 10 miles an hour ; a 
strong wind from 20 to 30 miles an hour ; a gale from 40 to 60 miles, 
and, in the case of very severe winds, as in tornadoes, the velocity is 
often as much as 100 or 200 miles an hour. 

Local Winds 

Land and Sea Breezes. — The circulation which is set up on the 
seashore in connection with the heating and cooling of the atmosphere 
illustrates one of the simplest causes of local winds. The same sort of 
thing may also take place on hot days along the shore of good-sized 




^ jM 




^% ik> m 




#4 <V w 










W 












f 





Fig. 446. — The sea breeze (left) and land breeze (right) on Cape Ann, Massachusetts. 



lakes. It should be recognized, however, that the land and sea breeze 
are not especially common in the temperate zone, and that the subject 
is discussed at some length, because it so well illustrates the principle of 
wind circulation under the simplest conditions. These breezes rarely 
affect more than 10 or 15 miles near the coast. 

On a fine, warm day in summer the air over the land is warmer than 
the air over the water . Early in the morning there may be no wind 
at alL Soon, however, the land is warmed by absorption. The air is 
warmed by radiation, conduction, and convection, which go on more 



748 



COLLEGE PHYSIOGRAPHY 



rapidly over the land than over the water. The pull of gravity on the 
heavier air over the cool sea then results in the flowing in of a cool 
refreshing sea breeze, which displaces the lighter warm air that rests on 
the land. This sea breeze thus brings in cool air from over the water 
and therefore lowers the temperature. Before the sea breeze begins to 
blow, the temperature on shore may have been 8o° or 90 , but, as a result 
of the sea breeze, the temperature falls and the rest of the day is pleas- 
antly cool. It is partly on this account that so many people go to the 
seashore to spend their summer vacations. In some parts of the 
tropics the contrast of temperature over land and sea is so great that 
sea breezes are very pronounced and occur nearly every day (Fig. 446). 
After the sun has set, the land radiates its heat much more rapidly 
than the water, and it is not very long before the temperature condi- 
tions are reversed, the land being cooler than the water and the air 
over the two surfaces varying accordingly. At the time when the air 
over the land and the air over the water reach approximately the same 
temperature, the sea breeze dies down because of the lack of a baro- 
metric gradient, and there is a calm. On this account sailboats are apt 

to be becalmed if they have not 
reached port before sunset. With 
the continuation of the process of 
radiation, the air over the water 
soon becomes warmer than the 
air over the land, and then grav- 
ity sets up the opposite circula- 
tion, because of the formation of 
a new barometric gradient. The 
cool air of the land slides out 
over the sea, causing the warmer 
air there to rise and setting up 
a land breeze. Sailboats which 
have been becalmed offshore 
when the sea breeze ceased may 
" tack " into port later in the 
evening when the land breeze 
begins to blow. The land breeze 
is usually warmer than the sea 
breeze, and on evenings and mornings when there is a land breeze, it ' 
may be uncomfortably warm even on the seacoast. 

Mountain and Valley Winds. — In hilly and mountainous regions 
there is sometimes a local circulation of the atmosphere similar to the 
land and sea breezes. This is due to the fact that the cool, heavy air 
slides down the slopes as the hilltops and slopes are cooled by radiation 
at night, thus causing the warmer air in the valley, where radiation is 
less, to be displaced. The cool air from the slopes is likely to move 
down the valley, often causing mountain winds that may locally gain 
considerable force during the night. 



{fl- 


4* 


8* 


NOON 


16!? 


20* 


24!? 




inches 
02 



02 




























Inches 
.02 


















& 












M.B 






















N 


.02 


















i S 











.02 


J02 










N 




.02 



.02 






vS^ 


























^B 








































B 


/ 


















\ 


















G 










\ 














*< 


















J3^ 










.04 


>5 


i 


5 


( 


h 


NO 


ON 


l( 


h 


2C 


S 


V 


i 



Fig. 447. — Daily range of pressure at various 
altitudes in the Alps. G = Geneva, 1339 
feet above sea level ; B — Berne, 1880 feet; 
S = Santis, 8093 feet ; M. B. = Mont Blanc, 
15,781 feet. (Angot.) 



WINDS 



749 



The opposite circulation takes place during the day when the hill 
slopes and the valleys are warmed and the air ascends the sides and 
heads of the valleys. The mountain wind down the valley at night 
gains strength from the fact that the winds from several tributary 
valleys are gathered into one main valley, while the valley wind, moving 
up the valley during the day, is apt to be weakened through being 
distributed into side valleys. Some valley winds, however, are very 
strong during the day, as where the topography is very marked and the 
altitude is great. An exception to the up- valley wind of the daytime 
is often noticed in the vicinity of glaciers, because there the air over 
the cold ice and the snow field settles under the influence of gravity and 
displaces the warmer air in the valley, so that glacier winds may blow 
down a valley from an ice tongue with considerable velocity. 

Monsoon Winds 

Relation to Seasonal Variations of Temperature. — The land and 
sea breezes, and the mountain and valley winds, are related to daily 
temperature changes. There are similar winds due to seasonal 
changes of temperature, and these, when well developed, are known as 
monsoons. They occur on some of the continents, especially in Asia, 
and cause a variation in direction of the wind from summer to winter. 

The Monsoons of India. — The monsoon winds are best developed 
in India. There the great land mass of southern Asia becomes much 




Fig. 448. — The summer monsoon of India on the left, and the winter monsoon on the 
right. The figures show the barometric pressures, and the lines of equal pressure, or 
isobars, indicate the reversed barometric gradients of the two seasons. 



warmer in summer than the adjacent Indian Ocean. The air therefore 
blows from the ocean towards the land and is known as the summer 
monsoon. In the winter the continent and the plateau of India are 
cooled by radiation until their temperature is less than that of the 
adjacent Indian Ocean. The heavy air over the land therefore moves 
outward and displaces the lighter air over the ocean, forming the 
winter monsoon (Fig. 448). 



75o COLLEGE PHYSIOGRAPHY 

Accordingly, the winds change twice a year, and the changes are so 
regular and the winds so steady that they were taken advantage of in 
early times by sailing vessels carrying cargoes to and from Europe. 
The ships planned to reach the Strait of Bab-el-Mandeb, at the en- 
trance of the Red Sea, in the spring, at the time of the beginning of the 
blowing of the summer monsoon. As India is directly northeast of the 
Strait of Bab-el-Mandeb, and as the summer monsoon blows from 
southwest to northeast, these trading vessels were able to proceed to 
India with a fair wind. They discharged their goods there and took 
on new cargoes for transportation back to Europe, waiting until fall, 
when the beginning of the winter monsoon, which blows from north- 
east to southwest, furnished a fair wind to take their ships directly 
southwestward to the Strait of Bab-el-Mandeb and the Red Sea. The 
sailing routes in the Indian Ocean, the Bay of Bengal, and the Arabian 
Sea are to-day markedly under the control of the monsoons. 

As winds are always named by the direction from which they blow, 
just as we call a man who comes from Germany a German, and one 
who comes from western United States a westerner, the summer mon- 
soon coming from the southwest is often spoken of as the Southwest 
Monsoon, and the winter monsoon coming from the northeast as the 
Northeast Monsoon. 

Monsoons in Other Regions. — There are similar monsoons in other 
parts of Asia, along the coast of China, for example, where the summer 
monsoon is a southeasterly wind rather than a southwest wind ; but 
the monsoon circulation is not so well developed as in India. 

The other continents, likewise, show some tendency to develop a 
monsoon circulation, but in most cases the regular winds are too well 
established to allow the recognition of the monsoons as independent 
winds. The seasonal changes in temperature over the continents 
usually result in slight changes in wind direction from season to season 
and not in a complete reversal of direction. Around the borders of 
most continents in the temperate zone the monsoon is not strong 
enough to destroy completely the westerly wind circulation and form 
regular monsoons. In the United States a fairly regular monsoon 
circulation can be recognized in the lower Mississippi valley, especially 
in Texas. 

Planetary Circulation 

Relation to Temperature. — The major wind systems of the earth 
are directly related to the distribution of temperature over the earth. 
In general there is a warm equatorial belt between the Tropic of 
Cancer and the Tropic of Capricorn, and cooler belts north and south 
of it. These are determined by the greater heating of the atmosphere 
(a) by the vertical rays of the sun between the tropics, and (b) the 
decreasing supply of heat with the lower and lower angle of the sun's 
rays to the north and south. 

There is of course a direct relation of temperature to barometric 



WINDS 



7Si 



pressure, the warm air in the tropical zone having low barometric 
pressure in contrast with the cool air and high barometric pressure to 
the north and south. There are also local areas of permanent high 
pressure and permanent low pressure at various points over the con- 
tinents and oceans as a result of this relation of pressure to tempera- 
ture (Fig. 449)- 

Accordingly, we find that the planetary circulation of the earth 
results in the formation of seven belts of varying atmospheric circula- 
tion. The first of these is (a) the belt of equatorial calms, north and 
south of which are pairs of belts one in each hemisphere which we 




Fig. 449. — Isobars or lines of equal pressure for the year. The darkest shading represents 
high pressure. The figures (29.85 for example) are inches to which the mercury in a 
barometer rises, being highest where the air pressure is greatest. In the dark zones 
of high pressure, the horse latitude belt, the air is settling ; it moves thence toward the 
low pressure belt of the warm tropical zone, forming the trade winds, and toward the 
low pressure areas near the poles, forming the prevailing westerlies. (Bartholomew.) 

know as (b) the trades, (c) the horse latitudes, and (d) the prevailing 
westerlies (Fig. 450). 

Comparison with the Circulation of Air in a Room. — The circula- 
tion of air on the earth may be compared roughly with the movement 
of air in a room heated by a stove. Near the stove the air is warmed, 
and the cooler and heavier air in other parts of the room crowds in and 
pushes the warm air upward. This sets up a circulation consisting of 
(1) a movement of air toward the stove, (2) a rise above it, (3) an 
upward current away from the stove, and (4) a settling of the air 
near the walls of the room. 

The heated belt of the equatorial regions corresponds to the part of 
the room near the stove. There is, therefore, (1) a movement 
of air along the surface of the earth from the tropics toward the 
equator, (2) a rising in the equatorial region, (3) a movement away 



752 



COLLEGE PHYSIOGRAPHY 



from the equator high above it, and (4) a settling some distance to the 
north and south. The winds thus set in motion affect all parts of the 
earth, in every zone and over every continent and ocean. All these 
winds are set in motion by the relations of gravity to differences in 
temperature between the warm tropical region and the cooler zones 
to the north and south. 

Belt of Equatorial Calms. — In the equatorial region there is a belt 
of equatorial calms or doldrums caused by the rising of the warm air 
which has been brought in from north and south in the trade winds. 
There is little wind in the belt of calms because the air movement is 
upward instead of horizontal. The belt of calms was named in the 




Fig. 450. — The belts of winds and calms on the earth in winter. 



days of sailing vessels because of the baffling winds there which blew 
from no persistent direction and for no great length of time, between 
the much longer intervals when the ship was becalmed. The doldrum 
belt does not remain stationary, but migrates northward at one season 
of the year and southward at another with the shifting of the belt of 
greatest heat which is known as the heat equator. 

Trade Winds. — The belts on either side of the doldrums are known 
as the trade wind belts because of the great steadiness of the winds over 
the ocean. They were named by sailors in the early days of ships 
propelled by the wind. Of course the wind does not blow steadily 
from one direction at all times in the trade wind belts; but, as is 
shown in Fig. 452, the prevailing wind is toward the equator. This 
results in (a) the cutting of steep cliffs on the windward sides of islands 
in the path of the trade winds by the surf which is always beating 
there, and (6) quiet harbours on the leeward sides. As is explained 
later (p. 756), the trade winds blow, not from the north and south 
directly toward the equator, but from the northeast in the north- 



WINDS 753 

ern hemisphere and from the southeast in the southern hemisphere 
because of the effect of rotation of the earth. They are therefore 
known as the northeast trades arid the southeast trades, respectively. 

With the shifting of the belt of calms northward and southward 
during each season, there is a corresponding shifting of the trade winds, 
which are farther north in summer than in winter. Places near the 
border of the trade wind and doldrum belts have alternate seasons of 
calms and steady northeast or southeast winds, with corresponding 
effects upon the vegetation (p. 794) and the physical comfort of the 
people living on the lands in these border belts. 

The unusual development of the monsoons in Asia, and particu- 
larly in India, results from the excessive warming of the southern 
portion of this great land area in the summer of the northern hemi- 
sphere, and the consequent migration of the heat equator, and the ac- 
companying low barometric pressure well up on the land. The out- 
flow of cold air from India in winter strengthens the northeast trade 
wind, but in summer the inflow of air from the Indian Ocean is reen- 
forced by the southeast trade wind, which extends across the equator 
and is there leflected so that it blows nearly parallel to the ordinary 
summer monsoon. At this season the northeast trades of the Indian 
Ocean are entirely nullified. 

The air that rises in the equatorial belt of calms is divided, some of it 
flowing northward and some southward, high above the trade winds. 
These antitrades blow in pmub 

the opposite direction from 
the trade winds. They 
may be observed on high 
peaks which rise above the 
trade winds, as in the Ha- 
waiian Islands, and also 
by the movement of high 
clouds and of dust from 
volcanic eruptions. "* 

tr^-^^T „+;+.., A^r, "MV^fV. Fig. 451. — The relationships of the air circulation 

Horse Latitudes.— North from the equator; £ to the poles 

of the belt of northeast 

trades and south of the belt of southeast trades are regions known as 
the horse latitudes. These are regions which have calm weather and 
light, variable winds during a large part of the time. The calm con- 
dition here is due to the facts that the air is settling down from aloft, 
and that the differences in pressure are so slight (Fig. 451). 

The belts of horse latitudes migrate northward and southward with 
the seasons, so that at their borders on the sides toward the equator 
are regions which are part of the year in the trade winds and part of the 
year in the horse latitude belt. At the opposite sides of the horse 
latitude belts, toward the poles, the borders are part of the year in 
the horse latitude calms and part of the year in the belts of prevailing 
westerly winds. 
3c 




754 



COLLEGE PHYSIOGRAPHY 




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WINDS 



755 




Prevailing Westerlies. — These belts occupy all of the northern and 
southern hemispheres between the horse latitude belts and the poles. 
They occupy most of the temperate zone on each of the continents, and 
the winds and climates in the prevailing westerlies are therefore of most 
interest to civilized man. 

The prevailing westerlies are supplied from the horse latitudes by 
air, some of which came from the equator and some from the poles. 
Accordingly there is a movement of air from a broad belt in the tem- 
perate zone toward the small area around each pole. This may be 
compared with the movement of 
water toward the small outlet of a 
wash-basin. In attempting to reach 
the outlet of the wash-basin, the 
water whirls about it. In a similar 
way the air whirls about each pole 
in what is known as the circumpolar 
whirl (Fig. 453). The direction in 
which this whirling air is turned 
from a north-south direction is tow- 
ard the east in each hemisphere, 
because of an influence of the earth's 
rotation (explained later, p. 756). 
The direction of movement of the 
air near the horse latitudes is, there- 
fore, converted to an easterly direc- 
tion near the earth's surface in each 
hemisphere ; and, since the air moves 
from west to east, these wind belts are called the prevailing westerlies. 
They cover the greater part of the two temperate and the two polar 
zones. 

Variations in the Prevailing Westerlies. — Various causes inter- 
fere with these winds. They are often strongest during fine summer 
days, for example, on account of the heating of the lower air which 
then rises, being displaced by faster-moving air from a short distance 
above the earth's surface. After sunset when the lower air is cooled, 
these winds die down. Sea breezes, storms, the influence of valleys 
and mountains, and other topographic features also modify the direc- 
tion and velocity of these winds. 

The topography and the variations in temperature of different soils 
and rocks, and of the surfaces with or without vegetation, interfere 
with the movement of the air, so.that the winds are usually weaker and 
less steady on land than over the ocean. The southern hemisphere, 
in which the areas of the ocean exceed those of the continents, has 
better-developed prevailing westerly winds than the northern hemi- 
sphere. The velocity of the winds in the southern hemisphere is such 
that the prevailing westerlies in and near latitude 40 south are some- 
times alluded to as the Roaring Forties, In the southern parts of the 



Fig. 453. — Diagram to show ideal wind 
circulation in the southern hemisphere 
near the earth's surface. Trade = 
trade wind belt ; H — horse latitudes ; 
C. W. = circumpolar whirl. 



756 



COLLEGE PHYSIOGRAPHY 



Pacific, Atlantic, and Indian oceans, it is possible for a vessel to sail 
eastward around the earth in the prevailing westerly wind belt with 
fair winds most of the way. 

In the northern hemisphere, where the westerlies are interfered with 
by the large proportion of land, there is nevertheless great steadiness 
and velocity at some height above the earth. This may be observed 
by watching the upper clouds which usually move rapidly eastward, 
even when the wind at the surface is from the opposite direction. 

Effect of Rotation on Winds. — As already mentioned, the trade 
winds are deflected from blowing directly northward and southward 

toward the equator, 
and the prevailing 
westerlies are also de- 
flected by the influ- 
ence of the earth's 
rotation. This is an 
application of what 
is known as Ferret's 
Law. A body mov- 
ing in any direction 
upon the earth's sur- 
face is deflected tow- 
ards the right, if 
in the northern hemi- 
sphere and toward the 
left, if in the southern 
hemisphere, by the 
rotation of the earth 
from west to east. 
This applies to objects 
at any point except 
directly on the equa- 
tor. Its most notable 
effect is upon the 
movements of the at- 
mosphere (Fig. 454). 
The trade wind in the northern hemisphere, which is moving toward 
the heated equatorial region, would move from north to south if the 
earth were not rotating. As a result of Ferrel's Law, however, this 
wind is deflected towards the right, and therefore blows from north- 
east to southwest and is known as the northeast trade wind. The 
deflection is spoken of as right-handed deflection, since the departure 
from movement directly towards the equator is toward the right hand 
as one faces the equator. The trade wind of the southern hemisphere, 
which is affected by left-handed deflection in a similar way, becomes a 
southeast wind rather than a south wind. The prevailing westerly 
winds of the northern and southern hemispheres are affected respec- 




FlG. 454. — ; Ferrel's ideal diagram to show the atmospheric 
circulation in plan and cross-section. Dotted arrows show 
upper air currents. 



WINDS 



757 



tively by right-hand and left-hand deflection and, therefore, become 
westerly winds. 

The effect of rotation upon the ocean currents is well established, as 
in the movement of the Gulf Stream in the North Atlantic Ocean (p. 
693), but the deflection of rivers is so slight that the application of 
Ferrel's Law is not fully accepted by all persons, although it has been 
pointed out that rivers in the northern hemisphere have cut higher 
banks on one side in some places, as if they were deflected by the earth's 
rotation. 

Seasonal Migration of Wind Belts. — The northward and southward 
migration of the wind belts results from the fact that the earth is 




Fig. 455. — Wind belts of the Atlantic in winter 
migration of wind belts with shifting of heat 
steadiness ; double line, strong winds ; circles, 
Atlantic are best developed in winter when cold 
(After Koppen.) 



(right) and in summer (left), showing 
equator. Length of arrows indicates 
calms. Prevailing westerlies in north 
air from North America flows outward. 



inclined at an angle of 23 J° on its axis and revolves about the sun in this 
inclined position, so that the sun's rays are vertical at the mathemati- 
cal equator only twice a year. This is at the time of the vernal and 
autumnal equinoxes ; and the sun's rays are vertical at one tropic 
three months earlier and at the other tropic three months later. Ac- 
cordingly, the zone of greatest heat migrates with the season of the 
year; and the heat equator is sometimes north of the mathematical 
equator and sometimes south of it. As a result of the unequal heating 
of the continents and oceans and of the air above them, the heat 
equator does not exactly correspond to the mathematical equator, 
even at the time of the equinoxes. It is north of the true equator in 
some parts of the world and south of it in others (Fig. 455). 



758 COLLEGE PHYSIOGRAPHY 

At all seasons, therefore, the widths of the belts of doldrums or 
equatorial calms, trade winds, horse latitudes, and prevailing west- 
erlies vary over the continents and over the oceans. These belts all 
shift with the season, being farther north in the northern hemisphere 
in our summer, and farther south in the southern hemisphere during 
our winter. The effect of this migration upon the border belts has 
already been noted. It causes some regions to have two seasons, one 
of them calm and the other windy, one wet and the other dry, depend- 
ing upon the time when these regions are in the belt of equatorial 
calms or horse latitudes, or in the belt of trade winds or of prevailing 
westerlies. The migrations are regular, however, so that these border 
regions have a recurrence of windy or calm, dry or rainy, seasons. 
The middle portion of each of these belts, however, and the larger part 
of the belts of prevailing westerly winds, have no corresponding change 
in their wind regime, although their temperatures and the regularitv 
of their winds vary more or less with the seasons. The seasonal 
migration of wind belts causes some exceedingly important variations 
in climatic conditions. 

Cyclonic and Anticyclonic Areas. — The development of extensive 
areas of cyclonic and anticyclonic character over oceans and lands has a 
notable effect upon the regular winds of the earth. The nature and 
cause of the cyclonic areas is explained more fully in the following 
chapter (p. 765). It is sufficient here to state that cyclonic areas are 
those in which the air moves around toward a region of low pressure, 
with the winds blowing in all directions toward the centre ; and that 
anticyclonic areas are areas in which the air moves outward from a 
region of high pressure, with winds blowing in all directions from the 
centre. The development of such areas results in an interference 
with the regular winds, but cyclonic development is relatively rare in 
the belts of trade winds and is found chiefly in the prevailing westerlies. 

These cyclonic and anticyclonic areas completely nullify the pre- 
vailing westerly circulation at times, and greatly strengthen it at 
others. They move in the general direction of the prevailing winds. 

For references to literature on Winds, see pp. 781-782. 



CHAPTER XXVI 
STORMS 

Cyclonic Storms 

Nature of an Area of Low Pressure. — The characteristics of a low 
pressure area are as follows, and may be illustrated by the study of a 
weather map of the United States for a typical day in winter. Figure 
456 shows an area where the barometric pressure is low. The longer 
name low pressure area is generally abbreviated to the term Low, and 
this is done on the map. Outside this centre of low pressure, or Low, 
the mercury in the barometer is higher. The distribution of pressure 
is indicated by lines of equal pressure, or isobars. Some distance south- 
east of the Low, which is in Canada near the Rocky Mountains, are two 
areas marked High, and these are areas of maximum pressure for that 
part of the United States. The direction in which the wind is blowing 
is indicated on Fig. 456 by arrows which fly with the wind, and the 
air is moving from all directions towards the low pressure area. In 
addition to the isobars — heavy lines on the map which indicate 
equal pressure — there are dashed lines which indicate equal tem- 
perature, — isotherms (p. 723). The relation between the isotherms 
and isobars and the precipitation of rain in the vicinity of areas of 
low pressure is important. 

Eastward Movement of an Area of Low Pressure. — On the weather 
map for the following day (Jan. 8) the Low has moved eastward, 
its path being indicated by a chain of arrows from southwestern 
Canada to Lake Superior. Near it the map is shaded to indicate that 
rain has fallen during the past 24 hours. The arrows show that the 
air is still blowing in towards the centre of this Low. 

In the weather map for the third day (Jan. 9, Fig. 456) the storm 
has moved still farther eastward and it is now central in the Province 
of Ontario, east of Lake Huron. If we had maps for other days, we 
should be able to trace this Low out over the Atlantic Ocean and possi- 
bly across the British Isles and Europe into Siberia, although it might 
merge with some other low pressure area on the way, or completely 
disappear. 

Such a low pressure area as is shown in these maps is known as a 
cyclone or cyclonic storm, which may be defined as an area of low air 
pressure toward which winds blow from all directions and in which rain 
frequently falls. 

759 



760 



COLLEGE PHYSIOGRAPHY 




Fig. 456. — Weather maps of three successive days in 1893, showing isobars (heavy lines), 
isotherms (dotted lines), wind direction (indicated by arrows), and areas of rain (shaded). 
The path of one Low from Western Canada to New England is indicated by a chain of 
arrows. (After U. S. Weather Bureau.) 



STORMS 



761 



Difference between Cyclonic Storms and Tornadoes. — A cyclone 
or cyclonic storm should be carefully distinguished from that type of 
violent wind of small area which is known as a tornado. We use the 
name tornado for a violent destructive storm of small area, and the 




^. p 




Fig. 457. — The winds blowing down a barometric gradient toward a cyclonic storm centre 
(right) with deflection due to the earth's rotation. The opposite wind circulation (left) 
in an anticyclone. (Milham.) 

name cyclone or cyclonic storm for the area of low pressure, which is 
of much larger size and in connection with which there may be nothing 
violent. The name cyclone comes from the fact that the winds blow 
inward and around the centre of an area of low pressure, which is 
itself moving. 

Relation of Pressure to Cyclonic Storms. — Within the Lows the 
barometric pressure does not vary greatly, the range in Fig. 457, 
for example, being from 29.7 inches at the centre of the Low to 30.3 



BROKEN CLOUDS 



^4fe 






CIRRUS CLOUDS 
cc" CLEAR 



-u*ks: 



)MER N, , ') 



SECTION 



EAST 



HEAVY STRATUS CLOUDS 



Fig. 458. — The inflowing and rising air in a cyclonic storm, with distribution of clouds 

and precipitation. 



inches at one side. The isobars always encircle areas of low and of 
high pressure. 

Relation of Winds to Cyclonic Storms. — The relation of winds 
to a Low is well indicated in the diagrammatic sketch (Fig. 458), 
where it is seen that the light air in the middle of the Low is forced to 



762 



COLLEGE PHYSIOGRAPHY 



rise by the flowing in of the heavier air on either side, which is descend- 
ing the barometric gradient toward the centre of the Low. 

A commonly observed phenomenon is the reversal of the direction 
of winds in connection with the passage of cyclonic storms. Suppose, 
for example, that we are some miles east or southeast of the centre of a 
Low. We then have a southeasterly or southerly wind, because the 
air is moving toward the centre of the cyclonic storm. Later the cy- 
clonic storm has moved far enough east or northeast of us to leave us 
some miles west or southwest of its centre. At that time we have a 



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90 
80 
70 

60 

SO 

40 

30 

20 
30.75 

.5 

25 
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Fig. 459. — Thermograph and barograph curves with indication of direction of winds and 
alternation of clear and cloudy weather. Normal diurnal curves, of large range, with 
a rise in the mean temperature and in the maxima and minima from day to day under 
the clear sky of a spring anticyclone. At the beginning of this spell (April 24) the cool 
wave in front of the approaching anticyclone brought lower temperatures, while the 
warming increased in the light winds near the centre of the High. The rise in the mean 
temperature from April 25 to 27 is shown by the rise of the dotted temperature belt. 
(Ward.) 



westerly wind, because the air is moving eastward toward the centre 
of the Low (Fig. 459). 

Relation of Clouds and Precipitation to Cyclonic Storms. — The 
relation of clouds to the cyclonic storm (Fig. 458) is a natural one, 
the centre of the Low having heavy nimbus clouds because the rising 
air is cooled sufficiently to have its relative humidity increased to the 
dew point at no great height above the earth's surface. After the air 
has moved upward over the centre of the Low, and outward in either 
direction, the clouds are likely to be more broken, and at some distance 
there will be high cirrus clouds and clear weather (Fig. 460). 

The relation of rain to a cyclonic storm is also a simple one. There is 
usually precipitation of rain near the centre of the cyclonic storm 
because of the continuous process of condensation which goes on in 
this cloud area, until drops of water too large to float are formed, and 
fall to the earth's surface. In eastern United States the rain is likely 
to be in the southeast quarter of the cyclonic storm, because the air 



STORMS 



763 



from the south and east is moving northward and is, therefore, being 
cooled. It will cause precipitation as rain or snow sooner than the air 
moving toward the centre of the Low from the north or west, because 
the latter is moving southward and is having its capacity for moisture 
increased by being warmed (Fig. 46 1) . The precipitation in connection 
with the eastward movement of the Low of Fig. 456 brings rainfall 
on successive days to Michigan, Ontario, and western New York. 

Relation of Temperature to Cyclonic Storms. — The relation of 
temperature to cyclonic storms is a result of the variations in wind 




Fig. 460. 



Diagram to show the relationships of clouds to the different parts of a low press- 
ure area. (Ward.) 



direction. It is apt to be warm when the southerly wind is blowing, 
and cool or cold when there is a northerly wind. 

Anticyclones. — The areas of high pressure marked High on Figs. 
456, 461, and 463 have the wind blowing outward in all directions 
from the centre and are usually called anticyclones, because the condi- 
tions within them are just the reverse of those in cyclones. The sky is 
generally clear or fair, and there is not likely to be rain. Anticyclones 
are cold in winter, but warm by day in summer, with cooler 
nights. Figure 462 shows the usual circulation of air in an anticyclone. 
The anticyclones move eastward as the cyclonic storms do, in many 
cases crossing the continent of North America and the Atlantic Ocean 
to Europe. 



764 



COLLEGE PHYSIOGRAPHY 



Normal Movements of Cyclones and Anticyclones. — In the temper- 
ate zones the belts of prevailing westerly winds are visited periodically 
by a succession of cyclones and anticyclones. Figure 463 shows the 




Fig. 461. — Upper map shows area of winter rainfall (stippled) in the eastern part of a 
low pressure area in Minnesota and North Dakota. Lower map shows the same 
low pressure area a day later when it has moved eastward, bringing rain to Wisconsin 
and adjacent states. (After U. S. Weather Bureau.) 



positions of a series of Lows and Highs in the northern hemisphere 
on January 30 and 31, 19 14. In the northern United States, two 
low pressure areas with an intervening High are likely to pass any 
given place at intervals of from 3 to 7 days in winter. If you 



STORMS 765 

watch the maps published by the United States Weather Bureau, you 
will see a movement of areas of high and low pressure similar to 
those in Figs. 456, 461, and 463'. The passage of these areas of low 
pressure is indicated by the rise and fall of the barometer, and the 
curve in Fig. 464 shows the change of pressure at one point during a 
week. It is by means of a study of the barometric pressure over the 
whole of the United States that it is possible to predict what the 
weather will be, because of the regularity of movement of these cy- 
clonic storms and their relationships to temperature, winds, and pre- 
cipitation. In winter, especially, cloudy weather, with rain and higher 
temperatures, usually accompanies the cyclonic storms and clear and 



CIRRUS CLOUDS 

CLEAR WEATHER 



EART5LY CLOUDY^ 



;>1^S 



DIRECTION 
OF MOVEMENT 



Fig. 462. — The descending and outflowing air in an anticyclone. 

colder weather the anticyclones, the wind direction of course varying 
as these Highs and Lows pass. 

There are several paths which are commonly followed by the low 
pressure areas of United States (Fig. 465). Most of the Lows seem to 
originate either in the northwest or in the southwest, but many doubt- 
less reach the western United States from the Pacific Ocean. In each 
case they move eastward, usually crossing the Middle West in the 
vicinity of the Great Lakes and following the St. Lawrence valley to 
the Atlantic Ocean. The average velocity of movement of the storm 
centre is between 500 and 1000 miles a day. 

It should not be thought that there is stormy weather in connection 
with all low pressure areas. Some cyclonic storms have light wind 
because the pressure is not very low, and the barometric gradient is not 
very steep. Such Lows are likely to have little if any rain. These 
weak cyclonic storms sometimes die out entirely, and sometimes 
develop into exceedingly vigorous storms. On this account the pre- 
diction of the weather is sometimes erroneous, but the movement of 
cyclonic storms is generally so regular that most storms are accurately 
forecasted. 

Reason for Cyclonic Storms in Prevailing Westerlies. — The reason 
for the development of the cyclonic storms in the west wind belts is not 
entirely understood, but it is clear that the cyclones and anticyclones 
move like great eddies in the prevailing westerlies of the northern and 
southern hemispheres. The blowing in of the air towards the centre 
of the Lows is like the movement of water in an eddy in a river. 

Likewise, while the cyclonic storm is moving eastward (Fig. 466) 



766 



COLLEGE PHYSIOGRAPHY 




Fig. 463. — Weather map of the northern hemisphere on two successive winter days 
in igi4. Continuous lines are isobars. Pressures are expressed in millibars (1000 
millibars = 29.53 inches). Dotted lines arc isotherms. Temperatures are expressed 
in absolute units (freezing point = 273 ). Note eastward progress of areas of high and 
low pressure. (After U. S. Weather Bureau.) 



STORMS 



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with the prevailing westerlies, the air within it is eddying from all 
sides towards its centre, and the air in the eastward-moving anticy- 
clone is moving outward in all directions from the centre. 

The development of cyclonic storms in the prevailing westerlies 
may sometimes be related to the heating of the air and its consequent 
rising over the heated place, somewhat as the air rises over a stove 
when the heavier air is drawn in towards the centre. The chief 
objection to this theory is the fact that cyclonic storms are most 
common and are best developed during the winter, when local excess 
of temperature should be less likely to occur. It seems likely, however, 
that the cause of most of our high and low pressure areas is to be sought 
in the conflict and congestion of air currents from different directions 
and with different conditions of temperature, — this conflict taking 
place above, it may be 
some miles above, the 
earth's surface. 

Whether the cyclones 
and anticyclones are due 
to warming of the air or 
to conflicts of air cur- 
rents or to some other 
cause, it is clear that 
some parts of the at- 
mosphere have a lower 
pressure than others, 
and, because of the in- 
fluence of gravity, the 
air flows toward these places of low pressure, starting a whirl which 
we known as the cyclonic storm. 

Relation of Cyclones and Anticyclones to Local Weather. — It 
has already been pointed out that the wind changes during the passage 
of cyclones and anticyclones, and that on the east side of a cyclonic 
storm the wind is generally easterly in direction, on the south side 
generally southerly, and between the cyclone and the succeeding anti- 
cyclone from a westerly direction. These winds, however, do not 
move straight towards the centre of the Low. They are turned by the 
effect of the earth's rotation so that they blow spirally ; and if the differ- 
ence in air pressure between the High and the Low is great, they blow 
with considerable force. The rising of the air near the centre of the 
Low and the settling of the air near the centre of the High separate the 
reversal of winds during the passage of cyclonic storms. 

The influence of the warm south wind and the cool north wind upon 
the weather during the passage of an area of low pressure has likewise 
been pointed out. During the passage of anticyclones the air settles 
near the centre, and, in moving away from the centre, usually causes 
pleasant weather in summer. During the passage of an area of high 
pressure in winter the weather may be extremely cold. It also happens 



Fig. 464. — Curve showing change of pressure for seven 
successive days in summer in central New York with 
the passage of two cyclonic storms. 



768 



COLLEGE PHYSIOGRAPHY 




Fig. 465. — Paths of cyclonic storms across the United States. (Van Cleef.) 




Fig. 466. — Diagram to show the average path of storms in the northern hemisphere. 
The figures show the total number of storms from 1878 to 1887. 



STORMS 



769 



that the ground is cooled far more by radiation through the clear air of 
the anticyclone than the cloudy air of the cyclone and that the summer 
weather is consequently cooler while a High is passing. Since descend- 
ing air is warmed, the cause of an ticy clonic cold is to be sought in active 
radiation, rather than in the descent of cold from aloft. 

Cyclonic Storms and Rain. — The reason for clear weather in con- 
nection with anticyclones is that when air is settling, it is growing drier. 
It is, therefore, having its capacity for moisture increased, as is always 
the case when descending air is warmed by compression. It is for this 
reason that there is 
usually little or no rain 
in connection with the 
passage of an area of 
high pressure. The op- 
posite is true, however, 
in the areas of low press- 
ure, where the cooling 
of the air as it rises 
causes condensation of 
vapour and the forma- 
tion of clouds and rain. 
In a well-developed cy- 
clonic storm the area 
of cloudy and rainy 
weather may be as 
much as a thousand 
miles in diameter (Fig. 
467). _ _ 

In addition to this 
there is also rain when 
the air is forced to rise 
over highlands (Fig. 
468). The west wind 
in a cyclonic storm 
may, therefore, bring 

rain when it strikes the western slope of the Appalachian High- 
land ; but there will, likewise, be rain if the east wind is forced to rise 
over the eastern slope of the Appalachians. In New England a Low 
may be situated just west of Boston, and the winds blowing in from 
the northeast, east, and southeast will be heavily laden with vapour 
because they have been blowing over the surface of the ocean. When 
these vapour-laden winds rise in the cyclonic storm, they are cooled 
sufficiently so that the dew point is reached and some of the vapour 
is condensed. It is very common in New England to have heavy 
rainfall and strong winds in connection with just these conditions, 
and because of a common northeast wind these are spoken of as 
northeast storms or northeasters. This prevalence of rain in connec- 
30 




Fig. 467. — Weather map for a winter day, showing rain- 
fall east and south of the Low and snow to the north- 
west (a) where it is cooler in North Dakota because 
farther north and (b) in Nebraska and South Dakota 
because it is higher than near the centre of the Low in 
Minnesota. (After U. S. Weather Bureau.) 



77o 



COLLEGE PHYSIOGRAPHY 



135 in. 



tion with the northeast quarter of the cyclonic area is a direct result 
of the influence of the ocean. 

In connection with the condensation of water vapour to form clouds 
and rain, there is a development of latent heat (p. 722), and this 
helps to keep the temperature of the air from falling. 
Very commonly, cyclonic storms increase in violence as 
they pass over the Great Lakes and out over the ocean. 
One reason for this probably is that so much more vapour 
is supplied over these bodies of water that the heat from 
its condensation helps to cause lower 
90 in. pressure and, therefore, a more 

rapid inflow and rising of air. In- 
deed, a cyclonic storm may be 




SIERRA NEVADA 
61.18in. 



8 to. I GREAT BASIN f 




PACIFIC 

0C Ei?. g SACRAMENTO VALLEY 

22.77 in. s ._ a 01 



.18.44 in, 



Sea Level 25 miles 50 



Fig. 468. — The relation of annual rainfall in the prevailing westerlies to the Coast Range 
and Cascades in Washington and the Sierra Nevada in California. (Bowman.) 



thought of as a great engine, because it furnishes some of its own 
energy as the vapour condenses. 

Cold Waves and Blizzards. — The west and northwest winds which 
follow in the rear of vigorous winter cyclones are often strong and 
very cold. Such cold winds accompanied by snow are called blizzards 
in Dakota and northers in Texas, though northers may take place with- 
out snow. The air moves with great velocity because there is a marked 
difference in the barometric pressure between the cyclone and the 
anticyclone, the barometric gradient sometimes being so steep that 
the wind blows 40 to 60 miles an hour. These blizzards and northers 
are often very destructive of life. Because of the intense cold and 



STORMS 



77i 



fierce snow squalls which accompany them, whole herds of sheep and 
cattle are often lost, and men sometimes lose their way in the blinding 
snow and are frozen by the fierce cold. In the northeastern United 
States there are occasionally mild forms of blizzard. 

During the approach of a well-developed anticyclone following a 
well-marked cyclone in the eastern United States, we sometimes 
have a very rapid fall in temperature during the winter. When 
the fall in temperature reaches or exceeds a certain definite number 
of degrees, this is known as a cold wave. During a cold wave, a great 
body of cold air spreads over the country, sometimes even extending 
southward to the Gulf of Mexico. The area covered by one cold wave 




Fig. 469. — Map of 



cold wave in the month of November. Arrows show wind direction. 
(After U. S. Weather Bureau.) 



is shown in Fig. 469, where a large portion of the United States 
west of the Mississippi River and north of St. Louis had temperatures 
from freezing to 20 below zero. During a cold wave a blanket of 
air descends from the cold northern portion of the continent and per- 
haps partly from aloft (Fig. 462) . Since it contains little water vapour 
and is warming as it spreads out, the weather accompanying it is 
clear and dry. Throughout a cold wave, radiation proceeds rapidly, 
causing very low temperatures in winter, and perhaps unseasonable 
frosts in fall and spring. 

In Europe, certain combinations of cyclonic and anticylonic condi- 
tions also give rise to cold winds. These winds are called by various 
names. The mistral in southern France and the bora along the 
eastern shore of the Adriatic are the best known examples. 

The Sirocco. — Another type of wind in connection with the passage 



772 



COLLEGE PHYSIOGRAPHY 



of cyclones and anticyclones in the eastern United States is also so 
distinctive as to deserve a special name. This is the opposite of the 
blizzard and is known as the sirocco. It is a southerly wind, which 







^^MflfiP^ 






I^kki " Bl 




"-" liK?'^ :- , - , vw^W: ^Tr^suKmi 












9 .'■('• K . - r'-if S? \> * ^Tij~ 1 • ^iv'Oj'WjV^' 




,;r: -'. •'-. : \' KfffcgHHHfiPlI •'••'; m ■ 'iAihsSRs 




•^.fo fej ^g«»a jsdaB 








■ ■ ^ , ^'*^V-A« ' -V ^V *' ^ "* k '^j&^Jfc * »^cH^EBB 







Fig. 



Photograph of a flash of lightning. (Milham.) 



causes oppressively warm weather in summer and unseasonable 
warmth in winter. In winter the blowing of the sirocco results in the 
so-called " January thaw " and in unseasonable melting of the snow. 
During the summer the sirocco often accompanies the development of 
thunderstorms and tornadoes. 



STORMS 



773 



Chinook and Foehn Winds. — Sometimes when a cyclonic storm is 
east of the northern Rocky Mountains, the air which is descending 
the eastern slope of the mountains, on its way toward a low-pressure 
centre, is felt as a brisk, dry, warm breeze. It is known as the chinook. 
In descending rapidly the air is warmed by compression, just as the 
air in a bicycle pump is warmed. This warming lowers the relative 
humidity until the air becomes drier than before. Some of the best 
winter ranges for cattle at the western edge of the Great Plains are at 
the mouths of valleys where the 
chinook winds from the mountains 
evaporate the snow and leave the 
grass uncovered for cattle when 
the rest of the plains are snow- 
covered. 

The foehn is of exactly the 
same nature. In Switzerland it 
was formerly believed that this 
wind came from the Sahara and 
was dry on that account. It has 
no relation to dry regions, how- 
ever, but is dry because it is 
warmed by compression and has 
its capacity for moisture increased 
and its relative humidity lowered 
in consequence. It not only 
evaporates water and removes 
the snow with remarkable rapid- 
ity, but it dries out the wood in 
buildings. If situated where the 
foehn wind blows frequently, 
houses may become so dry that 
fires are greatly to be feared. 
Whole villages in Switzerland 
have been wiped out by fire which 

started at a time of foehn winds when the buildings were exceedingly 
dry. 

Thunderstorms. — Thunderstorms commonly develop locally in 
areas of low pressure and usually in the southern portion of the Low, 
where warm, humid air is slowly moving northward (Fig. 471). A day 
with such weather conditions is muggy and oppressive. As the ground 
is warmed during the day and the air above the ground is warmed, 
the humid air rises and cumulus clouds appear. In the latter part of 
the day these clouds become larger and darker, changing to cumulo- 
nimbus, sometimes rising in rolling, surging masses a mile or more 
above a level base. Finally, rain will fall from such clouds, and thunder 
and lightning are produced. Lightning (Fig. 470) is electricity gener- 
ated in air currents which swirl about while the vapour is rapidly 




Fig. 471. — Weather map showing a Low in 
eastern Canada in July with thunder showers 
(small arrows west of Boston) in its south- 
ern portion in the afternoon. (After U. S. 
Weather Bureau.) 



774 



COLLEGE PHYSIOGRAPHY 



condensing. Electricity gathers in the clouds until a spark passes 
from one cloud to another, or from the clouds to the earth. Thunder 







. , .... . . , — 




* ' 






" 


- 




























*j^ — ~ l ,J ' " 


.. 


•:■ - - —...J ; '\ 
























. ^ 








..— — ^ 


i 




















.. ~ ; f JI N 








* ■ •»••* 






y&e$pr 














„ . ,i -- 




B '''l''=w 
































Fig. 472. — Destruction by tornadoes. Upper view (W. L. Ikenberry) shows laths driven 
into house at Mt. Morris, 111. Lower view (A. W. Dunwiddie) shows house turned 
upside down near Janesville, Wisconsin. 



is the noise caused by the explosive effect of the rapid heating of the 
air by the passage of the electric spark of the lightning. The rolling 



STORMS 



775 



of thunder is merely the result of echoes among the clouds or among 
mountains. 

Thunderstorms may cover an area as large as several states, but 
are sometimes only a mile across. They travel eastward in the pre- 




WILLIAMS EN6BAVIHS CO., N.Y, 

Fig. 473. — The paths of five tornadoes in Nebraska and Iowa in 1913. (Condra.) 



vailing westerlies at the rate of 20 to 50 miles an hour, and in extreme 
cases the thunderstorm may last from 2 to 10 hours. The thunder- 
storm differs from the ordinary rain storm in having very heavy rain, a 
strong wind squall, and lightning. Hail not infrequently falls during 
severe thunderstorms. 



776 



COLLEGE PHYSIOGRAPHY 



ll 



4 



I '! 



A* 



ff.lg 



SK*M|-*_ 




Fig. 474. 



A house after one wall was blown out dur- 
ing a tornado. 



Thunderstorms occur almost every day in the equatorial belt of 
calms. They are also frequent in mountains, for there the air may 

rise on a hot day in the 
ascending valley wind, 
until clouds gather and 
develop into thunder- 
storms. In arid lands 
such storms are some- 
times accompanied by 
rapid condensation of 
vapour and by such 
heavy rain that they are 
spoken of as cloudbursts. 
In cool northern lands, 
such as Alaska, thunder 
and lightning rarely oc- 
cur, because the tempera- 
ture is too low. 

Tornadoes. — In the 
southern parts of low- 
pressure areas the con- 
ditions which cause thunder showers also sometimes cause tornadoes. 
These occur when the warm, humid, lower layers of air brought by 
southerly winds have cooler air wedged in below them by winds from 
the west. As the lower air rises, a whirl starts near the centre of 
the rising air, and winds blow with great force. Heavy rain, and often 
hail, fall ; and there is thunder and lightning. Tornadoes sometimes 
occur in groups as thunderstorms do, several of them often developing 
in the same general district on the same day, as is shown in Fig. 473, 
which indicates the courses 
of five parallel tornadoes 
across Nebraska and Iowa 
in 1913. _ 

The winds accompanying 
the whirl of a well-developed 
tornado are so strong as to 
overturn houses (Fig. 472), 
pick up heavy" objects and 
carry them long distances, 
uproot trees, and cut broad 
swaths through a forest. 
At the centre of the tornado 
whirl there is usually a 
funnel-shaped cloud. Here 
a partial vacuum may be formed, and, as it passes, the air inside 
of houses sometimes expands with sufficient force to blow out windows 
and even the walls of houses (Fig. 474). The path of greatest de- 




H IG. 475. — Distribution of tornadoes in the United 
States from 1794 to 1881. Darkest shade more 
than 35, medium shade 25 to 35, lightest shade 
less than 25. 



STORMS 



777 



struction of the tornado may be only a few score yards in width, and 
its length not more than a few' miles, for, after travelling that dis- 




Fig. 476.- 



Upper view shows a tornado near Mt. Morris, 111. (W. L. Ikenberry.) Lower 
view shows a waterspout near Martha's Vineyard, Mass. 



tance, the wind may become less violent and the tornado no longer 
destructive. In spite of the fact that it takes only a minute or two 
for a tornado to pass, its work of destruction is very complete, and 



778 COLLEGE PHYSIOGRAPHY 

this is one of the most dreaded and destructive forces of nature. In 
regions of frequent tornadoes it is not uncommon to dig excavations 
in the ground called " cyclone cellars," in which the people may seek 
shelter. 

Tornadoes are rather abundant in the Mississippi valley (Fig. 475), 
for there the ground is level and open, and it is easy for the warm, humid 
air from the Gulf of Mexico to come into close juxtaposition with the 
cooler upper air from the Great Plains and thus bring about the condi- 
tions which favour the formation of a tornado. They are rarely formed 
east of the Appalachians. 

Waterspouts. — When a tornado occurs over the sea or any other 
body of water, the partial vacuum in the centre allows the water to 
rise in a low cone, and the spiral winds may actually carry some of 
the water up in a swirling waterspout (Fig. 476). The main part of 
the waterspout, however, is formed of a hanging cloud, funnel-shaped, 
as in the tornado. 

Hurricanes and Typhoons or Tropical Cyclones 

Localization in the Tropics. — A type of storm similar to the cy- 
clones of the belt of prevailing westerly winds is the hurricane, or 
typhoon, or tropical cyclone. These develop in certain parts of the 
tropical zone, at certain definite seasons, and move northward or 
southward into the temperate zone. Such storms are known in the 
North Atlantic Ocean as hurricanes and in the North Pacific as typhoons. 
In the Bay of Bengal, Arabian Sea, and South Indian Ocean the name 
cyclone is commonly used for these storms. The West Indian hurri- 
canes are typical of the group. 

West Indian Hurricanes. — The West Indian hurricanes commonly 
originate in or near the West Indies and pursue a curved path which 
takes them northwestward toward or into the Gulf of Mexico and 
then northeastward along the Atlantic coast of the United States. 
Here they soon become larger and less violent as they pass into the 
temperate zone. The reason that they recurve eastward may be 
partly due to the earth's rotation, but is chiefly because of the influence 
of the prevailing wind. The westward movement of the cyclones is 
in the trade winds, and the eastward movement is in the prevailing 
westerlies. Sometimes, however, the West Indian hurricanes con- 
tinue northwestward to the coast of the Gulf of Mexico and even to 
the Great Lakes, instead of recurving to the east of Florida. 

The West Indian hurricane of September 8, 1900, was accompanied 
by waves and high water which advanced over the low coast of Texas, 
submerging a large part of the city of Galveston. This wave and the 
accompanying violent winds killed 5 or 6 thousand people and de- 
stroyed 20 or 30 million dollars' worth of property. The wave was 
chiefly due to the violence of the storm winds (Figs. 477, 478). 

Another destructive West Indian hurricane occurred in 1899, when 



STORMS 



779 




Fig. 477. 



The path of the West Indian hurricane which devasted Galveston on Sept. 
8, igoo. (After U. S. Weather Bureau.) 




Fig. 478. — Wreckage in the city of Galveston, Texas, after the hurricane in 1900. 



780 



COLLEGE PHYSIOGRAPHY 




Fig. 479. — Ideal diagram to show movement of air 
in a hurricane or tropical cyclone. (E. Hayden.) 



the centre of the storm traversed the entire length of the island of 
Porto Rico. Three thousand people lost their lives, most of them being 
drowned by the storm waves on the coast of Porto Rico, and the winds 
and heavy rain completely destroyed a coffee crop which would have 
been worth $7,000,000 if it had matured. 

Cause of Tropical Cyclones. — Tropical cyclones are whirls, similar to 
those of the temperate zones, associated with the rising of warm, humid 
air in the tropical zone (Fig. 479). They are much larger than torna- 
does, but smaller than most extra-tropical cyclones, and originate over 
the ocean rather than on the land because the humid air over the sea 
supplies much more vapour. The condensation of this vapour liberates 
latent heat, which helps to keep up the temperature of the air and 
causes it to rise still more rapidly. In the centre of a tropical cyclone 
there is rather low pressure, although it is far from approaching 

a vacuum. The wind 
blows violently toward 
this centre, often having 
sufficient force to over- 
turn trees and houses. 
Towns have been devas- 
tated and many vessels 
lost during these severe 
winds, as at Samoa in 
1889, when a number of 
ships were destroyed during a tropical cyclone. Along the south- 
eastern Atlantic coast of the United States the West Indian hurricanes 
are often very violent storms, and leave the coast strewn with wreck- 
age. During these hurricanes the otating storm may only move for- 
ward at the rate of 8 to 12 miles an hour, but the wind blowing in 
toward the centre may have a velocity of 60 to 100 miles an hour. 
The slow rate of progress and the high wind velocities of the hurri- 
cane form a striking contrast with the usually non-destructive cyclonic 
storm of the temperate zone, which commonly moves eastward at the 
average rate of 30 miles an hour, while the winds which accompany 
it rarely have a great velocity. During the hurricane at Galveston 
in 1900 the anemometer registered a wind velocity of 96 miles an hour 
and then broke to pieces. 

The time of most frequent occurrence of tropical cyclones in the 
northern hemisphere is late summer and early fall, because this is 
when the belt of greatest heat is farthest north. A study of all 
the West Indian hurricanes from 1876 to 191 1 shows that the largest 
number of hurricanes came during the month of September, — the 
number begins to increase rapidly during the month of August, and 
the hurricane season is pretty much over at the end of October (Fig. 
480). 

Close to the equator no tropical cyclones can occur, because the 
influence of the earth's rotation (Ferrel's Law, see p. 756) is slight. 



STORMS 



781 



Whirls can develop only when the wind is turned to one side so as to 
start a spiral movement around the centre of rising. These great 
atmospheric whirls can start only in the hot belt when it has migrated 
far enough north or south to reach latitudes where the deflective force 
is sufficient to develop the whirl. It is for this reason that most of 
the West Indian hurricanes occur in August, September, and October, 
when the belt of calms is north of the equator. 

Hurricane Warnings. — Because of their violence and destructive- 
ness, it is of the greatest importance to issue warnings of the coming of 
tropical cyclones. With the increase in number of the stations for 
meteorological observation in the Lesser Antilles and Porto Rico and 
Cuba, and with additional observations upon vessels reporting regu- 




95° Longitude 90' 



Fig. 480. — The paths of West Indian hurricanes. (Fassig.) 

larly by wireless telegraph, the warnings of hurricanes are every year 
more satisfactory. This is of vast importance in connection with the 
greatly increased shipping which will traverse the hurricane belt of 
the Caribbean Sea and the West Indies after the opening of the 
Panama Canal. 



References to Literature 

Cleveland Abbe. The Progress of Science as Illustrated by the Development 
of Meteorology, Smithsonian Report for 1907, No. 1836, pp. 287-309. 

W. J. van Bebber. Die Wettervorhersage, Stuttgart, 1898, 219 pp. 

Frank H. Bigelow. Storms, Storm Tracks, and Weather Forecasting, Bull. 
20, U. S. Weather Bureau, 1897, 87 pp. 

A. T. Burrows. The Chinook Winds, Journ. Geog., Vol. 2, 1903, pp. 124-136. 

W. M. Davis. The Temperature Zones, Journ. School Geog., Vol. 1, 1897, 
pp. 139-143- 



782 COLLEGE PHYSIOGRAPHY 

O. L. Fassig. Hurricanes of the West Indies, Bull. X, U. S. Weather Bureau, 

Washington, 1913. 
William Ferrel. Popular Treatise on the Wind, New York, 1889; Recent 

Advances in Meteorology, Rept. Chief Signal Officer, Part II, 1885; see 

also Professional Papers of the Signal Service, No. VIII, 1882, and No. 

XII, 1882. 
J. P. Finley. Tornadoes, New York, 1887. 
E. B. Garriott. The West Indian Hurricane of September 1-12, 1900, Nat. 

Geog. Mag., Vol. n, 1900, pp. 384-392. 
Albert Gockel. Das Ge witter, Koln, 1905, 204 pp. 
A. W. Greely. Hurricanes on the Coast of Texas, Nat. Geog. Mag., Vol. 11, 

1900, pp. 442-445. 
G. Guilbert. Nouvelle Methode de Prevision du Temps, Paris, 1909, 343 pp. 
H. H. Hildebrandsson and Teisserenc de Bort. Les Bases de la Meteorologie 

Dynamique, 2 vols., Paris, 1898, 1907. 
Richard Inwards, Weather Lore, London, 1898, 233 pp. 
A. McAdie. The Clouds and Fogs of San Francisco, San Francisco, 191 2, 

106 pp. 
W J McGee. The Lessons of Galveston, Nat. Geog. Mag., Vol. 11, 1910, 

PP- 377-383- 
C. F. Marvin. Anemometry, Circular D, Instrument Division, U. S. Weather 

Bureau, Washington, 1907. 
W. L. Moore. Storms and Weather Forecasts, Nat., Geog. Mag., Vol. 8, 1897, 

pp. 65-82; ibid., Vol. 9. 1898, pp. 255-305. 
E. R. Van Cleef . Is There a Type of Storm Path ? Monthly Weather Review, 

Vol. 36, 1908, pp. 56-58. 
U. S. Weather Bureau. Pilot Charts ; Storm Bulletins, etc. 



CHAPTER XXVII 
CLIMATE 

Weather and Climate 

The difference between weather and climate is that weather includes 
the conditions of temperature, pressure, wind, clouds, and rain from 
day to day, and climate is the average of these weather conditions. 
We speak of having rainy weather when there is precipitation on 
several days in succession, but we should not speak of having a rainy 
climate in a region if it rained on three days in the month and were 
fair on the remaining twenty-seven. We properly say that certain 
parts of the tropical zone have a rainy climate. By this we mean that, 
although the weather on some days is clear, and although it does not 
rain every day, it is rainy on more days than it is pleasant. The aver- 
age condition, then, or the climate, is rainy. 

There is general popular belief that climates are changing at the 
present time. Accurate meteorological records show that there is no 
basis whatever for this belief, at least so far as the last century and a 
half are concerned. There are, however, well-established slight oscil- 
lations of climate, of which the 35-year periods of Bruckner are best 
known. There may possibly be shorter periods related in some 
way to the occurrence of sun spots. There are, of course, climatic 
variations in the remote geological past, such as (a)' those cold periods 
which resulted in the glacial and interglacial oscillations of the Pleis- 
tocene (p. 297) and the glacial periods in Southern Africa, India, 
Australia, and South America in the Permian (p. 299), and in other 
parts of the earth at still other periods ; (b) those dry periods when salt 
and gypsum were formed, as in New York during the Silurian, and 
(c) those moister and perhaps warmer periods when coal was formed 
in various parts of the earth. 

Some of the more important kinds of climate are dry, hot, desert 
climates ; damp, equable, marine, and littoral climates ; continental 
climates ; mountain climates ; and monsoon climates. The hot, rainy 
climate is quite characteristic of the equatorial belt of calms, and the 
extreme or continental climate of interiors of continents. 

Climatic Zones 

The Five Zones. — The five zones are dependent upon the inclina- 
tion of the sun's rays, as has already been indicated in connection 
with the discussion of temperature (p. 725). The distribution of 

783 



7 8 4 



COLLEGE PHYSIOGRAPHY 



solar heat between the equator and poles results from the different 
angles at which the sun's rays reach the earth in different latitudes. 
There has consequently arisen a division of the earth into five climatic 
zones, — two polar or frigid, two temperate, and one torrid or tropical 
zone (Fig. 481), or, more properly, two cold zones, two intermediate 
zones, and one tropical zone. 

The boundaries of these zones, as usually outlined, follow the 
parallels of latitude, but the irregularity of the actual boundaries is 
indicated in Fig. 481, where the white, which indicates the Arctic 
or polar zone, is also represented in the temperate and tropical zones, 
and likewise the conditions of the temperate and tropical zones extend 
outside their respective boundaries. This is because there are a 
number of influences which result in an extension of low temperatures 

into the equatorial region 
and of high temperatures 
into the temperate zones. 
Variation in Zones ac- 
cording to Altitude. — It 
is well known that the 
climate of highlands and 
mountains is cooler, on 
the average, than that of 
neighbouring lowlands, 
and one of the important 
causes of irregularities in 
the boundaries of the 
zones is altitude. On 
the isothermal charts of 
the United States (Fig. 
482), it is apparent that 
the isotherms are bent 
towards the equator in 
crossing highlands. This 
is well seen in the mean isothermal chart for July, where the line pass- 
ing through places having an average -July temperature of 70 is di- 
verted southward by the Rocky Mountains, so that instead of running 
east and west, as it does on the plains, it runs north and south from 
North Dakota to New Mexico. On the Pacific coast where winds 
from the equable ocean blow upon the slopes of the north-south 
mountains the influence of altitude is also apparent, for the isotherms 
extend north and south instead of east and west. This is because 
the climate is warm and equable near the coast and the temperature 
falls as the mountain slopes are ascended so that the climate of the 
mountain tops is very much cooler. 

Variation in Zones under the Influence of Water. — Another cause 
for modification of climate is distance from the ocean and from other 
large bodies of water, like the Great Lakes. Islands in the ocean, 




Fig. 481. — Diagram to show how altitude and other 
features cause the conditions of the polar zones to 
exist in the tropical zone, etc. 



CLIMATE 



785 



like Bermuda, have cooler summers and warmer winters than the 
mainland in the same latitude.. Likewise seacoasts, especially on 
the windward sides of continents, have less extremes of heat and cold 




Fig. 482. — Isothermal maps of the United States for January (upper) and for July (lower). 

and are said to have a more equable climate than points in the interior 
of continents. This is well shown in Figs. 428 and 482, where we 
may compare the temperatures of the state of Washington, Minnesota, 
and Nova Scotia, which are all in about the same latitude. 

3E 



786 COLLEGE PHYSIOGRAPHY 

In January the region near Puget Sound in Washington has a mean 
temperature of 30 to 35 , while North Dakota has a mean tempera- 
ture of from 5 above to 5 below zero, and Nova Scotia has a mean 
temperature of about 20 . The much higher winter temperatures in 
Washington and Nova Scotia are due to the fact that these regions 
are influenced by the ocean, while Minnesota, in the interior of the 
continent, has much lower temperatures because of its distance from 
the water. 

The isothermal charts of the world furnish many illustrations of the 
same sort of thing, and there is always a greater range of temperature 
in the interior of the continent and a smaller range of temperature 
near the ocean. By mean annual range we mean the difference be- 
tween the mean temperatures of the warmest and coldest months. 
If we contrast Asia, the Pacific Ocean, and America, for example 
(Fig. 497), we find that the mean annual range of temperature is 120 
in Siberia, 20 at the Aleutian Islands, and 8o° west of Hudson Bay. 
Likewise in the Atlantic Ocean we find that the mean annual range 
of temperature for Iceland on the Arctic Circle is exactly the same 
as that of Mexico on the Tropic of Cancer and of the British Isles 
halfway between. The mean annual temperature of Iceland is, there- 
fore, very mild for its northerly position, although in this case ocean 
currents also influence the temperature of this island. In the southern 
ocean, where there is relatively little land, the mean annual range of 
temperature is very slight indeed. 

Variation in Zones according to Winds. — The best illustration of 
the variation in zones because of the influence of winds is found 
where winds blow from water upon land, as in the northwestern United 
States and Europe (Figs. 428 to 482). The Pacific and Atlantic 
oceans are warmer in winter and cooler in summer than the continents, 
and the air over them in the prevailing westerlies is modified accord- 
ingly. The eastward movement of this air thus moderates the cold 
of winter and the heat of summer on the land. Accordingly, agri- 
culture thrives in Norway and Sweden, and large cities are found 
far north, while the eastern coasts of Labrador and Baffin Land, in 
the same latitude in eastern North America, are frigid and almost 
uninhabited. The western coast of North America in the same lati- 
tude in Alaska has the equable climate of the British Isles and 
the Scandinavian Peninsula. London is in the same latitude as 
southern Labrador, and St. Petersburg is in about the same latitude 
as southern Greenland. Because of the influence of onshore winds, 
the temperature at San Francisco in January is the same as that at 
Charleston, S. C, although the latter is 5 farther south; and the 
temperature of San Francisco in July is the same as that of Halifax, 
which is 6° farther north. 

The Influence of Ocean Currents. — Just as convectional currents of 
water in a tea-kettle carry the temperature of the hot surface of the 
stove to the cooler water at the top of the kettle, so the ocean currents 



CLIMATE 787 

and drifts bear water from the warm tropical zone to the cool temperate 
and polar zones ; and the cold water of the polar zones is carried into 
the temperate zones by the opposite currents. The temperature of 
the wind which is blowing over these ocean currents is increased or 
decreased, and as these winds blow upon the lands they carry with 
them some of the warmth or the cold which has been brought by the 
ocean currents from other zones. 

In the North Atlantic Ocean this influence of the currents is espe- 
cially notable (Figs. 416, 428, and 450). The great northward bend of 
the isotherms west of the British Isles and Scandinavia shows the in- 
fluence of the warm westerly winds (Fig. 428) in winter, but in summer, 
when the surface water is warmed by the sun, this influence is less 
noticeable. The Gulf Stream drift carries part of the isotherms 
north, and the equatorial return current carries their continuations 
south. The opposite condition is seen near Newfoundland and Nova 
Scotia, where the cold Labrador Current bends the isotherms toward 
the equator and the Gulf Stream crowds them toward our east coast. 
The isotherms are, therefore, crowded together on the American coast 
and spread apart in fan-shape on the coast of Europe, resulting in 
much greater difference of temperature in a short distance in eastern 
America than in western Europe. There is also similar influence of 
ocean currents on the isotherms along the west coasts of the United 
States, South America, and South Africa. 

Variation in Zones as a Result of Local Topography. — The eastern 
part of the state of Washington in the United States has hotter sum- 
mers and colder winters than the western part of the same state. 
This is because the Cascade Mountains cut off the winds from the ocean 
which give an equable climate to the Pacific slope of the state of Wash- 
ington, but keep it from the eastern part of the state. Throughout the 
world mountain barriers have a similar influence on the climate of 
places in their lee. Hills and valleys have slight local effects on cli- 
mate, chiefly by shutting off winds which may carry warm or cold tem- 
peratures. 

Other illustrations of the same principle are found in the subtropical 
climates of Italy, southern Spain, and France. The waters of the 
Mediterranean are warm, and the Alps, Pyrenees, and other moun- 
tains shut off the cold north winds. They also prevent the warm 
southerly winds from carrying their warmth any great distance away 
from the Mediterranean. Other factors than topography also in- 
fluence the mild climate of the Mediterranean countries, but oranges 
and palms grow in Italy in the latitude of Boston and New York, 
in the eastern United States. In the latitude of Italy, the eastern 
United States are visited by killing frosts for several months in 
the year, and frosts would be fatal to the orange trees of Italy if 
the protective influence of the Alps on the north did not act as 
an effective barrier against the incursions of severe cold from the 
north. Other influences are the lack of a near-by source of severe 



788 



COLLEGE PHYSIOGRAPHY 



cold to draw on and the different cyclonic and anticyclonic control 
to the north. 

Resulting Irregularity of the Five Zones. — As a result of the in- 
fluence of (a) altitude, (b) water, (c) winds, (d) ocean currents, and 
(e) local topography, the boundaries of the five zones are exceedingly 
irregular. Accordingly there have been various suggestions as to 
different means of drawing the boundaries of the zones. One is that 
the boundaries between the tropical and temperate zones should 
follow the limits of the growth of palm trees, and the boundary be- 
tween the temperate and polar zones should follow the limits of the 
growth of wheat. Another suggestion is that (a) the annual isotherms 




Fig. 483. — Supan's suggestion as to zones. (After Ward.) 



of 68° F., and (b) the temperature of 50 F. for the warmest month, 
should delimit the zones (Fig. 483). It has also been suggested that 
the temperate and tropical zones should be separated, not by the 
Tropics of Cancer and Capricorn, but by the northern and southern 
boundaries of the trade wind belts. None of these substitutes is a 
sufficient improvement to be generally adopted, and the boundaries 
which follow the tropics and Arctic and Antarctic circles are generally 
used. 

Climate of the Belt of Equatorial Calms 

The Hot, Rainy Belt. — The conditions of temperature, humidity 
and rainfall, and the absence of regular winds give the belt of calms 
a distinctive climate. They have a notable influence on life there. 
The climate of the belt of calms, however, is not characteristic of the 
whole tropical zone, and in the following pages it will be seen that the 
climates of the world are best described in relation to the atmospheric 
circulation rather than the zones of heat. 



CLIMATE 



789 



90° 



8.0° 



The climate of the doldrums (Fig. 484) is hot because of the great ab- 
sorption of heat by the earth under the direct rays of the sun at the 
times when they are vertical, and the consequent heating of the air 
through radiation, conduction, and convection. The air contains a 
great deal of moisture, and, as 

it rises, the water vapour is con- J. F. M. A. M. J. J. A. S. 0. N. D. J. 

densed into rain as soon as a 
sufficient elevation is reached. 
The belt of calms, therefore, has 
a very rainy climate (Fig. 485). 

One striking characteristic of 
the climate of the doldrum belt 
is its monotonous uniformity. 
The heat increases rapidly after 
sunrise, and clouds soon form. 
In the afternoon these often de- 
velop into violent thunderstorms, 
from which heavy rain falls. 
Radiation during the night is 
not sufficient to cool the humid 
air much. Because of the up- 
ward movement of the air there 
is an absence of steady wind dur- 
ing both night and day, and sail- 
ing vessels are often becalmed 
for days at a time. These con- 
ditions are repeated regularly. 
On the land the temperatures 
during the day are higher than 
over the water, and sea-breezes 
sometimes blow along the coast 
because of the differences of 
temperature there. 

Many parts of the doldrum 
belt may be spoken of as hav- 
ing a single hot rainy season. 
Some parts of it, however, be- 
cause of the shifting of the heat 
equator, may be said to have 
two rainy seasons and two less 
rainy seasons. 

Backwardness of Inhabitants. 
— Because of the heavy rainfall and the warmth, which permits trees 
to grow throughout the year, there are dense forests on the land. 
Within these forests the air is reeking with moisture. The climate 
is so warm and damp that it is difficult to work. It is so hard to 
clear away the vegetation that the task is not readily undertaken. 



UU" ■ ■ . - ... ... - 


\ \ \/ 


W\ iv 


7/TVV 

*no w 1 4 w 


1 f 

Rno — . 1 



70° 



60° 



J. F. M. A. M. J. J. A. S. 0. N. D. J. 



50° 



Fig. 484. — Seasonal range of temperature in 
the tropics. (Ward.), W — Wadi Haifa; 
N — Nagpur ; A - — Alice Springs ; H — 

. Honolulu ; J — Jamestown, St. Helena. 



79° 



COLLEGE PHYSIOGRAPHY 



This is especially true because the forest plants themselves yield 
abundant food with little labour. On this account the people who 
live in the tropical forest of the belt of equatorial calms are apt to 
depend directly upon nature for their food; and most of the in- 
habitants of the equatorial region have made little progress toward 
civilization because they have little ambition for improving their 
condition. 

Climate of the Trade Wind Belts 

Rainy Windward Coasts. — In the trade wind belt there is a differ- 
ence between (a) rising coasts which face the direction from which the 

trade winds come, (b) 
the lowland areas, and 
(c) the lee coasts. 

In the regions north 
and south of the belt of 
calms the trade winds 
are blowing towards a 
warmer region. They 
are, therefore, con- 
stantly evaporating 
water from the surface 
of the ocean and from 
streams and lakes on 
the land, because the 
warmer the air becomes, 
the more vapour it can 
contain. Indeed, so 
much fresh water is re- 
moved from the surface 
of the sea in the trade 
wind belts that the sea 
water becomes more saline in these regions. When the trade winds blow 
over rising land, the coast which faces them causes the air to rise. It 
is, therefore, cooled and has its vapour condensed as abundant rainfall. 
This is well shown on the eastern side of the Isthmus of Panama and 
the northeastern coast of Brazil (Fig. 486), which receive a heavy 
rainfall from the northeast trades. Likewise the southeastern coast 
of Brazil receives heavy rainfall from the southeast trades. East- 
facing coasts throughout the world in the trade wind belts are therefore 
usually rainy. Parts of the East and West Indies (Fig. 486), north- 
eastern Australia (Fig. 488), and southeastern Africa (Fig. 487) like- 
wise have heavy rains because the trade winds blow upon them from the 
sea. These places also have a tropical forest, resembling that of the 
belt of calms. The Hawaiian Islands in the Pacific Ocean have heavy 
rains on the eastern side and a dry climate on the opposite side. This 
is because they lie in the belt of northeast trades, and the same thing 




Fig. 485. — Contrast of summer (upper) and winter 
(lower) as to distribution of tropical rainfall in rela- 
tion to the shifting of the heat equator (stippled). 



CLIMATE 



791 




792 



COLLEGE PHYSIOGRAPHY 




,*k N^gofrTTy^sr TRADES 



■<^ 



EDDesert 

EUi/g/it Rainfall 

^Moderate 
Heauy " 

Very Heauy » 



r^ 



81)' Longitude !«WestfrointU°Gr(*nw: 



is characteristic of all mountainous islands in either of the trade wind 
belts. 

Desert Trade Wind Belts. — The largest and best known desert in 
the world is the Sahara in northern Africa. The cause of the Sahara 
is the blowing of the northeast trades across northern Africa. 
Throughout the other portions of the trade wind belts of the world, 
on land, conditions of aridity are far more common than rainy climates, 

and in fact the trade winds are 
the most important cause of 
deserts. 

The trade winds are ready to 
take up vapour in passing over 
the land, as they are in passing 
over the ocean, because the air 
is moving towards the warmer 
equatorial region and having 
its capacity for moisture in- 
creased. In contrast with the 
trade winds on the ocean, how- 
ever, those on the land can ob- 
tain so little moisture that they 
become very dry winds, with 
vapour rising into them wher- 
ever there is water to evaporate. 
This leaves so little water for 
plants that the land becomes 
a desert. It should not be 
thought, however, that it never 
rains in the Sahara, for the oc- 
casional mountains and hills 
there cause the trade winds to 
rise and precipitate moisture on 
the windward slopes. Thus 
rain sometimes falls in parts of 
the Sahara. The water is 
quickly evaporated, however, 
and rivers which flow down the 
desert slopes of the mountains wither a short distance from the base. 
Most of the Sahara has less than 10 inches of rainfall per year, and the 
Mohave desert of Arizona has a rainfall of less than 2 inches a year. 

These conditions cause a broad belt of arid and desert country both 
north and south of the equator. It extends completely across each of 
the continents in the trade wind belts, except on the east-facing coasts 
where the trade winds bring rain and the eastern slopes of mountains 
in the interiors of the continents. Such deserts are found in Australia 
(Fig. 488), South Africa (Fig. 490), South America (Fig. 487), and 
southwestern United States (Fig. 496) and Mexico. The largest desert 




Fig. 487. — The desert trade wind belts 
South America and Africa. 



CLIMATE 



793 



tract is in northern Africa and southwestern Asia. From the Atlantic 
Ocean near the Cape Verde Islands a series of deserts extend east- 
ward through Africa, Arabia, Persia, and southwestern Asia. The 
great Sahara is a part of this belt. 

In these deserts life is very different from that in the tropical forest. 
There are not many species of plants capable of adapting themselves 
to life among such unfavourable conditions, and even these are scattered. 
The desert is, therefore, a barren open country avoided by animals and 
by man and, consequently, among the most sparsely settled parts of 
the world (Fig. 489). 

Desert weather is nearly always dry, the sky is usually cloudless, 
and the prevalent winds often blow the sand about and even cause 
dangerous sand storms. In the temperate zone deserts, the days may 
not always be as warm as in the tropical zone, but even in the tem- 
perate zone they become very hot indeed. The desert of southern 
Arizona, although far north of the 
Tropic of Cancer, is sometimes so 
warm that, the thermometer rises 
to about ioo° or 120 in the shade. 
The highest air temperature ever 
" officially " recorded, said to be 
1 54 F., was in northern Africa in 
the central Sahara. Because of 
the lack of moisture in the air, 
radiation proceeds with great 
rapidity after the sun has set and 
the ground and the air cool so 
quickly that it is often necessary 
to keep covered with blankets at 
night in places where, during the 
day, it may become insufferably hot. 

Oases in Deserts. — Any area in a desert where water may be ob- 
tained is spoken of as an oasis. Oases are usually either scattered 
springs or places where streams descend from mountains and flow 
out upon alluvial fans. The typical oasis at a spring is illustrated 
by the number of isolated oases of this character in the Sahara, while 
the oasis upon a river is well seen at such cities as Merv and Bokhara, 
southeast of the Caspian Sea. Another type of oasis is represented 
by the narrow strip of country along the Nile in Egypt, with the Libyan 
and Nubian deserts on either side. These two deserts are really parts 
of the Sahara, and the oasis of the Nile is made possible by the water 
which flows across the desert to the Mediterranean. 

The importance of the oases is, of course, the opportunity to obtain 
water for men and animals as they travel across the desert, or to main- 
tain permanent homes in places where there is a water supply. In 
some respects oases seem to excel humid regions, probably because in 
places where water is obtainable the hot climate makes it possible to 




Fig. 488. 



The desert trade wind belt 
in Australia. 



794 



COLLEGE PHYSIOGRAPHY 



raise many crops each year. In any event, the large oases of the Nile 
and Euphrates rivers supported civilization long before there were 
civilized people in any part of Europe, and, at the oases in the south- 
western part of the United States, the Pueblo Indians developed a 
civilization far ahead of that of the aboriginal inhabitants of the rest 
of this country. The small oases, surrounding a single spring and 
with the date palm as the chief sort of vegetation, support a small 




Fig. 489. — The world's belts of sparce population. The lined areas show the regions which 
had an average of over 2\ persons to the square mile in 1905. White areas with less 
population are : (a) dry, as in Sahara, western United States, etc. ; (b) cold, as in 
Siberia, northern Canada, etc.; or (c) too hot and damp, as in part of Brazil. (Jef- 
ferson.) 

permanent population, as well as furnishing stopping places for cara- 
vans which must obtain water as they cross the desert. 

Dry Lee Coasts. — The dry climate of the leeward sides of continents 
and islands in the trade wind belts is the result of the fact that the 
wind does not blow from the ocean. This is the case on the western 
side of the Hawaiian Islands, on the west coast of Mexico, and the 
west coast of South Africa. The trade wind is there blowing off the 
land, and is dry. 

Savanna Belts 



Wet and Dry Seasons. — The savanna belts are located between the 
rainy belt of calms and the trade wind deserts, where the lands in 
each hemisphere have a region with alternate dry and wet seasons. 
This climate is due to the migration of the heat equator and the shifting 
of the borders of the belts of calms and the trade winds. The belt of 
calms moves northward in the hot season and the savannas of the north- 
ern hemisphere then have heavy rain (Figs. 485 and 490). The belt 
of calms migrates back toward the equator, and these savannas then 
come under the influence of the dry trade winds. In the southern 



CLIMATE 



795 



hemisphere the savannas similarly have their rainy season in their sum- 
mer, when the doldrums are south .of the equator. 

Plants and Animals of the Savannas. — The absence of trees in the 
savannas and the great abundance of grass marks them as gradation 
areas between the belt of calms with its heavy forest and the arid 
portions of the trade wind belts, with their sparse vegetation. During 
the rainy season the savannas have copious rainfall, and vegetation 
springs up and grows rapidly, but during the dry season the vegeta- 
tion withers because the 
ground is parched by 
evaporation. The ab- 
sence of trees is due 
to the severity of the 
drought, and the pres- 
ence of grass is the re- 
sult of the fact that the 
grass grows rapidly dur- 
ing the wet season and 
is able to survive a 
period of drought. 

The name savannas is 
not the only one ap- 
plied to the savanna 
belts, the name llanos 
being used in Venezuela 
and Columbia, campos 
in Brazil, downs in Aus- 
tralia, and park lands 
in Africa. In contrast 
with the absence of 
animal life in the 
desert, the savannas 
support great numbers of plant-eating animals and flesh-eating mam- 
mals which prey upon them. 

The Future of the Savannas. — There is no doubt that the savannas 
are destined to be the most productive and populous lands in the 
tropics. Agriculture is favoured by the absence of forest, and the sea- 
sonal drought makes it necessary to provide for that season, just as 
we must provide for the cold winter in the temperate zone. The 
inhabitants of the savannas are, therefore, forced to be industrious 
and thrifty ; and in Africa the negroes of the savanna belts raise crops 
and cattle and are the most civilized natives on the continent. 




Fig. 490. — The distribution of the savannas in Africa. 
Forested belts along rivers interrupt the general 
grassy condition. 



Monsoon Climates 

The influence of the monsoon winds in India produces a climate 
with three well-defined seasons, — (a) the hot season, (b) the rains, 
and (c) the cool winter. Southeastern Asia and most other monsoon 



7Q6 



COLLEGE PHYSIOGRAPHY 



countries have only two seasons, one cold or cool and dry, the other 
damp and warm. 

The Hot Season in India. — The hot season of India lasts from April 
to June. At that time the hot, dry air of the northeast monsoon, 
blowing over the land, may cause the temperature to rise above ioo° 
in the shade. Toward the end of the hot season in June, the north- 
east monsoon ceases to blow, because the temperatures of the land 
and ocean are approaching equality and the barometric gradient is 

annulled. A calm therefore ensues, 
and at this time the heat is almost 
suffocating (Fig. 491). 

The Rains. — The season known 
as the rains is the time of the sum- 
mer monsoon, when the southwest 
monsoon blows from the Indian 
Ocean to the land. Clouds ap- 
pear, rain falls almost every day, 
and for a few weeks vegetation 
flourishes. A short period of calm 
follows the summer monsoon. 

The Winter. — The winter is 
the time of the northeast monsoon, 
and the heat in the preceding period 
of calms is relieved by the flowing 
of cool air from the interior tow- 
ard the sea. The winter mon- 
soon becomes established early in 
October, and the air is then clear 
and cool, except during the period 
of winter rains in northern India. 
By January it is necessary to heat 
houses with fires in many parts of 
India. A sort of spring follows 
this winter during February and 
March, and vegetation may spring 
up, but it is soon withered by the 
scorching heat of the hot season. The real growing season comes 
later with the summer rains of the southwest monsoon. 

The Heaviest Rainfall in the World. — The rainfall on the mountain 
slopes of India at the base of the Himalayas is the heaviest in the world. 
At some points there is a rainfall of somewhat less than 500 inches, 
that is, an amount which would form a layer 40 feet deep if it remained 
where it fell. Of this amount about two-thirds comes during the five 
summer months ; and on a single day there maybe as much as 40 inches 
of rain, or more than falls in many parts of the United States in a year. 
Indeed, this rainfall is so heavy that on some of the mountain slopes 
in India the soil is completely removed by stream erosion. 




Fig. 491. — The seasonal distribution of 
rainfall in India. Upper map indicates 
the winter with outflowing or northeast 
monsoon shown by arrows. Lower 
map shows the rains during the sum- 
mer or southwest monsoon. 



CLIMATE 



797 



The Climates of Intermediate or Temperate Zones 



80° 



70° 



-60° 



J. F. M. A. M. J. X A. S. 0. N. D. J 



Intermediate Zones. — The so-called temperate zones are far from 
temperate in their climatic character, and it is much better to speak of 
them as the intermediate zones. The 
differences from north to south occur 
chiefly in connection with (a) tempera- 
ture, (b) rainfall. 

Variation from North to South. — 
Because of the varying inclination of 
the sun's rays there is a notable in- 
crease in the warmth of the intermedi- 
ate zones from the polar to the tropic 50° 
regions (Fig. 492). Near the tropics, 
however, there is no very decided dif- 
ference between the temperature of 
summer and winter ; but away from 
the tropics the summer and winter 
temperatures are so different that the 
year is naturally divided into four 
seasons of spring, summer, autumn, 
and winter. In the vicinity of the 
Arctic and Antarctic circles the winters 
are so extremely cold and the summers 
are so cool that the climate may be 
spoken of as subarctic. 

The subarctic portions of the inter- 
mediate zones have few trees and, at 
the extreme limits, no trees whatever. 
Where there are few trees, they are 
stunted individuals, and in the region 
near Hudson Bay, for example, there 
may be full-grown trees which are only 
2 or 3 feet in height. In certain of the 
lands near the polar circles, no agri- 
culture is possible ; and there are scarcely 
any human inhabitants except where 
mining camps, such as those of Alaska 
and the Klondike, or fishing towns along 
the seacoast, like some in Norway and 
Siberia, result in small centres of popu- 
lation. 

With the increase of temperature 
toward the tropics there is a change 
of vegetation. This is well illustrated in North America. There 
the treeless tundra of the north merges southward into a forest 
belt, and vegetation becomes more and more luxuriant in southern 




J. F. M. A. M. J. J. A. S. 0. N. D. J. 



Fig. 492. — Seasonal range of tem- 
perature in the intermediate or 
temperate zones. (Ward.) S. I. 
— Solly Isles ; P. — Prague ; C. — 
Charcow; S. — Semipalatinsk; K. — 
Kiakhta ; B. — Blagoweshtchensk ; 
Sa. — Sakhalin ; T. — Thorshavn ; 
Y.— Yakutsk. 



798 COLLEGE PHYSIOGRAPHY 

Canada and in the United States, so that the grains and temperate 
zone fruits are raised. In southern United States near the tropics the 
climate is so warm that it may be spoken of as subtropical. In this 
warm belt the plants useful to man produce cotton, sugar, oranges, 
and, near the warm ocean, bananas, pineapples, and cocoanuts. 

The differences in rainfall between the equatorial and the polar 
margins of the intermediate or temperate zones are related to the 
temperature as well. There is moderate rainfall throughout most of 
the temperate zones, but it decreases toward the polar zone because 
cool air has less capacity for vapour than warm air. The rainfall also 
decreases towards the tropics, because of the lessened capacity for 
moisture in the descending air of the arid horse latitude belts. 

Steppes. — Such regions as Spain, Italy, and Greece in Europe, 
and southern California in the United States, are in the horse latitude 
or subtropical belt. They grade on the one hand into the desert 
trade wind belts and on the other into the moist climate of the 
mid-temperate zone and may be called the belts of steppes. Not all 
of the horse latitude belts are arid, however, Florida, for example, 
having abundant rainfall because it projects into the Gulf of Mexico 
and the Atlantic Ocean, and has nearly all its winds blowing from 
over the water. Some parts of the horse latitudes, however, are 
true desert. 

Steppes are similar to savannas in having a limitation of plant 
growth because of the climate. The borders of the horse latitude 
belts have a migration of wind and of climatic conditions, some por- 
tions being reached by the prevailing westerlies when they shift 
southward in the winter of the northern hemisphere, bringing with 
them snow and rain. The converse applies in the southern hemi- 
sphere. The steppes are dry in summer, however, when they are in 
the belt of the descending air of the horse latitudes, or the northern 
edge of the drying trade winds. On this account it is necessary to 
practise irrigation in order to carry on agriculture, chiefly because 
the regions of steppes are apt to have their rainfall in the wrong 
season of the year. Italy, by way of illustration, has rainy winters 
and dry summers. Therefore the Italian farmers irrigate their crops, 
which are growing in summer at the time when the moisture is deficient. 
Steppes are usually too dry for trees, but grass grows upon them and 
the curing of this grass to natural hay during the warm, dry summer 
makes good ranges for cattle. The Great Plains in Texas furnish an 
illustration of steppes with a grazing industry. 

Variation from West to East. — There are likewise variations in 
the climate of the intermediate or temperate zones from west to east. 
These are also dependent on (a) temperature, and (b) rainfall, being 
directly determined by the fact that the prevailing winds of the in- 
termediate zones are from the west. These variations are best con- 
sidered by a discussion of (a) west coasts, (b) regions near meridional 
mountains, (c) the interiors of continents, and (d) east coasts. 



CLIMATE 



799 



West Coasts. — The west coast of the United States, from northern 
California to Puget Sound, and the, northwest coast of Europe have a 
humid, equable climate because of the warm damp winds which blow 
from the ocean against these west-facing coasts (Fig. 493) . Ireland, on 
the northwest coast of Europe, for example, is known as the Emerald 
Isle, because the damp air keeps the grass always green. It never 
has droughts, during which the grass is parched and turns brown, 
as in the eastern part of the United States. 

The heaviest rainfall in the United States is in the western parts 
of Washington and Oregon, and, in certain places, amounts to more 
than 100 inches a year. This is because the vapour in the damp air 
from the ocean 



is precipitated 
during the ris- 
ing of the pre- 
vailing wester- 
lies over the 
mountain slope. 
The winter of 
cities like Seat- 
tle and Port- 
land is not a 
cold season, as 
in eastern and 
central United 
States, but 
rather a damp 
and cool sea- 
son. This is 
because the pre- 
vailing wester- 
lies are strongest 
in winter and 
because there 
are more storms 

then. There is practically no precipitation in the form of snow be- 
cause of the warm temperature near the ocean. There is heavy rain- 
fall on the southwestern coast of Chile for the same reasons. Northern 
Chile and southern California, however, have an arid climate, even 
on the seacoast, because they are not well within the belt of the pre- 
vailing westerlies, and therefore have the characteristic conditions 
of the horse latitudes and trade wind belts. The coast of Norway and 
the British Isles has a prevalence of rain and cloudy weather during 
the winter, similar to that in Washington, Oregon, and southern 
Chile. 

Regions near Meridional Mountains. — The heavy rainfall of 
eastern Norway, Scotland, Wales, and Ireland is not limited by the 




p#] Arid 

Light.Rainfall 
Moderate * 
Heai 



Fig. 493. 



The heavy rainfall of the west coast of North America 
in the region of prevailing westerlies. 



8oo 



COLLEGE PHYSIOGRAPHY 



mountain crests, as is the case in Washington, Oregon, and Chile. 
This is because these American mountains are much more lofty and 
are continuous, while the highlands of western Europe are low and 
broken. Accordingly the winds on the west coast of Europe are able 
to carry vapour far inland and even across the plains of Russia into 
western Asia. Europe, therefore, is well watered, since most of it 
lies north of the horse latitude belt. It contains no desert and, except 
on the plateau of Spain and near the shores of the Caspian, no arid 
region (Fig. 494). This explains the extensive agriculture of Europe. 




Fig. 494. — Rainfall map of Europe. 

The western part of North America forms a decided contrast, be- 
cause the lofty, continuous Cascade and Sierra Nevada ranges pre- 
vent the wind from carrying vapour far inland, and so much vapour is 
condensed on the western slopes that the winds descend the eastern 
slopes as dry winds. It therefore happens that from the Sierra Nevada 
and Cascade ranges eastward to the 100th meridian most of the United 
States is arid or semi-arid (Fig. 495). Although the Mississippi valley 
is the part of North America which corresponds in position to well- 
watered Germany, Austria, and eastern Russia in Europe, destructive 
droughts frequently take place there. Within the arid belt to the west, 
however, there are local mountain ranges like the Rockies and Black 
Hills which have greater precipitation than the intervening plains 
and plateaus (Fig. 496). 



CLIMATE 



801 



Interiors of Continents. — Because of distance from the sea, the 
interiors of continents usually have less rainfall than the coasts. This 
is the cause of frequent droughts in central and western Asia and in 
the central United States. In the northern United States and southern 
Canada these droughts are less destructive than they are to the 
south, because light rainfall will support crops in a cool climate. 



6 
6 
4 
3 
2 
1 





6 
5 
4 
3 
2 
1 



Seattle 35.68 


New York 42.47 






Bismarck 17.50 Chicago 33.54 








































-el 


















h 


nE 


ill. 












JAN. 
FEB. 
MAR. 
APR. 
MAY 
JUNE 
JULY 
AUG. 
8EP. 
OCT. 
NOV. 
DEC. 

JAN. 
FEB. 
MAR. 
APR. 
MAY 
JUNE 
JULY 
AUG. 
SEP. 
OCT. 
NOV. 
DEC. 

JAN. 
FEB. 
MAR. 
APR. 
MAY 
JUNE 
JULY 
AUG. 
SEP. 
OCT, 
NOV, 
DEC. 
JAN. 
FEB. 
MAR. 
APR. 
MAY 
JUNE 


JULY 
AUG. 
SEP. 
OCT. 
NOV. 
DEC. 





St. Louis 40.10 


5 
4 

S 
2 

1 



5 


San Francisco 
22.96 




Washingtc 


n 40.80 


4 




Denver 14.05 






3 u 












2 " 






I 










1 .... 


JLL_ 


I. 


IlL. . 






i 














FEB 
MAR 
APR 
MAY 
JUNi 
JUL1 
AUG 
SEP 
OCT 
NOV 
DEC 

JAN 
FEB 

MAR 


MAY 
JUNE 
JULY 
AUG 
SEP 
OCT 
NOV 
DEC 

JAN 
FEB 
MAR 
APR 
MAY 
JUNE 
JULY 
AUG 
SEP. 
OCT 
NOV 
DEC 

JAN 
FEB 
MAR 
APR 
MAY 
JUNE 
JULY 
AUG 
SEP. 
OCT 
NOV 
DEC 








Jacksonville 


3 
7 
6 
6 
4 
8 
2 
1 



8 


New Orleans 55.63 52.5 


3 










6 








5 


Los Angeles 15.75 




























Phoenix 7-27 






a ' 


1 il 


III. ..III. II 








FEB. 
MAR. 
APR. 
MAY 
JUNE 
JULY 
AUG. 
SEP. 
OCT. 
NOV. 
DEC. 

JAN. 
FEB. 
MAR. 
APR. 
MAY 
JUNE 
JULY 
AUG. 
SEP. 
OCT. 
NOV. 
DEC. 

JAN. 
FEB. 
MAR. 
APR. 
MAY 


JULY 
AUS. 
SEP. 
OCT. 
NOV. 
DEC. 

JAN. 
FEB. 
MAR. 
APR. 


JULY 
AUG. 
SEP. 
OCT. 
NOV. 
DEC. 



Fig. 495. — Monthly precipitation at selected stations in United States, showing varia- 
tions from west to east and north to south. (After Milham.) 



Two factors enter into this : first, the smaller evaporation of the 
cool regions allows the dampness to remain in the ground for a longer 
time ; and, secondly, the melting of the frozen soil keeps the soil damp 
late into the summer. 

East Coasts. — Although windward coasts are rainy, and leeward 
coasts are dry, in the trade wind belt, it does not necessarily follow 
that because the west-facing or windward coasts of the prevailing 
westerlies are rainy, that the east-facing or leeward coasts of the pre- 

3F 



802 



COLLEGE PHYSIOGRAPHY 



vailing westerlies should be dry. The air in the prevailing westerlies 
has crossed the whole continent before coming to the east coast and 
has obtained little moisture on the way except such as might be evap- 
orated from lakes and rivers. What prevents aridity on east coasts, 
however, is the cyclonic storm eddies of the prevailing westerly wind 
belt. It will be recalled that in the prevailing westerlies the winds 
of these storms blow in from all sides toward the centre of low pressure. 
Consequently some of the winds of eastern and southeastern United 
States blow from the Atlantic Ocean and Gulf of Mexico. These winds 
bring abundant rainfall to the eastern United States, and the annual 




Fig. 496. — Rainfall map of the United States. (Gannett.) 



precipitation of parts of North Carolina, Tennessee, Florida, and the 
Gulf States (Fig. 496) is over 60 inches. 

East coasts have changeable weather on account of the influence 
of these cyclonic storms. In summer the northwest winds are dry 
and cool, in winter dry and cold. Whenever storm winds blow from 
the sea, the temperature and humidity are modified by the waters of 
the ocean. Thus the south winds are warmed in passing over the 
Gulf Stream or the Gulf of Mexico and carry warmth and dampness 
to the southern and eastern states. The east winds are cooled in the 
summer in blowing over the Labrador Current, and are damp and 
chilly, often bringing fogs to Nova Scotia and the New England 
states. Because of the influence of winds, the east coasts may have 
weather which during one day is like that of the interior of the con- 
tinent and on the next like that of the equable ocean. The north- 



CLIMATE 



803 



eastern coast of China has a climate similar to that of the eastern 
United States, being characterized by the seasonal contrasts which 
are typical of the eastern coasts in the temperate zones. 

Variation from Seacoast to Interior. — The variation in the climate 
of the temperate zones from seacoasts to the interior has been illus- 
trated by the contrasts of the west coast, interior, and east coast, but 
there is also a notable contrast between the seacoast with its more 
or less equable climate and the interior of the continent with what we 
call a continental climate, characterized by great extremes. 

Throughout the world we find that in the intermediate or temperate 
zones there is a considerable difference between (a) the interior of the 




eoRMAY & co., N.r. 



Scale of Temperature 



0to20.. 20to50. 50to 90. 90 to 110.110 to 120 



Fig. 497. — Map showing mean annual range of temperature for the world. (Barthol- 
omew.) 

continent, which has warm or hot summers and cool or cold winters, 
and (b) the seacoast, where the summers may not be oppressively hot 
and the winters not unbearably cold. During a summer day in middle 
latitudes of the intermediate zones the temperature in the interior of 
a continent may arise above ioo°, and in winter it may descend as 
much as 40 below zero. This is an extreme range of 140 in a year, 
and the summer has the climate of the tropical zone, and the winter 
tends toward the climate of a polar region. In United States, Minne- 
sota and the Dakotas have this extreme or continental climate. The 
same thing is found in north central Siberia near the Arctic Circle, 
where hot summers are followed by bitterly cold winters. This latter 
is the coldest part of the world in winter and is sometimes spoken of 
as a cold pole of the earth (Fig. 497). 

^ The extreme climate of the interior of a continent is chiefly due to 
distance from the sea and freedom from its influence. In summer, 



8o 4 COLLEGE PHYSIOGRAPHY 

the land warms because the sun stays a long time above the horizon, 
even though in the temperate zones it is not very high in the heavens 
except on the tropics, where it is overhead at noon on the summer 
solstice. In the winter the nights are very long and the sun is much 
lower in the heavens than during the summer. This results in such 
extreme radiation that the land loses the excessive heat which it has 
accumulated during the summer and becomes exceedingly cold. 

Certain seacoast regions are said to have an equable climate be- 
cause they are generally characterized by lack of extremes, though 
this is not true of all coastal regions, the northeastern parts of United 
States and China furnishing exceptions. Equability is likewise due 
to the influence of the ocean, which may be thought of as a stubborn 
medium, gaining heat more slowly than the land in summer and losing 
it more slowly than the land in winter. In summer or winter the winds 
carry the temperature of the ocean to the parts of the continents away 
from the sea, which have an equable climate throughout the year 
because the heat of summer and the cold of winter are ameliorated 
by the winds from the ocean. 

Climate of the United States. — The contrast of climates of the 
east and west coasts and of seacoasts and interiors is all well illustrated 
in the United States. We may speak of the climate of the west coast 
as equable, the western part of the interior as continental, the eastern 
interior and the east coast as somewhat-modified continental, and the 
southeast coast as equable. Eastern United States, meaning the 
region from the Great Plains and Mississippi valley to the Atlantic 
Ocean, is so important to a majority of users of this book that it seems 
profitable to consider its weather and climate in slightly greater detail. 

Summer Weather in Eastern United States. — The following is an 
actual illustration of typical summer weather in eastern United States. 
An anticyclone is passing over the region. The day is one of agreeable 
warmth, with a cool, dry, gentle west wind and a nearly cloudless 
sky. The following night is one of refreshing coolness. 

The anticyclone is followed by an area of moderately low pressure. 
With the approach of this Low the wind shifts from west to southeast, 
the temperature increases, the air becomes more humid, and both 
day and night are muggy and oppressive. 

The second day begins with the sky flecked by small clouds, which 
in the afternoon grow to thunder-heads. These may give rise to a 
thundershower in the afternoon. Just before the thundershower 
there is a sharp wind squall, and during the shower there is heavy rain 
and severe lightning and thunder (Fig. 498). 

As soon as the storm is over, the wind shifts to the west again, 
because another anticyclone has followed the Low. With the passage 
of this second anticylone the air is again dry and refreshing. 

Summer weather in the United States is commonly a succession of 
just such days as are described above, the cycle being repeated with 
regularity (Figs. 459, 499), although there are slight variations. For 



CLIMATE 



805 



DURATION OF RAINFALL 
A. 



7A.M. 9 11A.M. 1 3P.M. 3:30P.M. 4P.M. 4:30P.M. 5P.M. 7 9P.M. 

















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thunder 




96 
90 
84- 
















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TEMPERATURE 
























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78 
72 
66 


























































































































68 
29.82 
29.78 


■F— 
































































PF 


ESS 


URI 




























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29.74 
29.70 






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48 








































































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40 
32 
24 
16 
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ocity 


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60 V 

30 V 

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HL 


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inch 


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RA 


nf; 


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7A.M. 9 11A.M. 1 3P.M. 3.30P.M. 4P.M. 4.30P.M. 5P.M. 7 9P.M. 

Fig. 498. — The temperature, pressure, wind, relative humidity, and rainfall during a hot 
summer day with an afternoon thundershower. (Milham.) 



8o6 



COLLEGE PHYSIOGRAPHY 




Fig. 4Qg. — Weather maps of three successive autumn days in 1913, showing eastward 
progress of a Low from Montana to Nova Scotia and a High from Nevada to the Great 
Lakes. (After U. S. Weather Bureau.) 



CLIMATE 



807 



example, there are times when the low pressure areas are so poorly 
developed that little rain falls for several weeks. During such a 
drought the smaller streams disappear, wells run dry, vegetation 
withers, and crops are retarded. At other times the low pressure 
areas may be so well developed that instead of scattered thunder 
storms there is general cloudiness and rain. In late summer and 
early autumn, when hurricanes pass up the Atlantic coast accom- 
panied by strong winds and heavy rain, this condition of generally 
stormy weather is often developed. 

Winter Weather in Eastern United States. — It has already been 
stated that during the winter the cyclonic circulation of the prevailing 
westerlies is better developed than during the summer, both anticy- 
clones and cyclones being more frequent and more emphatic. As 



6 Noon 6 12 6 Noon 6 12 6 Noon 6 12 6 Noon 




Fig. 500. — Thermograph records, a to e at Nashua, N. H., / at Cambridge, Mass., g at 
Fort Assiniboine, Mont, a shows a period of clear warming weather in April ; b, cloudy 
weather accompanying a West Indian hurricane in September ; c, change from moderate 
winter weather to a cold spell in February ; d, steady fall of temperature from one Janu- 
ary night to the next during approach of a cold spell in winter ; e, steady rise of tem- 
perature in December; /, high temperature at night in November caused by warm 
southerly winds followed by cold westerly winds ; g, sudden rise in temperature with 
hot, dry, chinook wind. (After Davis.) 



they pass over the country (Fig. 501) they bring alternate clear and 
cloudy weather. The succession of cyclones and anticylones is some- 
times so regular in winter that dne day of the week may have nearly the 
same kind of weather for 2 or 3 successive weeks. It is this that gives 
rise to the belief that if it snows or rains on the first Sunday in a 
given month it will snow or rain on every Sunday in that month. 

As the winter cyclones pass they may bring rain, or snow (Fig. 443). 
The velocity of the wind varies and the direction of the wind shifts 
through several quarters, so that when the wind is from the north there 
will be chilly weather, and when it is from the south there will be warm 
weather (Fig. 500). A thaw often occurs when the south wind is blow- 
ing in midwinter and many even cause rain to fall as far north as 
Canada. Often there is a decided drop in temperature immediately 
after a thaw, because an anticyclone follows directly behind the 
cyclone. 

It is because of such changes as are outlined above that the climate 



8o8 



COLLEGE PHYSIOGRAPHY 




^rCST^O ^o c l'l5Sfo")^ 7 . V05- 20" 05" gS 5 75° 05 30 ^ ~.L s 5S 

'"S^^^^tWWas 29.6. L0W^ : ^fe£° = 




Fig 



• soi. 



Weather maps of three successive winter days in 191 4. Symbols as in Fig. 456. 
(After U. S. Weather Bureau.) 



CLIMATE 



809 



of eastern United States is spoken 
of as changeable and not temper- 
ate. There are few climates in 
the world characterized by such 
rapid changes of contrasting 
weather as the stormy west wind 
belts. These changes are trying 
to the health, and many diseases, 
such as grippe, pneumonia, and 
consumption are common in these 
severe climates. 

Climate of West Wind Belt in 
Southern Hemisphere. — There is 
a striking difference between the 
climate of the north temperate 
zone and that of the south tem- 
perate zone, because the prevail- 
ing westerlies differ somewhat in 
the northern and southern hemi- 
spheres. The difference is chiefly 
because of the fact that there is 
far less land than water in the 
southern hemisphere. The 
changes in temperature in the 
south temperate zone are, there- 
fore, less extreme than in the 
north. Over the smooth ocean, 
however, the winds blow with 
more strength and steadiness than 
over the irregular lands, so that 
they come to the land as stronger 
winds (Fig. 502). In other re- 
spects the climates of the north 
and south temperate zones are 
not very different. Over the 
southern ocean the weather is raw 
and cold in winter and damp and 
chilly in summer, although it does 
not have the extreme changes from 
warm to cold weather which we 
have in the northern hemisphere. 
In the south temperate zone the 
storms are frequent and fierce. 
It is because of this fact that the 
climate of the coast of southern 
Chile is rainier and more dis- 
agreeable than the climate of the 




Fig. 502. — Barograph record of a week's 
winter pressure in the South Pacific 
during a journey from Punta Arenas 
at the Strait of Magellan northward 
to Corral, Chile. (Ward.) 



8io COLLEGE PHYSIOGRAPHY 

coast of British Columbia and southeastern Alaska. Because of the 
velocity of the prevailing westerlies, or Brave West Winds, it is very 
difficult and often dangerous to go around Cape Horn at the extreme 
south end of South America, especially from east to west. 

Polar Climates 

Climate near the Arctic Circle. — North of the Arctic Circle the 
sun is above the horizon both night and day during the summer of the 
north polar zone. Accordingly, although the air is cool and sometimes 
raw, it is not very cold (Fig. 503). The ice melts out of the ground to 
a depth of 2 or 3 feet under the warmth of the sun, in places where the 
topography permits the thawing of soil, and the ground is therefore 
damp and swampy. In this season of the year in favourable places 
the grass becomes green, flowers blossom, and birds and insects 
appear. The summer weather is as changeable as in some parts of 
the belt of prevailing westerlies, however, because storms appear in 
fairly regular succession, bringing rain and snow squalls along the 
coast. On the sea, fogs are common at points where damp air is 
chilled in passing over cold water. 

During the late summer when the sun ceases to be continuously 
above the horizon, the days grow cooler and the nights become very 
cold. The insects disappear, the birds take their flight to the south- 
ward, and the land becomes covered with snow. The ground is again 
frozen clear to the surface, and a skim of ice may appear on the ocean, 
becoming thicker as the days grow shorter. At this season the Eskimo 
of the north polar zone gives up the use of his skin boat, or kayak, and 
commences to use a dog sledge in hunting the seal, which furnishes his 
chief food. 

When the time comes that the sun no longer rises above the horizon 
even at noon, the weather during both day and night is bitterly cold 
(Fig. 503). The principal changes in Arctic weather during the winter 
are those accompanying the passage of the cyclonic storms. Thaws 
are not unknown during the Arctic winter ; and, even in midwinter, 
the temperature may rise high enough so that the Eskimo snow 
houses or igloos begin to melt. 

When the sun reappears above the horizon in the spring, the snow 
melts and " frost " begins to go out of the ground. Then the Eskimo 
abandons his igloo for his skin tent or tupic. The floe ice in the ocean 
breaks up and floats away, so that the Eskimo uses his kayak instead of 
his sledge for hunting and travelling. This is the beginning of the long 
summer day of the Arctic. 

Climate nearer the North Pole. — All the way from the Arctic 
Circle to the North Pole the men who have traversed the region found 
the climate similar to that just describeS, although farther north the 
Arctic winter night is longer and colder and the summer is cooler. 
Even in the northernmost lands the sun supplies sufficient heat during 



CLIMATE 



811 



M. A..M. J. J. A. S. 0. N. D. J. 



the summer so that the snow melts from much of the low ground near 
the coast. In the extreme northern part of Greenland, for example, 
Peary found flowers in blossom, insects 
humming about, and many musk oxen 
roaming over the land in summer. 

The Arctic Ocean, the centre of which 
is the North Pole, is always covered with 
ice floes, even in summer, and it was over 
these that Abruzzi, Nansen, and other 
Arctic explorers travelled, and over which 
Peary finally reached the North Pole. He 
made his successful dash to the pole in 
early spring, because during the Arctic 
summer the ice is so much broken that it 
is difficult to cross it by sledges, and yet 
it is not broken enough so that ships may 
pass through. It was because of this fact 
that Peary learned through his many Arc- 
tic trips to go as far north as he could in 
a ship during one season and remain there 
during the cold Arctic night in order to 
be ready for an early start before the sun 
rose the second summer. The difficulties 
of travel over the ice and the exceedingly 
rigorous climate baffled the efforts of the 
hardiest and most venturesome explorers 
to reach the North Pole, until Peary was 
successful in doing so in 1908. The rigor- 
ous climate renders the lands nearest the 
North Pole an uninhabitable desert. 

Antarctic Climate. — The climate of the 
Antarctic continent has become fairly well 
known in recent years through the many 
south polar expeditions, particularly those 
of Shackleton, Scott, and Amundsen. The 
striking difference between the Arctic and 
Antarctic regions is that the average sum- 
mer temperature at the Antarctic Circle is 
about as cold as that at the North Pole 
in the Arctic Ocean. It should be stated, 
however, that both of these temperatures 
are rather moderate. The reason for the difference between the 
climate of the north and south polar regions is that the ice-covered 
land of the south polar continent gives very much lower tempera- 
tures than the slowly heating and slowly cooling water of the Arctic 
Ocean. The mean summer temperature on the coast of Antarctica is 
from 2 8° to 30 F., while the mean winter temperature is from 2 above 




J. F. M. A. M. J. J. A. S. 0. N. D. J. 



Fig. 503. — Seasonal range of 
temperature in the polar zone. 
(Ward.) N.Z. — Novaya 

Zembla; FJ. — Franz Jo- 
sef Land ; G.L. — Grinnell 
Land. 



8i2 COLLEGE PHYSIOGRAPHY 

zero to 1 5 below zero. These arc the average summer and winter 
temperatures of the coast, not the extremes. The interior of the con- 
tinent, rising to a height of 8000 or 10,000 feet, has a much more 
rigorous climate. At the South Pole there must be a continental 
climate, both because of altitude and because of the distance from 
the ocean. The summer temperatures encountered near the South 
Pole range from o° F. to 35 or 40 below zero. The winter tem- 
peratures near the South Pole are much lower than this. Nine 
hundred miles from the pole at a point not far from the seacoast 
Scott recorded 77 below zero, F., in August, 191 1, the heart of the 
Antarctic winter. 

The Antarctic snowfall is surprisingly light, the amount at sea level 
near the border of the continent being estimated as not more than the 
equivalent of 7 to 14 inches of rain annually. 

In the Antarctic region the winds of the prevailing westerly circula- 
tion attain great velocity at times, and these storms are followed by 
periods of calm. For example, when Amundsen was on his way to the 
South Pole in 191 1, he had pleasant weather. Only a few weeks later, 
however, Scott, who also attained the South Pole, encountered such 
stormy weather that he was greatly delayed. At the pole itself there 
should, theoretically, be fine calm weather, and indeed Amundsen and 
Scott both found that at the pole the snow lay in horizontal layers 
without drifting, as if calm weather were the general rule there. 

There seems to be a tendency to develop a foehn wind on the borders 
of the high plateau, with the air sliding outward down the slopes of the 
Antarctic continent. These winds often attain a velocity of 75 or 80 
miles an hour. There are also severe blizzards, and it was in one of 
these that Scott finally lost his life on the return trip from the South 
Pole, at a distance of only 12 miles from a depot of supplies. 

The severe cold, the blizzards, and the endless night of the polar 
winter limit the plant and animal life of the Antarctic continent to the 
very lowest forms ; and there have never been human inhabitants in 
this region, except when exploring parties have temporarily spent a 
short time at the margin of the continent 

References to Literature 

J. G. Andersson and Others. Die Veranderungen des Klimas seit dem 
Maximum der Letzten Eiszeit, Stockholm, 1910, 459 pp. 

H. Arctowski. Studies on Climate and Crops, Bull. Amer. Geog. Soc, Vol. 
42, 1910, pp. 270-282,480-495; ibid., Vol. 44, 191 2, pp. 598-606, 745-760; 
ibid., Vol. 45, 1913, pp. 117-131 ; ibid., Vol. 46, 1914, pp. 265-281. 

D. P. Barrows. The Colorado Desert, Nat. Geog. Mag., Vol. 11, 1900, pp. 

337-351- 

J. G. Bartholomew, W. E. Clarke, and P. H. Grimshaw. Atlas of Zoogeog- 
raphy, Bartholomew's Physical Atlas, Vol. 5, Edinburgh, 191 1. 

I. Bowman. Man and Climatic Changes in South America, Geog. Journ., 
Vol. 33, 1909, pp. 267-278. 

R. M. Brown. Indian Summer, Journ. Geog., Vol. 8, 1909, pp. 25-31. 



CLIMATE 813 

Edward Bruckner. Klimaschwankungen seit 1700, Penck's Geog. Abhand., 
Wien,' 1890, 324 pp. 

R. H. Chapman. The Deserts of Nevada and the Death Valley, Nat. Geog. 
Mag., Vol. 17, 1906, pp. 483-497. 

H. C. Cox. Frost and Temperature Conditions in the Cranberry Marshes of 
Wisconsin, Bull. T, U. S. Weather Bureau, 1910, 121 pp. 

J. Croll. Climate and Time, New York, 1890; Discussions on Climate and 
Cosmology, New York, 1886. 

P. C. Day. Frost Data of the United States and Length of the Crop Growing 
Season, Bull. V, U. S. Weather Bureau, 191 1, 5 pp. 

E. G. Dexter. Weather Influences, New York, 1904, 281 pp. 

H. N. Dickson. Climate and Weather, London, 1913, 256 pp. 

R.E.Dodge. Climate and Mankind, Columbia University, Extension Syllabi, 
Series B, No. 4, New York, 1903, 19 pp. 

H. Gannett. The Timber Line, Bull. Amer. Geog. Soc, Vol. 31, 1899, pp. 118- 
122. 

A. W. Greely. American Weather, New York, 1888. 

J. Hann. Handbuch der Klimatologie, 3d edition, Stuttgart, 1908; see also 
Ward's translation of Part I, New York, 1903. 

M.Hardy. Oxford Wall Maps, 1 909-1910 : The World — Vegetation Regions, 
and vegetation maps of each continent separately. 

M. W. Harrington. Rainfall and Snow of the United States compiled to the 
End of 1891, Bull. C, U. S. Weather Bureau, 1894, 80 pp. 

A. J. Henry. Rainfall of the United States, Bull. D, U. S. Weather Bureau, 
1897, 58 pp.; Climatology of the United States, Bull. Q, U. S. Weather 
Bureau, 1906, 1012 pp. 

E. W. Hilgard. A Report on the Relations of Soil to Climate, Bull. 3, U. S. 
Weather Bureau, 1892, 59 pp. 

E. Huntington. The Climate of the Historic Past, Monthly Weather Review, 
Vol. 36, 1908, pp. 359-364, 446-450; The Fluctuating Climate of North 
America, Smithsonian Report for 191 2, Publication 2206, Washington, 
1913, pp. 383-412 ; The Shifting of Climatic Zones as Illustrated in Mex- 
ico, Bull. Amer. Geog. Soc, Vol. 45, 1913, pp. 1-12, 107-116; The 
Pulse of Asia, Boston, 1907, 416 pp.; The Rivers of Chinese Turkestan 
and the Desiccation of Asia, Geog. Journ., Vol. 28, 1906, pp. 352-367. 

Mark Jefferson. The Culture of the Nations, Bull. Amer. Geog. Soc, Vol. 43, 
1911, pp. 241-265. 

W. Koppen. Klimakunde, 2d edition, Leipzig, 1906, 132 pp. 

C. H. Merriam. Life Zones and Crop Zones of the United States, Bull. 10, 
Division of Biological Survey, U. S. Department of Agriculture, Washing- 
ton, 1898. 

H. R. Mill. British Rainfall, annual volumes, 52d report, London, 1913. 

R. C. Mossman. The Greenland Sea, Its Summer Climate and Ice Distribu- 
tion, Scottish Geog. Mag., Vol. 25, 1909, pp. 281-291. 

A. Penck. The Shifting of the Climatic Belts, Scottish Geog. Mag., Vol. 30, 19 14, 
pp. 281-293. 

A. Supan. Die Verteilung der Niederschlags auf der Festen Erdoberflache, 
Gotha, 1898; Grundziige der Physischen Erdkunde, Leipzig, 191 1, pp. 
230-257. 

R. S. Tarr. Difference in the Climate of the Greenland and American Sides 
of Davis and Baffin Bay, Amer, Journ. Sci., Vol. 153, 1897, pp. 315-320. 

E. T. Turner. Climate of New York State, Chapter XI in Tarr's Physical 
Geography of New York State, New York, 1902. 

U. S. Weather Bureau. Climate and Crop Bulletins; Summaries of the 
Climatological Data for the United States by Sections. 

J. Walther. The North American Deserts, Nat. Geog. Mag., Vol. 4, 1892, 
163-176. 

R. de C. Ward. Climate, Considered Especially in Relation to Man, New 



814 COLLEGE PHYSIOGRAPHY 

York, 1908, 372 pp. ; A Year of Weather and Trade in the United States, 
Pop. Sci. Monthly, Vol. 61, 1902, pp. 439-448; Suggestions Concerning 
a More Rational Treatment of Climatology, Report 8th International 
Geographical Congress, Washington, 1905, pp. 277-293; Two Climatic 
Cross Sections of the United States, Monthly Weather Review, Vol. 40, 
I9i2,pp. 1909-1917; The Value of Non-instrumental Weather Observa- 
tions, Pop. Sci. Monthly, Vol. 80, 1912, pp. 129-137; Hann's Handbook 
of Climatology, Part I, New York, 1903, 437 pp. 
A. Woeikof. Die Klimate der Erde, Jena, 1887, 2 parts, 396, 445 pp. 






INDEX 



Aa, 440. 

Abandoned marginal gorges, 284. 
Abandoned shorelines, 383. 
Abandoned waterfalls, 125. 
Abbe, Cleveland, 744, 781. 

C, Jr., 384, S2i. 
Abbott, H. L., 139. 
Ablation, 205. 

Ablation, moraine, 221, 222. 
Abruzzi, Duke of, 811. 
Absolute humidity, 733. 
Absolute units, 766. 
Absorbed light, 716. 
Absorption, 716, 718, 721-722. 
Abysmal life in the sea, 678-680. 
Accidents, 187. 
Accordant valleys, 177, 233. 
Active volcanoes, 448. 
Adams, F. D., 12, 539, 614, 625. 
Adaptation, 670, 679. 
Adiabatic cooling, 719. 
Adjusted streams, 185, 560. 
Adolescence, 180, 182. 
Adria, 159. 

Advancing glaciers, 213-214. 
Advective zone, 729. 
Aeronauts, 709. 
Aeroplanes, 709. 
Africa, 535, 590-591. 
Aftonian, 298. 
Agassiz, Alexander, 384, 638, 658, 665, 680. 

Louis, 252, 258, 303, 339, 431. 
Agassiz, Lake, 281. 
Age of the earth, 623-625. 
Aggradation, 18. 
Aggrading streams, 114. 
Agonic line, 629. 
Agriculture, 55-56, 567. 
Aguilera, J. G., 435. 
Aiguille, 540. 
Air, 709. 

warming of, 722-723. 
Air pressure, 746. 
Alaskan earthquakes, 423-425. 
Albatross, 638. 

Alden, W. C, 263, 271, 274, 303, 305. 
Algae, 655. 
Algonkian, 32. 
Alkaline soil, 325. 



Allegheny Mountains, 511. 

Allegheny Plateau, 511-513. 

Alluvial fans, 162-165, 552. 

Alluvium, 32. 

Alongshore currents, 352-353. 

Alpine glaciers, 204. 

Alps, 600. 

maximum glaciation, 260. 
Altitude, and the zones, 784. 

effect on temperature, 728. 

results of high, 543. 
Amain landslide, 51. 
Amazon, grade of, no. 
Amengual, R., 665. 
America, mountains of, 535-536. 
American Fall, 128-129. 
Amundsen, R., 696, 812. 
Anastomosing channels, 142. 
Anderson, R., 493. 

T., 493- 
Andersson, T. G., 56, 812. 
Andes, 592-594. 
Andesite, 26. 
Andrews, E. C., 252. 
Anemometer, 746. 
Aneroid barometers, 71 1-7 12. 
Angot, Alfred, 726, 730, 744, 748. 
Animals, influence on coasts, 373-380. 

relation to plants, 670. 

work in weathering, 44. 
Antarctica, 535, 609, 811-812. 
Antarctic climate, 811. 
Antarctic ice sheet, 244-247. 
Antarctic Ocean, 639. 
Antecedent streams, 191, 558. 
Anthropogeography, see Man; also books 

listed in Introduction. 
Anticlines, 400. 
Anticlinoriun, 402. 
Anticyclones, 758, 763. 
Antillean mountains, 535, 597. 
Antitrades, 753. 
Ants, work in weathering, 44. 
Apogee, 703. 
Apollinaris, 83. 

Appalachian Mountains, 556-558, 576, 598. 
Ararat, 467. 
Archean, 32. 
Arched mountain type, 527. 

815 



8i6 



INDEX 



Archibald, D., 744. 

Arctic climate, 810-81 1, 

Arctic Ocean, 639, 696. 

Arctowski, H., 812. 

Arcuate mountains, 606. 

Ardennes, 577. 

Argillite, 28. 

Argon, 712, 713. 

Arid land deposits, 164-166. 

Arid land swamps, 337. 

Arnold, R., 358. 

Arreola, J. M., 493. 

Arretes, 545. 

Arrhenius, S., 493, 614, 625, 744. 

Artesian wells, 81-83. 

Artificial levers, 146. 

Artois, wells in, 81. 

Ash cones, 446. 

Ash, volcanic, 438, 443, 444. 

Asia, 603-607. 

Assam earthquake, 421. 

Asteroids, 1. 

Atlantic eddies, 693-694. 

Atlantic Ocean, 639, 640-644, 692-693. 

Atmosphere, 9, 709-814. 

composition, 712-714. 

relation to weathering, 37-38, 41-43. 

thickness, n. 
Atmospheric mixture, 712. 
Atmospheric pressure, 418, 709-712. 
Atmospheric protection, 8. 
Atolls, 378-379- 

Atwood, W. W., 50, 75, 252, 262, 305, 384. 
Augite, 19-20. 
Aurora Australis, 634-635. 
Aurora Borealis, 634-635, 709. . 
Australia, 535, 607-608. 
Autumn foliage, 737. 
Auvergne, 459. 
Avalanche lakes, 317. 
Avalanche waves, 686. 
Avalanches, 49-52, 206, 552. 
Aviators, 709. 

Axis of earth, shifted, 302, 621-623. 
Azoic, 35. 
Azores, volcanoes of, 467. 

Babylon, wind work near, 69. 
Bacteria, work in weathering, 44. 
Bad lands, 102-103. 
Baker, M., 471. 
Ball, R. S., 625. 

S. H., 303, 305. 
Balloons, 709. 
Baltic ice sheet, 258. 
Bancroft, J. A., 384. 
Banks of rivers, 143. 
Baratta, M., 435. 
Barbed tributaries, 177, 563. 



Barnes, A. H., 475. 

H. T., 664, 665. 
Barographs, 712. 
Barometers, 71 1-7 12. 
Barometric gradient, 746. 
Barometric pressure, 711. 
Barrell, J., 168, 530, 531, 578, 625, 665. 
Barrier beaches, 362, 368-370. 
Barrier reefs, 378. 
Barriers, canyons as, 123. 

lakes as, 334. 

mountains as, 572-575. 
Barringer, D. M., 493. 
Barrow, D. P., 812. 
Barrows, H. H., 521. 
Bars, offshore, 362, 363, 364-366. 
Bartholomew, J. G., 744, 751, 803, 812. 
Basal ice, 220. 
Basalt, 25, 26, 440. 
Baselevel, 119. 
Bastin, E. S., 305. 
Batholites, 485-486. 
Bauer, L. A., 633, 636. 
Bauxite, 20. 
Bayous, 150. 
Bays, .363-364, 380-382. 
Beaches, 360-370. 
Bean, E. F., 75. 
Bear den moraine, 272. 
Beardmore Glacier, 245-246. 
Becker, G. F., 625. 
Behrendt, G., 62. 
Belknap, G. E., 665. 
Bell, J. M., 493- 
Belted plains, 504-506. 
Bently, W. J., 252. 
Berghaus, H., 665. 
Bergschrund, 234. 
Bergwerk. 569. 
Bering Glacier, 243. 
Berkshire Hills, 525. 
Bertrand, M., 538, 539, 625. 
Bibliographies, see Introduction. 
Bigelow, F. H., 744, 781. 
Biosphere, 9. 
Biotite mica, 20. 
Birge, E. A., 339, 34°- 
Bitter lakes, 325. 
Black Forest, 548, 554, 568. 
Black Hills, 527. 

Black River Falls, flood at, 107-108. 
Black Sea, 691. 
Blackwelder, E., 305. 
Blake, 638. 
Blake, W. P., 73- 
Blatchley, W. S., 98. 
Blizzards, 770-771. 
Block Mountains, 526. 
Blood-rain, 58. 






INDEX 



817 



Blow holes, 357. 

Blue clay, 265. 

Blue Grass region, soil of . 54. 

Blue Ridge, 557. 

Blue veins, 216. 

Blue water, 654. 

Bocas, 456. 

Bog iron ore, 21. 

Bogoslof, 468-469. 

Bogs, 334, 337-338. 

Bombs, volcanic, 443-444. 

Bonney, T. G., 252, 303, 493. 

Bonsteel, J. A., 249. 

Bora, 771. 

Bore, 707. 

Borings, 612. 

Boscotrecase, 456. 

Bosses, 25, 485-486. 

Botn, 234. 

Boulder clay, 223-224. 

Boulder pavements, 331-332- 

Boulder train, 266. 

Boundaries, mountains as, 572. 

Bowie, W., 625, 626. 

Bowman, Isaiah, 521, 578, 770, 812. 

Brabazon, A. J., 201. 

Bradwell, H. J. L., 73- 

Brahmaputra delta, 159. 

Braided streams, 142. 

Bramer, J. C., 56, 303, 369, 384. 

Braun, G., 75. 

Brave west winds, 810. 

Brazil, weathering in, 37. 

Brazilian Highland, 594. 

Breakers, 352. 

Breccia, fault, 403. 

Breeze, 747. 

Brigham, A. P., 56, 303, 339, 384. 

British Admiralty Office, 668. 

British Meteorological Office, 668. 

Brock, R. W., 53, 56. 

Brooks, A. H., 578. 

Brown, R. M., 138, 168, 812. 

Browne, R. E., 194. 

Bruckner, E., 242, 252, 254, 260, 783, 813. 

Brun, A., 493. 

Bryant, H. G., 666. 

Buchan, A., 665, 744. 

Buchanan, J. V., 252. 

J. Y., 665. 
Buckley, E. R., 332, 339. 
Buckman, H. O., 56. 
Bulb glaciers, 204. 
Buried valleys, 288-291. 
Burrow, A. T., 781. 
Bursting bogs, 338. 
Burton, W. K., 424, 436. 
Butte, 503. 

Button-shaped folds, 538-539. 
3 G 



Cadell, H. M., 384. 

Calabrian earthquakes, 420-421. 

Calcareous tufa, 24, 83-84. 

Calcite, 19-21. 

Caldare, 234. 

Calhoun, F. H. H., 305- 

California earthquake, 430-432. 

California, valley of, 166, 521. 

Calkins, F. C, 580. 

Calms, equatorial, 752, 788-790. 

horse latitude, 753. 
Calvin, C, 303. 
Cambrian, 32. 

Campbell, M. R., 186, 194, 522, 578, 579. 
Campos, 795. 
Canadian Fall, 129. 
Canadian Geological Survey, 52. 
Canals, artificial, 138. 

glacial, 230. 
Cahoe-shaped valleys, 558. 
Canyons, n 7-1 23, 176. 
Cape Cod, 373. 
Cappello, H. C, 447. 
Capps, S. R., 305. 
Carbonates, 20. 
Carbon dioxide, 445, 712-714. 

in relation to glaciation, 301. 
Carboniferous, 32. 
Carlsbad, 84. 
Carmen, J. E., 168. 
Carnegie Institution, 633, 634. 
Carney, F., 98, 290. 
Carpathians, 600. 
Carpenter, diagram by, 487. 
Cascades, 123. 
Cascading Glacier, 207. 
Case, E. C, 252. 
Cataracts, 123. 
Catskill Mountains, 512, 525. 
Caucasus, 600. 
Caverns, 89-95. 
Cavern deposits, 93-94. 
Caves, sea, 357. 

solution, 89-95. 
Celsius scale, 720. 
Cementation, 96-97. 
Cement rock, 24. 
Cenozoic, 32. 
Centigrade scale, 720. 
Centrifuge, 672. 
Chains, mountain, 541. 
Chalcopyrite, 21. 
Chalk, 649. 
Challenger, 637. 
Challenger Plateau, 643. 
Challenger Reports, 643, 647, 648, 657, 665. 
Chalmers, R., 665. 

Chamberlin, T. C, 35, 73, 98, 202, 252 
253, 296, 303, 384, 493, 625, 665. 



8i8 



INDEX 



Champlain Sea, 283. 

Change of earth's axis, 621-623. 

Change of level, 382-383, 389-399, 586- 

587, 615-616. 
Changes in climate, 783. 
Chapman, R. H., 813. 
Charleston earthquake, 428-429. 
Chasms, 358. 
Chemical change, 620. 
Chemical load, in. 
Chemically-formed rocks, 24. 
Chert, 20. 

Childs Glacier, 208, 212, 213. 
China, loess in, 71-73. 
China's sorrow, 160. 
Chinook, 773. 
Chree, Charles, 636. 
Chun, C, 665. 
Cinder Cone, Cal., 472, 474. 
Circulation, planetary, 750-758. 
Circumpolar whirl, 755. 
Cirque, 234-236, 545. 
Cirrus clouds, 739, 740. 
Clapp, F. G., 523. 
Clark, W. B., 522. 
Clarke, F. W., 35, 339. 

W. E., 812. 
Clastic rocks, 23-24. 
Clay, 295. 
Clay rock, 23-24. 
Clayden, A. W., 744. 
Clayton, H. EL, 744. 
Cleland, H. F., 95, 98. 
Cliff glaciers, 207. 
Cliffs, sea, 355-357- 
Climate, 699, 783-814. 

affecting weathering, 45. 

effect on plateaus, 506-507. 

mountain, 575~576. 

relation to lakes, 333. 
Climatic relationships of streams, 186-187. 
Climatic zones, 783-788. 
Climbing bogs, 338. 
Close, H. M., 304. 
Cloudbursts, 776. 
Clouds, 738-739, 762. 
-Cgjil, 23 ^338, 783. 
Coastal file in the sea, 672-675. 
Coastal plain swamps, 336. 
Coastal plains, 500, 504, 508-511. 
Coast line development, 370-373. 
Coast lines, 342-388. 
Coast of Africa, 590-592. 
Coast of Australia, 607. 
Coast of Europe, 601-602. 
Coast of North America, 595-597. 
Cobb, Collier, 522. 

Coffin's Beach, dune encroachment at, 61. 
Coldness, in ocean, 650. 



Cold pole, 803. 
Cold waves, 770-771. 
Cole, L. J., 98, 168. 
Coleman, A. P., 303. 
Collet, L. W., 665. 
Collie, G. L., 75. 
Colorado Canyon, 120-123. 
Colorado Plateau, 516-519. 
Colorado River, grade of, no. 
Colour, of ocean water, 654-655. 

of organisms, 676. 
Colours, 715, 716. 

Columbia lava plateau, 480-481, 516. 
Columbia River, 153. 
Columbus, 635, 699. 
Columns, 94. 
Columnar structure, 484. 
Comanchean, 32. 
Combers, 685. 
Comets, 1. 
Compass, 629. 

dipping, 633. 
Compensation, zone of, 615. 
Complexity of mountain structure, 542-543, 
Composition of atmosphere, 712-714. 
Condra, G. E., 522, 775. 
Conduction, 719, 723. 
Confluence step, 550. 
Conflict of activities, 15. 
Conglomerate, 23-24. 
Conical projection, 33. 
Cone ash, 446. 
Cone deltas, 163. 
Cone of dejection, 163. 
Cone, volcanic, 439, 446-449. 
Cones, parasitic, 448. 
Connecticut Geological Survey, 530, 531. 
Connecting Plateau, 643. 
Consequent coasts, 370-372. 
Consequent falls, 130, 134. 
Consequent lakes, 311. 
Consequent streams, 171-172, 560. 
Constructional plains, 501. 
Continental climates, 783, 803. 
Continental glaciers, 203-204, 243-252, 296. 
Continental islands, 380. 
Continental plateaus, 585-586. 
Continental shelf, 508, 584, 640-641. 
Continental slope, 584, 642. 
Continent, south polar, 589. 
Continents, 12-14. 

areas, 585, 589. 

distribution, 587. 

form, 589. 

heights, 585. 

south-pointing, 588-589. 
Contour interval, 35. 
Contours, 34~35- 
Contractional hypothesis, 618-620. 



INDEX 



ii 9 



Convection, 719, 72 3. 

Convective zone, 729. 

Cook, G. H., 384. 

Coon Butte, 470. 

Coral reefs, 375-380. 

Cordillera, North American, 597. 

Cordilleras, 540-542. 

Cornell Glacier, 249-251. 

Cornice glaciers, 207. 

Cornish, V., 73, 138, 384, 665. 

Corrasion, 11 4-1 15. 

Corrie, 234. 

Corrosion, 114-115. 

Coseguina, 443. 

Coseismals, 412. 

Cotopaxi, 443. 

Coulees, 481. 

Counter current, 692. 

Cowles, H. C, 73. 

Cox, H. C, 744, 813. 

Crag and tail, 267. 

Crater Lake, Oregon, 474-475. 

Craterlets, 413. 

Craters, 447. 

Crazy Mountains, 483. 

Credner, H., 168. 

Creep, 49-50. 

Crescent beaches, 360. 

Crest of wave, 683. 

Cretaceous, 32. 

Crevasse, glacial, 217-218. 

Crevasse river, 159. 

Crinoids, 680. 

Critical point, of water, 443. 

Croll, J., 303, 625, 813. 

Croll's hypothesis of glaciation, 301-302. 

Crosby, F. W., 666. 

W. O., 253, 303, 306, 625, 666. 
Cross, W., 48, 73, 305, 493. 
Crumpling, 401. 
Crustal movements, 348, 621. 
Cuestas, 504-506. 
Cumberland Mountains, 512. 
Cumberland Plateau, 512. 
Cummings, B., 98. 
Cumulus clouds, 739, 740. 
Cumulo-nimbus clouds, 773. 
Currents, alongshore, 352-353. 

in lakes, 328-330. 

ocean, 354-355, 682, 687-700. 
Cushing, H. P., 194. 
Cuspate forelands, 364-365. 
Cut-offs, 150. 
Cwm, 234. 
Cycle, geographical, 171. 

incomplete, 191. 

of mountain development, 553-556. 

second, 556. 
Cycle river, 171-196. 



Cyclone, 759. 
Cyclone cellars, 778. 
Cyclones, tropical, 778-781. 
Cyclonic areas, 758. 
Cyclonic storms, 759-778. 

Daily range, 730-731. 

Dale, T. N., 560, 578, 580. 

Daly, R. A., 56, 194, 378, 384, 441, 465, 

486, 493, 578. 
Dana, J. D., 35, 36, 378, 384, 402, 493, 578, 

625. 
Danube delta, 157. 

Darton, N. H., 98, 138, 234, 522, 578, 626. 
Darwin, Charles, 56, 303, 378, 384. 

G. H., 626, 666. 
Daubree, A., 578. 
Davey, F., 431. 
David, T. W. E., 303. 
Davis, A. P., 139. 
C. A., 339, 385. 

W. M., 73, 74, 75, 98, 139, 168, 171, 
194, 206, 232, 253, 254, 303, 311, 
370, 385, 407, 435, 493, 522, 557, 
578, 626, 725, 744, 781, 807. 
Davison, C, 56, 435. 
Dawson, G. M., 303, 578. 

J- W., 303, 385. 
Day, 2. 
Day, A. L., 493. 

P. C, 813. 
Dayton flood, 107. 
Dead Sea, 325. 
de Beaumont, E., 625. 
de Bort, Teisserenc, 752. 
Debris-covered ice, 220-222. 
Deccan, 482. 
Decken, 538. 
Declination, 629, 631. 
Deeps, 640, 644, 645. 
Deep sea conditions, 637. 
Deep sea sediments, 649. 
Deformation, relation to vulcanism, 490. 
de Geer, G., 253, 303, 316, 385, 407. 
Degree, length of, 5. 
Degradation, 18. 
Degrading streams, 114. 
Dejection, cone of, 163. 
de Lapparent, A., 579, 627, 666. 
de la Beche, H. T., 625. 
Delaware Water Gap, 565, 566. 
Delebecque, A., 339. 
Deltas, 152-160, 318-319. 
Deluge, supposed relation to erratics, 257-258. 
de Margerie, E., 495. 
de Martonne, E., 75, 253, 579, 580, 587, 

641, 666, 684. 
de Montessus de Ballore, F., 416, 417, 427, 
429, 436, 666. 



820 



INDEX 



Density, of air, 709-710. 

of water, 688. 
Denudation, 17-18, 546. 

plains of, 500. 
Depression, effect of, 191. 
Deposits, avalanche, 552. 

eolian, 65-69, 72-73. 

glacial, 223, 264-279, 552. 

in salt lakes, 324. 

lake bottom, 319. 

marine, 584, 646-649. 

mountain, 552. 

river, 141-170. 

terrestrial, 23-24, 65-69, 72-73, 141- 
170, 264-279, 552. 
De Quervain, A., 247. 
Derby, O. A., 56. 
Derelicts, 699. 
Desert, 64, 792-794. 

of Greenland, 247-249. 

transportation of dust from, 70-71. 

wind work in, 65-73. 
Desert climates, 783. 
Desert valley filling, 165. 
Desiccation, 321. 
Destructional plains, 501. 
Destructive effects of earthquakes, 412-413. 
Devonian, 32. 
Dew, 736. 
Dew point, 736. 
Dexter, E. G., 813. 
Diabase, 25, 26. 

Diastrophism, 17, 389-437, 586, 622, 645. 
Diathermanous substances, 717. 
Diatomaceous ooze, 648. 
Dickson, H. N., 666, 813. 
Dietz, E. A., 522. 

Differential gradation, plains of, 500. 
Dike, sandstone, 413. 

volcanic, 25, 477, 482-483. 
Diller, J. S., 493, 522, 578. 
Diluvium, 32. 
Diorite, 26. 
Dip, 400. 
Dip circle, 633. 
Dip needle, 631. 
Dipping compass, 633. 
Discordant valleys, 230-233. 
Disease, 714. 
Dismal swamp, 336. 
Dissection of plains, 501-502. 
Dissipator, 205. 
Distortion of hydrosphere, 700. 
Distortion of sea level, 640. 
Distributaries, 156. 
Distributary, glacial, 249-251. 
Disturbance of the strata, 400-409. 
Dittmar, W., 666. 
Diverting stream, 186. 



Divides, migration of, 562-563. 

shifting, 185-186. 
Dodge, R. E., 168, 813. 
Doldrums, 752, 789. 
Dolomite, 19-21, 24. 
Dolomites, the, 548. 
Dolphin Plateau, 643. 
Domes, 402, 540. 
Dormant volcanoes, 448. 
Double folds, 539. 
Down folding in mountains, 537. 
Downs, 795. 
Downthrow, 403. 
Dragging, 100, 11 2-1 13. 
Drainage, in mountains, 558-567. 

of swamps, 335. 

preglacial, 290. 

reversal of, 291-292. 
Dredging, 638. 
Drift, glacial, 223, 265. 

marine, 695. 
Driftless Area, 263, 296. 
Drikanter, 69-70. 
Drong mountains, 538. 
Drowned valleys, 190. 
Drumlins, 268-269. 
Dry bulb thermometer, 734. 
Dryer, C. R., 522. 
Dry season, 794. 
Duclos, photograph by, 233. 
Dunes, 59-62, 66-68. 
Dunwiddie, A. W., 774. 
du Pasquier, L., 254. 
Dust, 57-58. 

atmospheric, 714. 
Dust wells, 59. 
Dust whirls, 66. 

Dutton, C. E., 139, 435, 494, 522, 614, 626. 
Dyas, 32. 

Eakin, H. M., 194. 
Ear bones of whales, 649. 
Earth activities, 15-18. 
Earth, age of, 623-625. 

as a planet, 1-8. 

form of, 12-15. 

in solar system, 4-7. 

magnetic survey of, 633. 

model of, 14. 

origin of, 617-620. 

relief features of, 583-610. 
Earth elements, 9-12. 
Earth in space, 8-9. 
Earth movements, 400, 622. 
Earth pillars, 103. 
Earth's axis, change of, 621-623. 
Earth's crust, 10. 

instability of, 399-400. 

movements of, 389-437. 



INDEX 



821 



Earth's interior, 611-628. 

Earth's magnetism, 629-635. 

Earth's surface, relation to interior, 611-612. 

Earthquake prediction, 418-420. 

Earthquake water waves, 433-435, 687. 

Earthquakes, 8, 409-435. 

distribution of, 415-417. 

relation to landslides, 51-52. 

specific instances, 420-432. 
Earthworms, work in weathering, 44. 
East coasts, climate of, 801-803. 
East declination, 629. 
East Haddam earthquakes, 426. 
Eastman, C. R., 494. 
Eclipse, 1, 7-8. 
Ecliptic, plane of, 4. 
Eddies in Atlantic, 693-694. 
Eddies in rivers, 113. 
Edmunds, C. K., 666. 
Eifel, 458-459, 480. 
Elastic rebound, 419. 
Elements, 18-19. 
Elevation, 345, 506. 
Elevations, measurement, 712. 
Elevated shorelines, 358, 382, 390-391. 
Elm landslide, 52. 
Emergence, 390. 
Emerson, B. K., 626. 

F. V., 75, 151, 168, 522. 
Emmons, S. F., 194, 522, 578. 
Empress of Ireland, wreck of, 738. 
Enclosed seas, circulation in, 690-691. 
Encyclopaedia Britannica, 7, 136, 259,632, 

700, 740. 
Energy, radiant, 716-717. 
Entrenched meanders, 188-190. 
Eocene, 32. 

Epicentrum, 411-412, 415. 
Epicontinental seas, 583-584. 
Equable climates, 803. 
Equator, 5. 

heat, 752. 

magnetic, 633. 
Equatorial calms, 752, 788-790. 
Equidistant projection, 33. 
Erosion, 18. 

along coasts, 351, 353, 355. 

by glaciers, 228-237, 252, 286-287, 350- 
35i, 546. 

headwater, 175. 

relation of joints to, 409. 

stream, 114-117, 544-545. 
Erosion features, 1 5 . 
Erratics, 267. 
Eruptions, fissure, 480-482 . 

instances of volcanic, 440-475. 
Erzgebirge, 569. 

Escarpments, 130, 406-407, 503-505. 
Escher, A., 539. 



Eskers, 273-274. 

Esker deltas, 275-277. 

Estuaries, 190, 349-350. 

Ether, 709. 

Etna, 441, 451-453. 

Eurasia, 534-535, 599-607. 

Europe, 600-602. 

Evaporating pan, 734 _ 735- 

Evaporation, 76, 733-734- 

Everglades, 336. 

Excelsior hot spring, 84. 

Exfoliation, 43. 

Exploration, 635. 

Extended rivers, 190. 

Extinct volcanoes, 448, 477. 

Extra-terrestrial processes, 15-17- 

Fahrenheit scale, 720. 

Fairbanks, H. W., 385, 579- 

Fairchild, H. L., 35, 194, 285, 286, 303. 

Fall line, 511. 

Falsan, A., 303. 

Fans, alluvial, 162-165. 

Fan-shaped folds, 538-539. 

Fassig, O. L., 781, 782. 

Fault block lakes, 316. 

Fault block mountains, 526. 

Faulting, 622-623. 

nature of, 403. 

relation to topography, 407-408. 
Faulting accomplished slowly, 406. 
Faulting in mountains, 526. 
Fault-line scarps, 407. 
Fault planes, 403. 
Fault scarps, 406-407. 
Fault trace, 430. 
Faults, 30. 

gravity, 403. 

horizontal movements along, 405-406. 

normal, 403. 

overthrust, 405. 

relation of springs to, 80. 

reversed, 405. 

step, 403. 
Feldspar, 19-20. 

Fenneman, N.M., 339, 385, 522. 
Ferguson, S. P., 744. 
Ferrel's Law, 693, 756, 780. 
Ferrel, William, 782. 
Filhol, H., 666. 
Filled lake plains, 499. 
Fingal's Cave, 484. 
Finley, J. P., 782. 
Finsterwalder, S., 253. 
Fiords, 236, 350-35 1- 
Firn, 206. 
Fisher, E. F., 169. 

O., 626. 
Fish Hawk, 638. 



822 



INDEX 



Fissure eruptions, 480-48.'?. 

Flint, 20. 

Flint, J. W., 666. 

Floe ice, 662. 

Floodplains, 143-146, 180. 

Flood, supposed relation to erratics, 257-258. 

Flood warnings, 109. 

Floods, 104-109. 

Floods on deltas, 160. 

Florida, 172. 

Florida coast railway, 384. 

Flow, lava, 438. 

Flowage, in earth's interior, 12. 

in ice, 202. 

of rocks, 539. 

zone of, 12. 

zone of, in ice, 203. 
Focus, 411. 
Foehn winds, 773. 
Fog, 700, 714, 737-738. 
Folded structures in mountains, 525-526. 
Folding, 622-623. 

in mountains, 526. . 

nature of, 400. 

Folds, 30. 

forms of, 400-401. 

relation to topography, 402. 
Follansbee, R., 339. 
Fool's gold, 21. 
Foraminifera, 647. - 

Force, lines of, 633. 
Forel, F. A., 253, 339. 
Foreset beds, 154. 
Forest on glaciers, 240-241. 
Forests, relation to rivers, jo6. 
Fort Wayne Outlet, 282. 
Fossils, 23, 31. 
Foucault's pendulum, 3, 4. 
Fouque,< F., 494. 
Fracture, zone of, 10. .,..._ 

zone of, in ice, 203. 
Fragmental rocks, 23-24. 
Fram, 662, 696. 
Frankenfield, H. C, 139. 
Frank landslide, 52-53. 
Franklin, Benjamin, 699. 
Freezing temperatures, 197. 
Friedlander, I., 494. 
Fringing reefs, 377~378. 
Frost, 736. 
Frost action, 41. 
Fujiyama, 468. 

Fuller, M. L., 98, 303, 339, 435. 
Funafuti, 378. 

Gale, 747- 

(ialilco, 3. , 

Galveston, location on sand bar, 371. 
destruction of, 778^779. 



Ganges delta, 159. 

Ganges, flood in, 110. 

Gannett, Henry, 36, 75, 253, 509, 522, 543, 

744, 802, 813. 
Gardiner, J. S., 666. 
Gardner, J. L., 268, 702. 
Garriott, E. B., 782. 
Garwood, E. J., 253. 
Gas volcano, 487. 
Gases, in lavas, 439-440, 491. 

in sea water, 653-654. 

poisonous, 445. 

volcanic, 444. 
Gazelle, 637. 
Geanticline, 401-402. 
Geikie, A., 35, 56, 139, 385, 579, 626. 

J-, 303, 435, 494, 579- 
Gems, 22. 
Geographical changes in relation to glacia- 

tion, 301. 
Geographical cycle, 171. 
Geoid, 12. 
Geological ages, 31. 
Geological column, 32. 
Geologicaltime, 623. 
Geologists, estimates of earth's age by, 624- 

625. 
Geomorphology, — for definition, see In- 
troduction. 
Geosyncline, 401-402. 
Geyser basins, 86. 
Geyser eruptions, cause of, 87-89. 
Geysers, 85-89, 486. 
Giant's Causeway, 484. 
Gibbs, G. S., 139. 
Gibraltar, 364. _ 

Gilbert, G. K., 43, 73, 102, 126, 128, 129, 139, 
169, 194, 215, 220, 229, 236, 253, 304, 
322, 323, 339, 382, 383,.3 8 ,5,.398, 399, 
406, 414, 433, 435, 485, 494, 528, 579, 
626, 744. 
Glacial deposits, 223, 264-279. 
Glacial drift, 223. 
Glacial erosion, 228-237, 252, 286-287, 35°- 

351, 546, 549-550. 
Glacial Great Lake's, 280-284. 
Glacial Lake Agassiz, 281. 
Glacial. lakes ; 227. . 

Glacial Period, 32, 256-307. 

complexity of, 297-209. 

time since, 130. 
Glacial Periods, pre-Pleistocene, 299. 
Glacial plains, 499. 
Glacial swamps, 337. 
Glaciation, 187, 197-307, 621-622, 783. 

early explanations of, 257 -258. 

evidence of former, 256-2,5^7. 

extent oi 258-263,., 

hypotheses for, 299-302. ' 



INDEX 



823 



Glaciation, influence on topography, 293-294. 

Glacier milk, 225. 

Glacier National Park, 545, 548, 549. 

Glacier Peak, 472. 

Glacier, railway on, 574, 575. 

Glacier reservoir, 205. 

Glacier tables, 217. 

Glacier types, 203-204. 

Glacier wells, 217. 

Glacier wind, 749. 

Glaciers, 197-255. 

continental, 243-252. 

of Antarctica, 609. 

piedmont, 239-243. 

rate of motion, 212. 

size of, 209. 

thickness of, 211. 
Glacio-fluviatile deposits, 223, 225, 272. 
Glauconite, 652. 
Glenn, L. C., 139, 522. 
Globigerina, 647. 
Globular projection, ss- 
Gneiss, 27-28. 
Gobi, desert of, 67. 
Gockel, Albert, 782. 

Goldthwait, J. W., 195, 304, 384, 385, 435. 
Goode, J. P., 195, 385, 744. 
Gorges, 117-120, 132, 176. - . 

Grabau, A. W., 35, 139, 169. 
Graben, 403, 407. 
Grade, 1 19-120. 

correction of, 179-180. 
Gradient, stream, 119. 

barometric, 746. 
Graham Island, 450-451. 
Graham, J. C., 98. 
Grand Banks, fogs at, 738. 
Grand Canyon of the Colorado, 120-123. 
Grand Coulee, 313, 517. 
Granite, 25-27. ... 

Granite soils,- 54: 
Graphite, 28. 

Gravel, 295. 

Gravitation, 4, 7, 16-17. .-. .- 
Gravity, 711. 

Gravity faults, 403. ■:-,.... 
Great Basin, 321-322, 519-521. 
Great circle routes, 754. 
Great Ice Barrier, 246. 
Great Lakes, 326-328. 

currents in, 330. 

glacial, 280-284. 
Great Salt Lake, 322, 325,. 334. 
Greely, A. W., 782, 813,-0- ^-. 
Greenland Ice Sheet, 247-252. 
Green water, 654. ■•■'- - ■ 

Green, W. L., 62.6. . • ■ 
Gregory, H. C, 522. 

J. W., 139, 385, 435,, 626,-666 ....... 



Grimshaw, P. H., 812. 

Grooch, F. A., 98. 

Ground moraine, 222-223, 22 4- 

Ground swell, 684. 

Ground water, 76-99, 197. 

Growth of mountains, 536. 

Guiana Highland, 594. 

Guilbert, G., 782. 

Gulch, 117. 

Gulf Stream, 694-695, 787. 

Gulliver, F. P., 385. 

Giinther, S., 36. 

Guyot, Arnold, 522. 

Gypsum, 19, 21, 324, 783. 

Habitability long maintained, 8-9. 

Hachures, 35. 

Hade, 403. 

Haeckel, E., 666. 

Hague, A., 522, 579. 

Hahn, F. G., 385. 

Hail, 743-744- 

Hair hygrometer, 734. 

Halemaumau, 466. „_j 

Hall, C. W., 522. 

Halos, 716. ... ..'.'. 

Hanging valleys,. 210, 230-233, 287, 293 
3Si, 550. 

submerged, 236. • . 
Hann,J., 744, 813. 
Hansen, A. M., 304. 
Harbours, 380-382. 
Harbours, crater, 488. 
Hardin, photograph by, 50. 
Hard pan, 265. 
Hard water, 83. 
Hardy, M., 813. 
Harker, A., 494. 

Harper's Ferry, 566. „ 

Harrington, M. W., 330, 339, 813. 
Harris, R. A., 666. '• 

Hatch, L., 404 '1 

Haug, E., 626. 

Haulalai, 464. 

Haupt, L. M., 383. 

Hawaiian volcanoes, 464-466. . 

Hayden, E.,.780.. : 

F. V., 579- 
Hayes, C. W., 579. 
Hayford, J. D., 435, 614, 626. 
Headwater erosion, 175. 
Heat, sources of local, 620-621. 
Heat equator, 752. 
Heat of vaporization, 722. 
Heated interior of earth, 11, 612. 

sources, 617-618. 
Heave, 405. '.'■>- 

Hedin, Sven, 74. 
Heilprin, A., 462, 494,-522. ,. 



824 



INDEX 



Heim, A., 56, 253, 538, 539, 579- 

Hekla, 467. 

Helderberg Mountain, 506, 525. 

Heligoland, 383. 

Helium, 713. 

Helland-Hansen, B., 655, 666. 

Hematite, 19, 21. 

Hemispheres, 33. 

Henderson, J., 253. 

Henry, A. J., 744, 813. 

Herbertson, A. J., 744, 791. 

Herculaneum, 454-455. 

Hess, H., 253. 

Hicks, L. E., 139. 

Hidden Glacier, 214. 

High, 711, 759- 

High tide, 701. 

Hildebrandsson, H. H., 782. 

Hilgard, E. W., 56, 158, 169, 813. 

Hill, R. T., 75, 522. 

Hillers, 121, 137, 428, 512. 

Hills, 525. 

Himalayas, 603. 

Hinde, G. J., 385. 

Hinge lines, 283. 

Hitchcock, A. S., 74. 

C. H., 304, 466, 494, 554- 
Hjort, J., 666, 667. 
Hoang Ho, 109, 160. 
Hoang Ho delta, 152. 

Hobbs, W. H., 35, 245, 253, 269, 304, 339, 
429, 435, 437, 494, 579, 626, 666, 729. 
Hodge, F. M., 522. 
Hoernes, R., 435. 
Hogbacks, 546. 
Holden, E. S., 377. 
Hole, A. D., 305. 
Hollister, G. B., 98. 
Holmes, A., 626. 

W. H., 98, 122. 
Holtenberger, M., 74. 
Homolographic projection, 33. 
Hooks, 366-368. 
Hopkins, C. G., 56. 
Hornblende, 19-20. 

Horizontal orogenic movements, 537-539. 
Horizontal movements along faults, 405-406. 
Horn, J., 538, 580. 
Horner, M., 438. 
Horns, 540. 
Horse latitudes, 753. 
Horseshoe Fall, 129. 
Horsts, 406-407. 
Hot season, 7g6. 
Hot springs, 83-85, 612. 
Hot Springs, Ark., 84. 
Hovey, E. O., 494. 

H. C., 92, 98. 
Howe, E., 56, 305, 494. 



Howe's Cave, 93. 

Howchin, W., 304. 

Howell, E. E., 296. 

Hoyt, W. G., 116, 139. 

Hubbard, G. D., 304, 385. 

Hugues, L., 666. 

Hull, E., 494- 

Human Geography, see Man; also books 

listed in Introduction. 
Humboldt Glacier, 250. 
Humidity, 733~735- 
Humphreys, A. A., 139. 

W. J., 713, 744- 
Hunter, J. F., 385. 
Huntington, E., 74, 304, 435, 813. 
Hunt, T. S., 494. 
Hurricane warnings, 781. 
Hurricanes, 778-781. 
Hydration, 39, 77. 
Hydrogen, 713. 
Hydrosphere, 9-10, 637-708. 
Hygrometer, 734. 

Ice, in lakes, 197. 

in rivers, 186-187. 

in the ocean, 661-665. 

in the sea, 197-198. 

work of river, 115. 
Ice apron, 242. 
Ice caps, 203-204, 243-245. 
Ice cascades, 218. 
Ice falls, 218. 
Ice foot, 662-663. 
Ice gorges, 115. 
Ice ramparts, 331-332. 
Ice sheets, 204, 243-252. 
Ice stream, 206. 
Ice structure, 216. 
Ice-dammed lakes, 279-280. 
Ice-sculptured valleys, 230. 
Icebergs, 209, 246-247, 250-251, 663-665. 
Iceberg waves, 687. 
Icelandic volcanoes, 467. 
Iddings, J. P., 494. 
Igloo, 810. 

Igneous rocks, 22, 24-26, 482, 612. 
Ikenberry, W. L., 774, 777- 
Illinoian drift, 298. 
Incised meanders, 189, 564. 
India, 795-796. 

monsoons of, 749. 
Indian earthquakes, 421. 
Indian Ocean, 639, 644-645, 691-692. 
Ingrafted rivers, 190. 
Inlets, 370. 
Inlets of lakes, 310. 
Insequent streams, 186, 559. 
Inside Passage, 595. 
Insolation, 717, 725, 726. 



INDEX 



825 



Instability of the earth's crust, 399-400. 

Instrument shelters, 721. 

Intensity, magnetic, 631. 

Interglacial gorges, 293. 

Interglacial stages, 298. 

Interior of earth, 611-628. 

Interiors of continents, climate of, 801, 

803-804. 
Interlaken, 159. 
Interlobate moraine, 272. 
Intermediate zones, 797-810. 
Intermittent streams, 106. 
International Boundary Survey, 201, 217. 
International 1 : 1,000,000 map of the 

world, 609-610. 
Interruptions, 187-192. 
Intruded lavas, 482-486. 
Invertebrates, age of, 32. 
Inwards, Richard, 782. 
Iowan drift, 298. 
Iron ores, 21, 97. 

Irregular coasts, showing submergence, 393. 
Ischia, 453. 
Isinglass, 20. 
Islands, 380, 589. 

of the Pacific, 609. 
Isobars, 759. 
Isoclinal folding, 401. 
Isogonic maps, 629. 
Isoseismals, 412. 
Isostasy, 389, 614, 615. 
Isothermal charts, 723-724. 
Isothermal layer, 728-729. 
Isotherms, 723. 
Italian earthquakes, 420-421. 
Italian volcanoes, 449-458. 
Ithaca Falls, 105. 

Jackson, photograph by, 85. 
Jaggar, T. A., Jr., 98, 469, 494. 
Jamaica earthquakes, 426. 
Jamieson, T. T., 304. 
January thaw, 772. 
Japanese Current, 697. 
Japanese earthquakes, 422-423. 
Jeanette, 696. 

Jefferson, Mark, 75, 169, 744, 794, 813. 
Jersey an drift, 298. 
Joerg, W., 579- 

Johnson, Douglas W., 139, 195, 253, 363, 
385, 494, 519, 579- 

L. C, 169. 

Willard D., 253, 429, 437, 515, 522. 

W. E., 36. 
Johnstone, J., 666. 
Johnston-Lavis, H. J., 494. 
Johnstown flood, 107, 317. 
Joint planes, 29, 134, 408-409, 484. 
Joly, J., 626. 



Jones, Thomas, 14. 

Joubin, L., 666. 

Juday, C, 339, 34<>. 

Judd, J. W., 494, 495- 

Julien, A. A., 56. 

Jupiter, 2. 

Jupiter Serapis, temple of, 395. 

Jura, 528, 600. 

Jurassic, 32. 

Kalahari desert, 67. 

Kame moraine, 275. 

Karnes, 274-275. 

Kangra earthquake, 421. 

Kansan drift, 298. 

Kant, Immanuel, 626. 

Kaolin, 19-20. 

Kar, 234-236. 

Karluk, 696. 

Karst topography, 92. 

Kaskaskia, abandonment of, 151. 

Katmai volcano, 443, 469. 

Kauai volcano, 476. 

Kayak, 810. 

Keewatin ice sheet, 259-262. 

Keith, A., 538, 579- 

Kelvin, Lord, 628. 

Kemp, J. F., 36, 522, 579. 

Kettle moraine, 275. 

Keyes, C. R., 74. 

Keys, Florida, 378. 

Key West railway, 384. 

Kikuchi, Y., 495. 

Kilauea, 464-466. 

Killing frosts, 737. 

Kinahan, G. H., 304. 

King, Clarence, 579, 626. 

C F., 75- 

F. H., 56, 98. 
Kingston earthquake, 426. 
Kirchhoff, A., 666. 
Kirkfield outlet, 283. 
Kiruna, 488. 
Klippen, 538. 
Knekel, W. von, 98. 
Knob-and-basin topography, 271. 
Knott, C. G., 435. 
Kobayashi, K., 435. 
Koch, J. P., 247. 
Koppen, W., 666, 757, 813. 
Koto, B., 435. 
Krakatoa, 58, 462-464. 
Krummel, O., 666. 
Kuen Lun Mountains, 602 . 
Klimmel, H. B., 195. 
Kuro Shi wo, 697. 

Labrador Current, 696-697, 787. 
Labrador ice sheet, 259-262. 



826 



INDEX 



Laccolites, 485. 

Laccolitic mountains, 52L. 

Lachine Rapids, 124. 

Lacroix, A., 494. 

Lacustrine plains, 499. 

Lagoons, 368-370. 

Lake Agassiz, 398, 501. 

Lake Algonquin, 283. 

Lake basins, causes, 311. 

Lake Bonneville, 322-324, 382-383, 398. 

Lake Chicago, 282. 

Lake clay, 227, 280. 

Lake dams, stability, 317. 

Lake deposits, removal, 321. 

Lake Drummond, 315. 

Lake Duluth, 282. 

Lake ice, 197. 

Lake Iroquois, 280, 283, 398. 

Lake Lahontan, 324. 

Lake level, variations in, 309-310. 

Lake Lundy, 282. 

Lake Maumee, 282. 

Lake Nipissing, 283. 

Lake Pepin, 313. 

Lake plains, 499. 

Lake Pontchartrain, 315. 

Lakes, 174-175, 295, 308-341. 

areas, elevations, and depths, 309. 

classification of, 311, 317. 

filling of, 318. 

formation of, 288. 

life history, 320. 
Lake Saginaw, 282. 
Lake shores, 331-332. 
Lake Van, 325. 
Lake Warren, 282. 

Lake waters, movements oi x 328-331. 
Lake Whittlesey, 282. 
Lamplugh, G. W., 139, 304. 
Land breeze, 747-748. .. 
Land-derived deposits, 646-647. 
Land hemisphere, 587. 
Lands, the, 1-636. 
Landslides, 49-52, 97. 
Langley, S. P., 744. 
Language, in mountains, 571.. 
Lapilli, 443-444. 
Laplace, P. S., 617, 626. 
Lassen Peak, 472, 474. 
Latent heat, 722. i( 
Lateral moraine, 222, 225, 270. 
Lateral thrust, 619. 
Latitude, 5. 

Laurentian Highlands, 598. 
Lava, 25, 438. 
Lava flows, 430. 

rapidity, 440 

cooling, 441. 

size, 1 1 1 l ' 



Lava plains, 499. 
Lava plateau, 480-482. 
Lavas, intruded, 482-486. 

mud, 445. 
Law, Ferrel's, 756. 

of migrating divides, 186. 

of waterfall extinction, 138. 

of waterfall formation, 137. 

of weathering, 47. 

Playf air's, 177. 
Lawson, A. C, 253, 305, 385, 430, 435, 436, 

579- 
Leads, 662. 

Le Conte, J., 35, 385, 579, 627. 
Lee coasts, 794. 
Lee, W. T., 195. 
Left-handed deflection, 756 
Leith, C. K., 436. 
Lesquereux, L., 523. 
Levees, natural, 146. 

Leverett, Frank, 260, 273, 282, 283, 304, 306. 
Lewis, H., 627. 

H. C, 304. 
Liassic, 32. 
Libbey, W., 666. 
Lichens, work in weathering, 43. 
Life, abundance of marine, 671. 
Life in the ocean, 669-681. 
Light, 715-716. 
Light in the ocean, 655-656. 
Lightning, 772, 7 73-774- 
Lime, 652. 
Limestone, 23. 
Limestone soils, 53 _ 54- 
Limonite, 19, 21. 
Lincoln, D. F., 253. 
Lindenkohl, A., 385. 
Lindgren, W., 195, 305, 579. 
Linnaeus, 397, 720. 
Lipari Islands, 449-450. 
Lisbon earthquake, 420. 
Lithodomus, 373, 395- 
Lithosphere, 1-636, 9-10. 
Littoral climates, 783. 
Littoral deposits, 646-647. 
Littoral life in the sea, 672-675. 
Llanos, 795. 

Load, of rivers, 111-114. 
Lobate moraine, 272. 
Lockyer, J. N., 627. 
Loess, 71-73. 

glacial, 277. 
London fog, 738. 
Longitude, 5. 

Longitudinal drainage, 560. 
Looming, 716. 

Louderback, G. D., 339, 495, 579. 
Love, A. E. H., 627- 
Low, 711, 759. ;.: ' •. 



INDEX 



82; 



Lowl, F., 195. 

Low pressure areas, 759. 

Low tide, 701. 

Lucin cut-off, 334. 

Lugeon, M., 253, 538, 579. 

Lumbering, mountain, 568. 

Lunar craters, 487. 

Lunar distortion, 700. 

Luray Cavern, 93-94. 

Lyell, Sir Charles, 131, 158, 169, 436, 495, 

627. 
Lyons, H. G., 139. 

Maare, 459. 

McAdie, A., 782. 

McCall, R. E., 98. 

McConnell, R. G., 53, 56, 303, 538. 

McDougal, D. T., 64, 74, 169. 

McGee, W J, 98, 139, 253, 304, 523, 627, 

782. 
Mackinder, H. J., 523. 
Magma, 489. 
Magnetic equator, 633. 
Magnetic meridians, 631. 
Magnetic pole, 631. 
Magnetism, terrestrial, 629-636. 
Magnetite, 19, 21, 633. 
Malaspina Glacier, 239-243. 
Malladra, descent into Vesuvius by, 447. 
Mammals, age of, 32. 
Mammoth Cave, 92-93. 
Mammoth Hot Springs, 85. 
Man and — 

alluvial fans, 164-165. 

barrier beaches, 370. 

deltas, 159. 

earthquakes, 409, 413, 434-435. 

floodplains, 145. 

food fish, 680-681. 

glaciation, 256, 294-297. 

lakes, 332-334- 

loess, 72-73. ._-, 

mountains, 567-577. 

plains, 507-508. 

rivers, 100-101, 150-15 1. 

salt marshes, 375. 

sea coasts, 360, 383T384. . 

soil, 55-56. 

swamps, 334-335- 

terrestrial magnetism, 635. 

tides, 708. 

valleys, 192-194. 

vulcanism, 467, 487-489. 

waterfalls, 138. 
Mangrove swamps, 375 . 
Mantle rock, 30-31. 
Manufacturing, in mountains, 570.. 
Maps, 33-35, 635. 
Marbut, C. F., 523. 



Marble, 27-28. 

Marginal channels, 284, 286. 

Marginal lakes, 227, 279-284. 

Marine climates, 783. 

Marine denudation, plains of, 500. 

Marine deposition, 645-649. 

Marine life, 697. 

Marine organisms, showing uplift, 391. 

Marine plains, 499-500. 

Markham, C, 629. 

Marl, 320. 

Marr, J. E., 580. 

Mars, 3. 

Marshall, W., 666. 

Marshes, 334. 

salt, 374-375- 
Martel, E. A., 98. 
Martin, G. C, 470, 495. 

J. O., 386. 
Martinique, 459-462. 
Marvin, C. F., 745, 782. 

R. G., 662. 
Mascaret, 707. 
Mato Tepee, 482. 
Matson, G. C, 523. 
Matthes, F. E., 253, 437. 
Matthews, E. B., 522. 
Mature coasts, 372. 
Mature mountains, 554. 
Mature valleys, 179-182. 
Maturity, 179-182. 
Mauna Loa, 441, 464. 
Maury, M. F., 638, 666. 
Maximum thermometers, 720. 
Mead, D. W., 139- 
Meander River, 148. 
Meanders, 146-15 1, 180, 321. 

entrenched, 189-190. 
Mechanical load, 111-112. 
Medial moraine, 208, 222. 
Medicinal springs, 83. 
Mediterranean, 690. 
Mediterranean seas, 346, 644. 
Meereskunde, Institut fiir, 668. 
Melville, G. W., 666. 
Menauer, J., 169. 
Mendenhall, A. C, 522. 

W. C, 495- 
Mercalli, G., 495. 
Mercator's projection, 33. 
Mercurial barometers, 711. 
Mercury, 2. 
Mer de Glace, 212. 
Meridians, magnetic, 631. 
Meridional mountains, climate of, 799-800. 
Merriam, C. H., 813. 
Merrill, G. P., 56, 495- 
Mesa, 503. 
Mesozoic, 32, 



828 



INDEX 



Metal thermometers, 720. 

Metamorphic rocks, 22, 27-28. 

Metamorphism, 27. 

Meteor crater, 470. 

Meteorites, 1, 649. 

Meteoritic hypothesis, 618. 

Meteors, 709. 

Mica, 19-20. 

Michael Sars, 638. 

Michelson, A. A., 627. 

Microthermometer, 663. 

Mid-Atlantic Ridge, 642-644. 

Middlemiss, C. S., 436. 

Migration of divides, 562-563. 

Migration, seasonal, 758. 

Milham, W. I., 745, 761, 772, 801, 805. 

Mill, H. R., 339, 523, 609, 813. 

Miller, W. G., 56. 

Millibars, 766. 

Millionth map of the world, 609-610. 

Milne, J., 419, 424, 436. 

Mindel, 260. 

Mineral load of rivers, 111-114. 

Mineral matter in the sea, 650-654. 

Mineral springs, 83. 

Minerals, 18-22. 

composition, 19. 

defined, 19. 

in rocks, 21-22. 
Mine water, 77. 
Minimum thermometers, 721. 
Mining, mountain, 569-570. 
Mino-Owari earthquake, 422. 
Miocene, 32. 
Mirage, 716. 
Mississippian, 32. 
Mississippi River, 149. 
Mississippi River Commission, 149, 170. 
Mississippi River, delta, 154-155, 159. 

floods, 108-109. 

load of, 114. 
Mississippi valley plains, 513-516. 
Missouri River Commission, 170. 
Mistral, 771. 
Modified drift, 272. 
Mohave desert, 792. 
Mohawk outlet, 283. 
Molten rock, 489. 
Molyneux, A. J. C, 139. 
Monaco, Prince of, 638, 666. 
Monadnocks, 554-555- 
Monoclinal ridges, 546. 
Monoclinal shifting, 547. 
Monocline, 81-82, 401, 402. 
Monomoy, wind erosion at, 61. 
Monsoon climates, 783, 795-796. 
Monsoon winds, 749-750. 
Monte Nuovo, 453. 
Monte Somma, 454, 456. 



Moon, 1. 

craters on, 487. 

earth and, 7-8. 

relation to tides, 702-703. 

size, 7. 
Moore, J. W., 745- 

W. L., 745, 782. 
Moraine, 222-223, 269-272. 
Moraine bar, 237. 
Moraine terraces, 284. 
Moraine-headed terraces, 277. 
Moseley, W., 666. 
Mossman, R. C, 813. 
Mougin, M., 212, 253. 
Moulins, 216. 

Moulton, F. R., 3, 4, 35, 625, 627, 634. 
Mt. Adams, 472. 
Mt. Baker, 470. 
Mt. Edgecumbe, 468. 
Mt. Erebus, 479. 
Mt. Hood, 470, 472, 480. 
Mt. Mazama, 475. 
Mt. Rainier, 470. 
Mt. Royal, 483. 
Mt. St. Helens, 469. 
Mt. Shasta, 471-472. 
Mt. Taylor, 482. 
Mt. Terror, 479. 
Mt. Wrangell, 468. 
Mountain belts, 529, 532. 
Mountain climates, 783. 
Mountain glaciers, 261-262. 

of Europe, 258-259. 
Mountain growth, 345-348. 
Mountain-making, relation to earthquakes, 

417-418. 
Mountains, 525-582. 

altitudes, 533~534- 

growing, 394"395- 

of Asia, 603-606. 

of Australia, 608. 

of New Zealand, 608-609. 

of Europe, 600. 
Mountain types, 525-529. 
Mountain wind, 748-749. 
Movements of ocean water, 682-708. 
Movements of the earth's crust, 389-437. 
Mud flows, 445. 
Mud volcanoes, 486. 
Mud-lumps, 156, 158, 486-487. 
Muggy weather, 723. 
Muir Glacier, 209, 211. 
Muir, John, 236. 

Murray, Sir John, 104, 339, 378, 386, 637, 
639, 641, 646, 648, 667, 671, 672, 676, 
677, 690. 
Murz line, 417. 
Muscovite mica, 20. 
Muskegs, 334. 



INDEX 



829 



Names applied to mountains, 539-542. 
Nansen, F., 247, 254, 386, 588, 638, 644, 662, 

666, 667, 811. 
Nasmyth, diagram by, 487. 
Natural bridges, 91, 94-96. 
Natural gas, 23. 
Natural levees, 146, 498. 
Navigation, 635, 697-699. 

obstacles to, 142. 
Neap tides, 703. 
Nebraskan drift, 298. 
Nebula, 617. 

Nebular hypothesis, 617-618. 
Neck, volcanic, 477, 482. 
Needles, 540. 
Neocene, 32. 
Neptune, 2. 
Neve, 205-206. 
Newberry, J. S., 304. 
Newburyport earthquakes, 427. 
Newcomb, Simon, 35. 
New Madrid earthquake, 427-428. 
Newton, H., 580. 
New World plateau, 588. 
New Zealand, 480, 608-609. 
Niagara, 126-134, 280. 
Nicolson, J. T., 625. 
Nile delta, 152. 
Nile, flood in, no. 
Nimbus clouds, 739. 
Nineveh, wind work near, 69. 
Nippoldt, A., 636. 
Nitrogen, 712, 713. 
Nordenskjold, O., 254. 
Norlind, A., 169. 

Normal development, lakes of, 311. 
Normal faults, 403. 
Normal mountains, 529, 530-531. 
North America, 595-599. 

maximum glaciation, 261. 
Northeast Monsoon, 750. 
Northeast storms, 769. 
Northeast trades, 753. 
Northeasters, 769. 
Northern Lights, 634-635. 
Northers, 770. 
North polar basin, 587-588. 
North pole, climate near, 810-81 1. 
North Star, 635. 
Nunatak Glacier, 215-216. 
Nunataks, 250. 
Nutting, P. G., 627. 

Oases, 793-794- 

Obelisk in New York, weathering of, 37. 

Oblate form of earth, 583. 

Oblate spheroid, 12. 

Obsequent streams, 186. 

Obsidian, 25. 



Ocean, 637-708. 

extent, 639. 

life in, 669-681. 
Ocean basins, 12-14, 583-584, 589- 
Ocean bottom life, 678-680. 
Ocean bottom plain, 584. 
Ocean bottom topography, 640. 
Ocean currents, 354-355, 687-700, 726, 786- 

787- 
Oceanica, 607-609. 
Oceanic islands, 380. 
Oceanic water, movements, 682-708. 
Ocean level, changes in, 615-616. 
Oceanography, 637. 
Ocean surface, 640. 
Oceans, distribution, 587. 
Offshore bars, 364-366. 
Offshore benches, 359. 
Offshore sand bars, 368-370. 
Ogilvie, N. J., 217. 
O'Hara, C. C., 139. 
Old age, 180. 
Old coasts, 373. 
Older drift, 296, 297-299. 
Old Faithful geyser, 86-87. 
Oldham, R. D., 422, 436, 495, 614, 627. 
Old mountains, 554. 
Old Red Sandstone, 32. 
Old valleys, 182-183. 
Old volcanoes, 477-478. 
Old World plateau, 588. 
Oligocene, 32. 
Olsson-Seffer, P., 74- 
Omori, F., 419, 436, 495. 
Oolite, 24. 
Oozes, 647-648. 
Oule, 234. 

Outlets of lakes, 310. 
Outwash gravel plains, 226, 242, 277. 
Orbit, 2. 

Ordonez, E., 495. 
Ordovician, 32. 

Ore deposits, 22, 84-85, 97, 488. 
Organic rock, 24. 
Organisms, variety of marine, 669-670. 

work in weathering, 43-44. 
Origin of earth, 617-620. 
Orinoco delta, 152. 
Orogeny, 537. 
Orthoclase feldspar, 20. 
Orthographic projection, 33. 
Osar, 273. 
Ostia, 159. 

Overburdened streams, 114. 
Overlapping spurs, 177. 
Oversteepened slopes, 231. 
Overthrust faults, 405. 
Overturned folds, 400. 
Owens Valley earthquake, 430. 



INDEX 



Ox-bow lakes, 149, 150. 
Oxidation, 39. 
Oxygen, 678, 712, 713. 
Ozark Plateau, 514. 

Pacific coastal plain, 521. 

Pacific Islands, 609. 

Pacific Ocean, 639, 644-645, 697. 

Pahoehoe, 440. 

Paint pots, 486. 

Paleogeography, 31. 

Paleozoic, 32. 

Palisades, 483, 484. 

Palmer, A. H., 745. 

Pamir, 603. 

Panama Canal, 781. 

Panama, Isthmus of, 593. 

Parasitic cones, 448. 

Paris, floods at, 109. 

Park lands, 795. 

Parks, mountain, 537, 558. 

Paschinger, V., 199, 254. 

Passarge, S., 74. 

Passes, 540, 550-552. 

Paths, storm, 768. 

Pavlow, A. P., 465. 

Peach, B. N., 538, 580. 

Peaks, 540, 547-549- 

Peale, A. C, 98. 

Peary, R. E., 247, 254, 588, 638, 644, 662, 
811. 

Peat, 338. 

Peat beds, showing submergence, 391-393. 

Pelagic life in the sea, 675-677. 

Pele, 459-462. 

Pele's hair, 466. 

Pelseneer, P., 667. 

Pence, W. D., 107. 

Penck, A., 13, 56, 98, 242, 254, 260, 277, 304, 
580, 667, 813. 

Peneplains, 183, 500, 554-556. 

Pennsylvania!!, 32. 

Peorian, 298. 

Percolation, 76. 

Perigee, 703. 

Period between tides, 701. 

Periodicals, — for general list, see Introduc- 
tion. 

Periodicity of earthquakes, 418-419. 

Permian, 32. 

Permian glaciation, 299-300. 

Perrett, F. A., 495. 

Perspiration, effect of, 734. 

Petrifaction, 97. 

Petrified wood, 97. 

Petroleum, 23. 

Petterson, O., 667. 

Philipp, II., 254. 
Philippson, A., 105. 



Phillips, O. P., 386. 

J-, 495- 
Phlegraean Fields, 453. 
Phosphate rock, 24. 
Phosphorescence, 656. 
Photography, submarine, 638. 
Physical Geography, — for definition and 

general references, see Introduction. 
Physicists, estimates of earth's age by, 624. 
Physiographic provinces, 520. 
Physiography, — for definition and general 

references, see Introduction. 
Piedmont bulbs, 204. 
Piedmont glaciers, 203-204, 239-243. 
Piedmont Plateau, 511, 525, 557. 
Pillsbury, J. E., 667. 
Piracy, river, 186, 566. 
Pirsson, L. F., 36. 
Pisa, 159. 

Leaning Tower of, 3. 
Pitch, 401, 557, 558. 
Pitted plain, 227. 
Plagioclase feldspar, 20. 
Planetary, circulation of atmosphere, 750-758. 

of ocean water, 688-691. 
Planet Deep, 645. 
Planet, earth as a, 1-10. 
Planetesimal hypothesis, 618. 
Planets, 1. 

distances, 3. 

sizes of, 2. 
Plain, ocean bottom, 584, 642. 
Plains, 497-524. 

life history, 501-507. 

of Asia, 605. 

of Europe, 600. 

of North America, 598. 

of South America, 594. 
Plankton, 672. 
Plants, influence on coasts, 373-380. 

relation to animals, 670. 

use of ground water by, 76-77. 

work in weathering, 43-44. 
Plasticity of earth, 613-614. 
Plateau of Africa, 591. 
Plateau of Australia, 607. 
Plateaus, 497-525. 

continental, 585. 

life history, 501-507. 

ocean bottom, 643. 
Platte River, 141-142. 
Play a lakes, 325. 
Playfair's Law, 177. 
Pleistocene, 32-33. 
Pliny, G., 455, 495- 
Pliocene, 32. 
Plucking, 228-229. 
Plug, volcanic, 477. 
Pocket beaches, 360. 



INDEX 



831 



Poisonous gases, 445. 
Polar climates, 810-812. 
Polaris, 635. 
Pole, cold, 803. 

magnetic, 631. 
Poles, geographical, 5. 

shifted, 621-623. 
Pollution of wells, 79. 
Polyconic projection, 33. 
Polyps, 377- 
Pompeii, 454-455. 
Porphyry, 26. 
Postglacial drainage, 291. 
Pot holes, 116-117, 125-126. 
Powcoa, 707. 

Powell, J. W., 139, 403, 523, 527, 580. 
Powers, S., 386. 
Pozzuoli, 395. 
Prairies, 514. 
Pratt, W. E., 495. 
Pre-Cambrian, 32-33. 
Precession of the equinoxes, 302. 
Precipitation, 733-745, 762. 
Prediction of tides, 708. 
Preglacial drainage, 290. 
Pre-Kansan drift, 298. 
Pre-Pleistocene Glacial Periods, 299. 
Pressure, 761. 

atmospheric, 709-712. 

in ice, 202. 

in ocean, 650. 
Pressure ridges, 662. 
Prevailing westerlies, 755, 765-767. 
Primary, 32. 
Prime meridian, 5. 
Projection, map, 33. 
Proterozoic, 33. 
Psychrometer, 734. 
Pteropod ooze, 648. 
Pueblo Indians, 794. 
Puget Sound, 521. 
Puller, L., 339. 
Pumice, 25, 27, 442. 
Pumpelly, R., 72, 74, 580. 
Push moraine, 223. 

Putnam, G. R., 153, 159, 367, 368, 371, 386. 
Pyrite, 19, 21. 

Quaking bogs, 337. 
Quartz, 19-20. 
Quartzite, 27-28. 
Quaternary, 32-33. 
Quebec landslide, 51. 
Quincke, G., 254. 
Quinton, R., 667. 

Rabot, C., 254, 339. 
Races, tidal, 705. 
Radial drainage, 558. 



Radiant energy, 16, 716-717. 
Radiating valley glaciers, 238. 
Radiation, 717, 718, 721-722, 723. 
Radiator, 718. 
Radioactivity, 492, 620. 
Radiolarian ooze, 648. 
Raft lakes, 311-312. 
Railway on glacier, 574-575. 
Railways, mountain, 573, 574, 575. 
Rain, 714, 733-745, 769, 7Q6. 

after volcanic eruptions, 444-445. 
Rainbow, 715. 
Rainfall, 700, 741. 
Rain gauge, 741. 
Rain sculpturing, 10 1. 
Rains, the, 796. 
Rainy season, 789, 794. 
Ramparts, ice, 331-332. 
Ramsay, A. C., 304, 339, 580. 
Range of temperature, 730-732. 
Ranges, 540-542. 
Ransome, F. L., 305, 495, 627. 
Rapids, 123. 
Rasmussen, K., 247. 
Ravine, 117. 

Reade, T. M., in, 139, 580, 627. 
Reaumur scale, 720. 
Rebound, elastic, 419. 
Receding glaciers, 213-216. 
Recemented glaciers, 208. 
Recent Period, 32-33. 
Recessional moraines, 272. 
Recession of waterfalls, 127-128. 
Reck, H., 254. 
Reclaimed swamps, 335. 
Reconstructed glaciers, 208. 
Recumbent folds, 400. 
Red clay, 648-649. 
Red River of the North, 172. 
Red Sea, 655, 690. 
Reed, W. G„ 385. 
Reefs, coral, 375-380. 

sand, 368-370. 

stone, 369-370. 
Reelfoot Lake, 316. 
Reeves, E. A., 36. 
Reflection, 715-716, 718. 
Refraction, 715. 
Regolith, 30-31- 
Reid, C., 386. 

H. F., 211, 212, 254, 419, 436. 
Rejuvenated streams, 187. 
Relative humidity, 733. 
Relief features of the earth, 14-15, 583-610. 
Relief, representation of, 34. 
Renard, A. F., 667. 
Replacement, 97. 
Reptiles, age of, 32. 
Residual soil, 53. 



832 



INDEX 



Resistant rock, 29, 503. 
Resorts, mountains as, 570. 
Retreats, mountains as, 571-572. 
Reusch, H., 386, 523. 
Reversed faults, 405. 
Revived mountains, 556. 
Revived streams, 187-190. 
Revolution, 1, 2, 3. 
Rhaeto-Romansh language, 571. 
Rhine River, 187-188. 
Rhyolite, 26. 
Rice, G. S., 56. 

W. N., 580. 
Richard, J., 667. 
Richardson, O. W., 633. 
Rich, J. L., 304. 
Richthofen, F. von, 72, 74. 
Ridges, 540, 546. 
Ries, H., 523. 
Rift valley lakes, 316. 
Rift valleys, 406-407. 
Right-handed deflection, 756. 
Rill work, 101. 
Ripple marks, 59-60. 
Ripples, 113. 
Rip, tide, 706. 
Riss, 260. 

Ritchie, J., Jr., 265. 
River deposits, 141-170. 
River piracy, 566. 
River plains, 497-499. 
Rivers, 100-196, 349. 

dunes near, 67. 

in mountains, 558-567. 

nature of, 100. 

of Africa, 591. 

of Asia, 606-607. 

of Australia, 607. 

of Europe, 600-601. 

of North America, 599. 

of South America, 594. 

salts in, 652. 

the enemies of lakes, 318. 

work of, 100. 
River valley cycle, 1 71-196. 
River valley swamps, 336. 
River water, sources of, 100, 104-109. 
Roaring Forties, 755. 
Robinson, H. H., 495. 
Roches mouton6es, 228-229, 299. 
Rockaway beach, 367. 
Rock basins, 231, 287. 
Rock benches, 359. 
Rock-defended terraces, 167. 
Rock, defined, 22. 
Rock disintegration, 18, 37-56. 
Rock flour, 225. 
Rock-forming minerals, 19-22. 
Rock glaciers, 208. 



Rocks, classes of, 22. 

Rocks of earth's crust, 18-31. 

Rock sheets, 538-539- 

Rock structure, effect of uniform, 502. 

Rock tables, 217. 

Rocky Mountains, 558. 

Rogers, H. D., 580. 

Rollers, 684. 

Roorbach, G. B., 356, 386. 

Roosevelt Dam, 577. 

Ropy structure, 440. 

Ross Barrier, 245. 

Rossberg landslide, 52. 

Ross, J. C., 631. 

Rotation, 1, 2. 

effect on streams, 148. 
Rotch, A. Lawrence, 745. 
Royal Gorge, 544, 558. 
Ruedemann, R., 194. 
Run-off, 76, 101, 106. 

Russell, I. C., 56, 74, 139, 239, 241, 254, 
305, 339, 474, 481, 495, 523, 580. 

Thomas, 745. 
Rust, 39. 

Sahara, 65, 67, 792-793- 
Sailing routes, 754. 
St. Lawrence system, 327-328. 
St. Martin, V., 606. 
St. Pierre, destruction of, 459. 
St. Vincent, 460. 
Sakurajima, 468. 
Salinity, 651-652, 688. 
Salisbury, R. D., 35, 36, 75, 202, 253, 254, 
262, 271, 303, 305, 493, 523, 625, 667. 
Salt, 19, 21, 89-90, 324, 651, 783. 
Salt lakes, 321-326. 
Salt marshes, 374~375- 
Sand, 295. 

Sand bars, 63, 141-142. 
Sand dunes, 59-62, 66-68. 
Sand grains, size, 61. 
Sand plains, 277. 
Sand reefs, 368-370. 
Sand storms, 65. 
Sandstone, 23-24. 
Sandstone dikes, 413. 
Sandy Hook, 366. 
Sanford, S., 523. 

San Francisco earthquake, 430-432. 
San Francisco Mountain, 471. 
Sangamon, 298. 
Sapper, K., 442, 495. 
Sapping, 504. 
Sarape, use of, 728. 
Saratoga Springs, 84. 
Sargasso Sea, 694. 
Sargassum, 671. 
Sargent, R. H., 71. 



INDEX 



833 



Satellites, 1. 

Sato, D., 495. 

Saturated air, 733, 

Saturn, 1. 

Savaii, 442. 

Savanna belts, 794-795. 

Saville-Kent, W., 386. 

Scale, 33-34- 

Scandinavia, submergence in, 397 

Scandinavian ice sheet, 258. 

Scattering, selective, 715. 

Schist, 28. 

Schneider, K., 495. 

Schott, G., 667. 

Schrader, F., 606. 

Schuchert, C, 36. 

Schwarz, E. H. L., 305, 627. 

Schwartzwald, 568. 

Scidmore, E. R., 436. 

Scott, R. F., 245, 247, 254, 812. 

W. B., 35, 580, 627. 
Scrope, P., 495. 
Sculpturing during uplift, 553. 
Sculpturing of mountains, 543-546. 
Sea breeze, 747-748. 
Sea caves, 357. 
Sea cliffs, 355-357- 
Seacoast climate, 803. 
Sea ice, 197-198, 661-662. 
Sea level, 640. 

changes in, 389-390, 615-616. 
Seasonal migration of winds, 757. 
Seasonal range, 731-732. 
Seasons, 5, 6. 
Secondary, 32. 
Sederholm, J. J., 269. 
Sedimentary rocks, 22-24. 
Sediments, 160-162. 
See, T. J. J., 627. 
Seiches, 330, 685. 
Seismic belts, 417. 
Seismographic records, 413-415. 
Seismographs, 409. 
Sekya, S., 495. 
Selective scattering, 715. 
Sellards, E. H., 339. 
Semple, E. C, 386, 580. 
Seracs, 218. 
Serpent kames, 273. 
Serpula atolls, 379. 
Shackleton, E. H., 245. 
Shaking, nature of earthquake, 411-412. 
Shale, 23-24. 
Shaler, M. K., 303. 
N. S., 56, 74, 91, 98, 139, 195, 206, 254, 
305, 306, 337, 340, 356, 386, 427, 
436, 455, 495, 523, 580, 627, 667. 
Sharks' teeth, 649. 
Sharp's Island, 384. 
3 H 



Shastina, 472. 
Shattuck, G. B., 523. 
Shaw, E. W., 154, 156, 169. 
Sheetfloods, 116. 
Sheets, volcanic, 25, 483-484. 
Shepherd, E. S., 493. 
Sheppard, T., 386. 
Sherzer, W. H., 254. 
Shifted poles, 621-623. 

relation of, to earthquakes, 418-419. 
Shimek, B., 74, 305. 
Shorelines, 342-388. 

elevated, 390-391. 

wind work near, 59-64. 
Shoshone Falls, 134. 
Shreve, F., 64. 
Siderite, 19, 21. 
Siebenthal, C. E., 67. 
Sieberg, A., 436. 
Sigsbee, C. D., 667. 
Silica, 652. 
Silicates, 20. 

Silicious sinter, 24, 86-87. 
Sills, 483-484- 
Silurian, 32. 
Simplon avalanche, 52. 
Sinclair, W. J., 432. 
Sink holes, 90-91. 
Sirocco, 771-772. 
Skaptar Jokull, 441. 
Skerries, 359. 
Sky, colour of, 654. 
Slate, 27-28. 
Sleet, 741, 743. 
Slichter, C. S., 98. 
Slickensides, 403. 
Sling psychrometer, 734. 
Slope, variations of, 1 10. 
Smith, A. L., 169. 

E. A., 523. 

G. O., 495, 580. 

J- R, 523. 

L. S., 340- 

W. S. T., 195. 
Smyth, C. H., Jr., 194. 
Snake River lavas, 481. 
Snicker's Gap, 566. 
Snow, 742-743. 

work of, 198. 
Snowfall, amount, 202. 
Snow fields, 198-202, 205. 

relation to glaciers, 202-203. 
Snow line, 198-199. 
Snow supply, 205. 
Soft water, 83. 
Soil, 30-31- 

formation of, 52-56. 

glacial, 294-295. 

importance, 52-53. 



^34 



INDEX 



Soil, lava, 482. 

residual, 53. 

volcanic, 487-488. 
Soil flow, 49. 
Solar distortion, 700. 
Solar system, 1-4. 
Solfatara, 480. 
Solidity of earth, 613. 
Solifluction, 49. 
Sollas, W. J., 386, 627. 
Solution, 100. 
Sonora earthquake, 430. 
Soufriere, 460. 
Sounding, 638. 
Sounding balloons, 709. 
South America, 592-594. 
South American earthquakes, 425-426. 
Southeast trades, 753. 
Southern hemisphere, climate of, 809-810. 
Southern Ocean, 639, 692. 
South pole, climate near, 812. 
South Sea islands, 609. 
Southwest Monsoon, 750. 
Space, cold of, 8. 
Specific gravity, 10-11. 
Spectrum, 715. 
Specular iron ore, 21. 
Spencer, J. W., 139, 386, 398, 627, 667. 
Sphagnum moss, 337, 338. 
Spine, volcanic, 460. 
Spiral nebula hypothesis, 618. 
Spits, 364. 

hooked, 366-368. 
Spouting horns, 357. 
Spring floods, 106-109. 
Springs, 80-81. 
Spring tides, 703. 
Spurr, J. E., 523, 580. 
Spurs, overlapping, 177. 
Stacks, 359. 
Stage, in cycle, 177. 
Stalactite, 24, 93-94. 
Stalagmite, 94. 
Stand Rock, 262. 
Stanford, E., 609. 
Stanton, T. W., 402. 
Step faults, 403. 
Steppes, 798. 
Steptoe, 481. 

Stereographic projection, 33. 
Steuer, A., 667. 
Stone, G. H., 74, 305. 
Stone reefs, 369-370. 
Storms, atmospheric, 759-782. 

magnetic, 631, 634. 
Storms, W. H., 195. 
Storz, descent into Vesuvius by, 447. 
Standard time, 5, 7. 
Strahan, A., 305. 



Strata, 23. 

disturbance of, 400-409. 
Stratification, 23. 
Stratified rocks, 23. 
Stratosphere, 728-729. 
Stratus clouds, 739, 740. 
Stream beds, 141. 
Stream, in ocean, 695. 
Stream junctions, 177. 
Stream load, excessive, 144. 
Stream piracy, 186. 
Streams, diversion of, 288-293, 
Striae, 224, 228. 
Strike, 401. 
Stromboli, 449-450. 
Structure, complex, effect of, 502. 

process, and stage, 370. 
Structures, rock, 29. 
Strutt, R. J., 627. 

Stumps, showing submergence, 391-393. 
Submarine canyon of Hudson, 190. 
Submerged hanging valleys, 236, 351. 
Submergence, 342-344, 391. 
Subsequent streams, 184-185. 
Subsidence, 619. 
Subsoil, 31, 54~55- 

Suess, Eduard, 386, 417, 539, 580, 609, 627. 
Summer monsoon, 749. 
Summer weather in United States, 804-807. 
Sun, and earth, 4. 

position of, 727. 
Sunrise, 6. 
Sunset, 6. 

Sun spots, 634, 783. 
Supan, A., 788, 813. 
Superimposed streams, 184-185, 558. 
Surf, 352. 

Surface currents in sea, 691-697. 
Surface life in the sea, 675-677. 
Surveying, 635. 
Suspension, 100. 
Swamps, 320, 334-338. 

mangrove, 375- 
Syenite, 26. 

Symmetrical folds, 400-401, 528. 
Symons, G. J., 463, 495. 
Synclinal mountains, 561-562. 
Syncline, 81, 400. 
Synclinorium, 402. 
Systems, mountain, 540. 

Taal, 467- 

Tablelands, 503-504. 
Talus, 48-49. 
Tanner, Z. L., 667. 
Taughannock Falls, 134-135. 
Taylor, E. G. R., 744, 7Qi- 

F. B., 128, 131, 133, i3Q» 266, 272, 273, 
280, 282, 283, 305, 533, 627. 



INDEX 



835 



Tectonic earthquakes, 410-41 1. 
Temperate zones, 797-810. 
Temperature, 710, 750-751, 762. 

distribution, 723-729. 

ocean, 656-661, 688. 
Temperature changes, 730-732. 

gradient, 729. 

measurement, 720. 
Terminal moraine, 223-225. 
Terraces, 166-168, 183, 321. 

moraine, 284. 
Terrestrial deposits, 23-24, 65-69, 72-73, 

141-170, 264-279, 552. 
Terrestrial magnetism, 629-636. 
Terrestrial processes, 15-16. 
Terrestrial tides, relation to earthquakes, 418. 
Tertiary, 32. 
Tertiary mountains, 533. 
Thames, load of, in. 
Thawing and freezing, 40. 
Thaw, January, 772. 
Thermographs, 720. 
Thermometers, 720. 
Thirty-five year periods, 783. 
Thomas, C, 580. 
Thomson, William, 628. 

Wyville, 667. 
Thoroddsen, T., 255, 495. 
Thoulet, J., 667. 
Through glaciers, 208. 
Throw, 405. 
Thunder-heads, 739. 
Thunderstorms, 773-776, 805. 
Thwaites, F. T., 263, 290. 
Tidal glaciers, 209-210. 
Tidal prediction, 708. 
Tidal races, 705. 
Tidal range, 702. 
Tidal waves, 433~435- 
Tide rip, 706. 
Tides, 7, 354, 700-708. 

in lakes, 330. 
Tied islands, 364. 
Tien Shan, 603. 
Tight, W. G., 74, 292, 306. 
Till, 223-224. 

composition, 265-266. 
Tillite, 224, 299. 
Till plains, 499. 
Till sheet, 265. 
Till, thickness, 266-267. 
Tilting, 191. 
Timber line, 543-544. 
Titanic, wreck of, 251, 663. 
Todd, David, 6, 35. 

J. E., 98, 306. 
Tolman, C. F., Jr., 306. 
Tomboro, 443. 
Tools, used by rivers, 115. 



Topographic forms, ocean bottom, 645-646. 

Tornadoes, 761, 776-778. 

Tower, W. S., 169, 523, 580. 

Townley, S. D., 628. 

Trachyte, 26. 

Trade wind belts, 790-794. 

Trade winds, 752-753. 

Transparent substances, 717. 

Transportation, 18. 

alongshore, 362. 

of rock, 218. 
Transported soils, 55. 
Transporting power of rivers, 113. 
Transverse drainage, 560. 
Trap hills, 484. 
Trellis drainage, 185, 559. 
Triassic, 32. 

Tropical cyclones, 778-781. 
Tropical swamps, 337. 
Trough of wave, 683. 
Trowbridge, A. C, 75, 169. 
True, A. C, 339. 
Tsunami, 425, 433-435, 687. 
Tuff, volcanic, 444. 
Tulare Lake, 315. 
Tundra, 334. 
Tupic, 810. 
Turnagain Arm, 705. 
Turner, E. T., 813. 
Twain, Mark, 142. 
Twilight, 709, 714. 
Tyndall, J., 212-213, 255, 745. 
Typhoons, 778-781. 
Tyrrell, J. B., 306, 340. 

Udden, J. A., 74. 
Uinta type of mountains, 527. 
Unconformity, 30. 
Underground drainage, 91. 
Underground reservoirs, 77. 
Underground rivers, 192. 
Underground water, 76-99, 104. 
Undertow, on beaches, 354. 

isostatic, 615. 
Uniform conditions, 8. 
United States, climate of, 804-809. 
earthquakes in, 426-432. 
plains and plateaus of, 508-521. 
volcanoes of, 469-475. 
United States Bureau of Fisheries, 340, 668, 

679. 
United States Census, 667. 
United States Coast and Geodetic Survey, 
35, 75, 90, 255, 387-388, 634, 668, 700, 
705- 
United States Geological Survey, 35, 43, 48, 
50, 67, 68, 75, 85, 90, 94, 99, 102, 107, 
116, 125, 137, 140, 145, 165, 169-170, 
185, 195, 238, 255, 267, 307, 34Q-34 1 , 



8 3 6 



INDEX 



358, 387-388, 402, 406, 414, 428, 433, 

437, 472, 473, 474, 476, 4Q6, 498, 

509, 512, 523-524, .545, 559, 564, 

581-582. 
United States Hydrographic Office, 664, 668, 

745, 754- 
United States Lake Survey, 340, 387. 
United States Signal Service, 745. 
United States Weather Bureau, 734, 737, 

742, 745, 76o, 764, 766, 769, 771, 773, 

779, 782, 806/808, 813. 
University of Wisconsin, 240. 
Unsymmetrical folds, 400-401. 
Upernavik Glacier, 250. 
Upham, W., 255, 281, 306, 580, 628. 
Uplifted sea bottoms, 171. 
Uplift, in mountains, 537, 556. 

local, 191. 
Upthrow, 403. 
U-shaped valleys, 230. 

Vale, 505. 

Valley forms, variations in, 183. 

Valley glaciers, 203-239. 

distribution, 237-238. 
Valleys, 100-140, 549-55°- 

drowned, 190. 
Valley train, 226. 
Valley wind, 748-749. 
Van Bebber, W. J., 781. 
Van Cleef, E. R., 768, 782. 
Vane, wind, 746. 
Van Hise, C. R., 12, 42, 56, 195, 436, 580, 

628. 
Vaporization, heat of, 722-723. 
Vapour, water, 714, 733~734- 
Variation, compass, 629-631. 
Vatna Jokull, 204, 243. 
Vaughn, T. W., 386. 

Veatch, A. C, 78, 306, 312, 340, 505, 523. 
Vedel, P., 326, 340. 
Vegetation, affecting weathering, 45-46. 

checking dunes, 62. 

influence on ccasts, 373-380. 

in swamps, 336. 

relation to wind work, 57. 
Veins, 84-85, 97. 
Velocity, of rivers, 109-111. 
Venezuelan Highland, 594. 
Vernier, 711. 
Vesuvius, 447, 453-458. 
Vichy, 83. 

Vicksburg, site of, 151. 
Victoria Falls, 134, 136. 
Viezzoli, F., 667. 
Viscosity, 203. 
Volcanic activity, 622. 
Volcanic ash, 438. 
Volcanic belts, 478-479. 



Volcanic bombs, 443-444. 
Volcanic caps, 478. 
Volcanic cones, 446-449. 

life history, 475-478. 
Volcanic dust, 58. 
Volcanic earthquakes, 410-41 1. 
Volcanic eruptions, instances, 440-475. 
Volcanic mountains, 526. 
Volcanic necks, 477, 482. 
Volcanoes, 17, 438-496, 586, 612, 645. 

imitative forms, 486-487. 

oceanic, 644. 

of Asia, 605. 
Volume, of rivers, 109-111. 
von Bohmersheim, A., 303. 
von Drygalski, E., 244, 253. 
von Engeln, O. D., 169, 523. 
von Richthofen, F., 195. 
Vosges, 554. 

Vulcanism, 17, 438-496, 586, 612, 
645- 

cause, 489-493. 

relation to earthquakes, 417-419. 
Vulcano, 449. 

Wahnschaffe, F., 306. 
Walcott, C. D., 94, 98, 628. 
Waldo, Frank, 745. 
Walther, J., 74, 667, 813. 
War, mountains in, 577. 
Ward, L. F., 98. 

R. de C, 745, 762, 763, 788, 789, 797, 809, 
811, 813-814. 
Warming of air, 723. 
Warming of land, 721-722. 
Warming of water, 722-723. 
Warmth, 715-732. 
Warnings, of floods, 109. 

of hurricanes, 781. 
Wasting of snow, 205. 
Water, and the zones, 784-785. 

chemical work of, 38-40. 

forms of, 733-745- 

in its solid form, 197-198. 

mechanical work, in weathering, 40-41. 

mineral load of, 83. 

warming of, 722-723. 

work of, in weathering, 38-41. 
Water ee, wreck of, 434. \ 
Waterfall extinction, law of, 138. 
Waterfall formation, law of, 137. 
Waterfalls, 123-126, 134-138, 174, 295. 
Water gaps, 557, 564, 565, 566-567. 
Water hemisphere, 587. 
Water power, 138, 193, 295. 
Waterspouts, 778. 
Water supply, 295. 
Water table, 77-78. 
Water vapour, 714, 733-734. 



INDEX 



837 



Water waves, earthquake, 433~435- 
Watson, T. L., 56, 289, 340, 386, 628. 
Wave-cut arches, 358-359- 
Wave-eroded material, 354. 
Wave erosion, 353. 
Waves, 352, 682-687. 

cold, 770-771. 

earthquake, 433~435- 
Weak rock, 29, 503. 
Weather, 767, 783-814. 
Weathering, 18, 37-56, 96, 118, 348, 544. 

•agents of, 37-38. 

results of, 46-52. 

variations in rate, 44-46. 
Weed, W. H., 99, 305. 
Weidman, S., 581. 
Wells, 79. 
Werth, E., 386. 
West coasts, climate of, 799. 
West declination, 629. 
Westerlies, prevailing, 755. 
West Indian earthquakes, 426. 
West Indian hurricanes, 778-780. 
Westminster Abbey, weathering at, 37. 
West wind belt, climate of, 809-810. 
Wet bulb thermometer, 734. 
Wet season, 794. 
Wheeler, G. M., 139. 

W. H., 386. 
Whirlpool Rapids, 132-134. 
Whitbeck, R. H., 523. 
White, C. A., 195. 

David, 306. 
White, photograph by, 361. 
Whitecaps, 684-685. 
Whitfield, J. E., 98. 
Whitney, J. D., 581. 
Wild, J. J., 667. 
Wilkes, C, 664, 667. 
Willamette Valley, 521. 
Willard, D. E., 523. 
Williams, F. E., 75. 
Willis, B., 36, 71, 74, 75, 103, 169, 306, 401, 

436, 538, 540, 576, 581, 628. 
Wilson, A. W. G., 195, 387, 581. 

H. M., 581. 

J. H., 306. 
Winchell, Alexander, 628. 
Wind, activities of, 57. 
Wind drift, 688. 
Wind-drift currents, 684-685. 
Wind-drift-structure, 61 . 



Wind erosion, 69-70. 
Wind gaps, 540, 567. 
Wind roses, 754. 
Winds, 726, 746-758, 761. 

and the zones, 786. 
Wind vane, 746. 
Windward coasts, 790. 
Wind waves, 352, 682-685. 
Wind work, 57-75, 349, 546. 

along shorelines, 59-64. 

in arid countries and deserts, 65-73. 

in humid lands, 57-58. 

on mountains, 58-59. 
Winter, in India, 796. 
Winter monsoon, 749. 
Winter weather in United States, 807-809. 
Wireless telegraphy, 631. 
Wisconsin drift, 298. 
Wisconsin Geological Survey, 340, 362. 
Wisconsin glaciation, 296. 
Wizard Island, 475. 
Woeikof, A., 814. 
Wolff, J. E., 580. 
Woodman, J. E., 387. 
Woodward, R. S., 628, 667. 
Woodworth, J. B., 74, 300, 306, 387, 436. 
Wright, C. W., 436. 

G. F., 73, 74, 255, 306. 
Wiirm, 260. 
Wyandotte Cave, 93. 



Yakutat Bay, 394, 423-425. 
Yarmouth, 298. 
Yazoo River, 146. 
Year, 2. 

Yellow Sea, 655. 
Yellowstone Falls, 137. 
Yellowstone Park, 84-87, 480, 482. 
Yosemite Valley, 548-549. 
Young, C. A., 35. 

R. B., 299: 
Youth, 118-119, 126, 131-134, 171-179, 

180, 288, 370-372, 554- 
Yukon delta, 152. 



Zanoga, 234. 

Zeigler, V., 74. 

Zone of compensation, 615. 

Zone of flowage, 12. 

Zone of fracture, 10. 

Zones, climatic, 725, 783-788. 






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